[v1] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

[PATCH -next] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

Posted by Baisong Zhong 3 years, 5 months ago

Recently, we got a syzkaller problem because of aarch64
alignment fault if KFENCE enabled.

When the size from user bpf program is an odd number, like
399, 407, etc, it will cause skb shard info's alignment access,
as seen below:

BUG: KFENCE: use-after-free read in __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032

Use-after-free read at 0xffff6254fffac077 (in kfence-#213):
 __lse_atomic_add arch/arm64/include/asm/atomic_lse.h:26 [inline]
 arch_atomic_add arch/arm64/include/asm/atomic.h:28 [inline]
 arch_atomic_inc include/linux/atomic-arch-fallback.h:270 [inline]
 atomic_inc include/asm-generic/atomic-instrumented.h:241 [inline]
 __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
 skb_clone+0xf4/0x214 net/core/skbuff.c:1481
 ____bpf_clone_redirect net/core/filter.c:2433 [inline]
 bpf_clone_redirect+0x78/0x1c0 net/core/filter.c:2420
 bpf_prog_d3839dd9068ceb51+0x80/0x330
 bpf_dispatcher_nop_func include/linux/bpf.h:728 [inline]
 bpf_test_run+0x3c0/0x6c0 net/bpf/test_run.c:53
 bpf_prog_test_run_skb+0x638/0xa7c net/bpf/test_run.c:594
 bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
 __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
 __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381

kfence-#213: 0xffff6254fffac000-0xffff6254fffac196, size=407, cache=kmalloc-512

allocated by task 15074 on cpu 0 at 1342.585390s:
 kmalloc include/linux/slab.h:568 [inline]
 kzalloc include/linux/slab.h:675 [inline]
 bpf_test_init.isra.0+0xac/0x290 net/bpf/test_run.c:191
 bpf_prog_test_run_skb+0x11c/0xa7c net/bpf/test_run.c:512
 bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
 __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
 __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
 __arm64_sys_bpf+0x50/0x60 kernel/bpf/syscall.c:4381

To fix the problem, we round up allocations with kmalloc_size_roundup()
so that build_skb()'s use of kize() is always alignment and no special
handling of the memory is needed by KFENCE.

Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
---
 net/bpf/test_run.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 13d578ce2a09..058b67108873 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -774,6 +774,7 @@ static void *bpf_test_init(const union bpf_attr *kattr, u32 user_size,
 	if (user_size > size)
 		return ERR_PTR(-EMSGSIZE);
 
+	size = kmalloc_size_roundup(size);
 	data = kzalloc(size + headroom + tailroom, GFP_USER);
 	if (!data)
 		return ERR_PTR(-ENOMEM);
-- 
2.25.1

Re: [PATCH -next] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

Posted by Daniel Borkmann 3 years, 5 months ago

[ +kfence folks ]

On 11/1/22 5:04 AM, Baisong Zhong wrote:
> Recently, we got a syzkaller problem because of aarch64
> alignment fault if KFENCE enabled.
> 
> When the size from user bpf program is an odd number, like
> 399, 407, etc, it will cause skb shard info's alignment access,
> as seen below:
> 
> BUG: KFENCE: use-after-free read in __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
> 
> Use-after-free read at 0xffff6254fffac077 (in kfence-#213):
>   __lse_atomic_add arch/arm64/include/asm/atomic_lse.h:26 [inline]
>   arch_atomic_add arch/arm64/include/asm/atomic.h:28 [inline]
>   arch_atomic_inc include/linux/atomic-arch-fallback.h:270 [inline]
>   atomic_inc include/asm-generic/atomic-instrumented.h:241 [inline]
>   __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
>   skb_clone+0xf4/0x214 net/core/skbuff.c:1481
>   ____bpf_clone_redirect net/core/filter.c:2433 [inline]
>   bpf_clone_redirect+0x78/0x1c0 net/core/filter.c:2420
>   bpf_prog_d3839dd9068ceb51+0x80/0x330
>   bpf_dispatcher_nop_func include/linux/bpf.h:728 [inline]
>   bpf_test_run+0x3c0/0x6c0 net/bpf/test_run.c:53
>   bpf_prog_test_run_skb+0x638/0xa7c net/bpf/test_run.c:594
>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
> 
> kfence-#213: 0xffff6254fffac000-0xffff6254fffac196, size=407, cache=kmalloc-512
> 
> allocated by task 15074 on cpu 0 at 1342.585390s:
>   kmalloc include/linux/slab.h:568 [inline]
>   kzalloc include/linux/slab.h:675 [inline]
>   bpf_test_init.isra.0+0xac/0x290 net/bpf/test_run.c:191
>   bpf_prog_test_run_skb+0x11c/0xa7c net/bpf/test_run.c:512
>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
>   __arm64_sys_bpf+0x50/0x60 kernel/bpf/syscall.c:4381
> 
> To fix the problem, we round up allocations with kmalloc_size_roundup()
> so that build_skb()'s use of kize() is always alignment and no special
> handling of the memory is needed by KFENCE.
> 
> Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
> Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
> ---
>   net/bpf/test_run.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 13d578ce2a09..058b67108873 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -774,6 +774,7 @@ static void *bpf_test_init(const union bpf_attr *kattr, u32 user_size,
>   	if (user_size > size)
>   		return ERR_PTR(-EMSGSIZE);
>   
> +	size = kmalloc_size_roundup(size);
>   	data = kzalloc(size + headroom + tailroom, GFP_USER);

The fact that you need to do this roundup on call sites feels broken, no?
Was there some discussion / consensus that now all k*alloc() call sites
would need to be fixed up? Couldn't this be done transparently in k*alloc()
when KFENCE is enabled? I presume there may be lots of other such occasions
in the kernel where similar issue triggers, fixing up all call-sites feels
like ton of churn compared to api-internal, generic fix.

>   	if (!data)
>   		return ERR_PTR(-ENOMEM);
> 

Thanks,
Daniel

Re: [PATCH -next] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

Posted by zhongbaisong 3 years, 5 months ago


On 2022/11/2 0:45, Daniel Borkmann wrote:
> [ +kfence folks ]

+ cc: Alexander Potapenko, Marco Elver, Dmitry Vyukov

Do you have any suggestions about this problem?

Thanks,

.

> 
> On 11/1/22 5:04 AM, Baisong Zhong wrote:
>> Recently, we got a syzkaller problem because of aarch64
>> alignment fault if KFENCE enabled.
>>
>> When the size from user bpf program is an odd number, like
>> 399, 407, etc, it will cause skb shard info's alignment access,
>> as seen below:
>>
>> BUG: KFENCE: use-after-free read in __skb_clone+0x23c/0x2a0 
>> net/core/skbuff.c:1032
>>
>> Use-after-free read at 0xffff6254fffac077 (in kfence-#213):
>>   __lse_atomic_add arch/arm64/include/asm/atomic_lse.h:26 [inline]
>>   arch_atomic_add arch/arm64/include/asm/atomic.h:28 [inline]
>>   arch_atomic_inc include/linux/atomic-arch-fallback.h:270 [inline]
>>   atomic_inc include/asm-generic/atomic-instrumented.h:241 [inline]
>>   __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
>>   skb_clone+0xf4/0x214 net/core/skbuff.c:1481
>>   ____bpf_clone_redirect net/core/filter.c:2433 [inline]
>>   bpf_clone_redirect+0x78/0x1c0 net/core/filter.c:2420
>>   bpf_prog_d3839dd9068ceb51+0x80/0x330
>>   bpf_dispatcher_nop_func include/linux/bpf.h:728 [inline]
>>   bpf_test_run+0x3c0/0x6c0 net/bpf/test_run.c:53
>>   bpf_prog_test_run_skb+0x638/0xa7c net/bpf/test_run.c:594
>>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
>>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
>>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
>>
>> kfence-#213: 0xffff6254fffac000-0xffff6254fffac196, size=407, 
>> cache=kmalloc-512
>>
>> allocated by task 15074 on cpu 0 at 1342.585390s:
>>   kmalloc include/linux/slab.h:568 [inline]
>>   kzalloc include/linux/slab.h:675 [inline]
>>   bpf_test_init.isra.0+0xac/0x290 net/bpf/test_run.c:191
>>   bpf_prog_test_run_skb+0x11c/0xa7c net/bpf/test_run.c:512
>>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
>>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
>>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
>>   __arm64_sys_bpf+0x50/0x60 kernel/bpf/syscall.c:4381
>>
>> To fix the problem, we round up allocations with kmalloc_size_roundup()
>> so that build_skb()'s use of kize() is always alignment and no special
>> handling of the memory is needed by KFENCE.
>>
>> Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
>> Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
>> ---
>>   net/bpf/test_run.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
>> index 13d578ce2a09..058b67108873 100644
>> --- a/net/bpf/test_run.c
>> +++ b/net/bpf/test_run.c
>> @@ -774,6 +774,7 @@ static void *bpf_test_init(const union bpf_attr 
>> *kattr, u32 user_size,
>>       if (user_size > size)
>>           return ERR_PTR(-EMSGSIZE);
>> +    size = kmalloc_size_roundup(size);
>>       data = kzalloc(size + headroom + tailroom, GFP_USER);
> 
> The fact that you need to do this roundup on call sites feels broken, no?
> Was there some discussion / consensus that now all k*alloc() call sites
> would need to be fixed up? Couldn't this be done transparently in k*alloc()
> when KFENCE is enabled? I presume there may be lots of other such occasions
> in the kernel where similar issue triggers, fixing up all call-sites feels
> like ton of churn compared to api-internal, generic fix.
> 
>>       if (!data)
>>           return ERR_PTR(-ENOMEM);
>>
> 
> Thanks,
> Daniel
>

Re: [PATCH -next] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

Posted by Jakub Kicinski 3 years, 5 months ago

On Wed, 2 Nov 2022 10:59:44 +0800 zhongbaisong wrote:
> On 2022/11/2 0:45, Daniel Borkmann wrote:
> > [ +kfence folks ]  
> 
> + cc: Alexander Potapenko, Marco Elver, Dmitry Vyukov
> 
> Do you have any suggestions about this problem?

+ Kees who has been sending similar patches for drivers

> > On 11/1/22 5:04 AM, Baisong Zhong wrote:  
> >> Recently, we got a syzkaller problem because of aarch64
> >> alignment fault if KFENCE enabled.
> >>
> >> When the size from user bpf program is an odd number, like
> >> 399, 407, etc, it will cause skb shard info's alignment access,
> >> as seen below:
> >>
> >> BUG: KFENCE: use-after-free read in __skb_clone+0x23c/0x2a0 
> >> net/core/skbuff.c:1032
> >>
> >> Use-after-free read at 0xffff6254fffac077 (in kfence-#213):
> >>   __lse_atomic_add arch/arm64/include/asm/atomic_lse.h:26 [inline]
> >>   arch_atomic_add arch/arm64/include/asm/atomic.h:28 [inline]
> >>   arch_atomic_inc include/linux/atomic-arch-fallback.h:270 [inline]
> >>   atomic_inc include/asm-generic/atomic-instrumented.h:241 [inline]
> >>   __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
> >>   skb_clone+0xf4/0x214 net/core/skbuff.c:1481
> >>   ____bpf_clone_redirect net/core/filter.c:2433 [inline]
> >>   bpf_clone_redirect+0x78/0x1c0 net/core/filter.c:2420
> >>   bpf_prog_d3839dd9068ceb51+0x80/0x330
> >>   bpf_dispatcher_nop_func include/linux/bpf.h:728 [inline]
> >>   bpf_test_run+0x3c0/0x6c0 net/bpf/test_run.c:53
> >>   bpf_prog_test_run_skb+0x638/0xa7c net/bpf/test_run.c:594
> >>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
> >>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
> >>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
> >>
> >> kfence-#213: 0xffff6254fffac000-0xffff6254fffac196, size=407, 
> >> cache=kmalloc-512
> >>
> >> allocated by task 15074 on cpu 0 at 1342.585390s:
> >>   kmalloc include/linux/slab.h:568 [inline]
> >>   kzalloc include/linux/slab.h:675 [inline]
> >>   bpf_test_init.isra.0+0xac/0x290 net/bpf/test_run.c:191
> >>   bpf_prog_test_run_skb+0x11c/0xa7c net/bpf/test_run.c:512
> >>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
> >>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
> >>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
> >>   __arm64_sys_bpf+0x50/0x60 kernel/bpf/syscall.c:4381
> >>
> >> To fix the problem, we round up allocations with kmalloc_size_roundup()
> >> so that build_skb()'s use of kize() is always alignment and no special
> >> handling of the memory is needed by KFENCE.
> >>
> >> Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
> >> Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
> >> ---
> >>   net/bpf/test_run.c | 1 +
> >>   1 file changed, 1 insertion(+)
> >>
> >> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> >> index 13d578ce2a09..058b67108873 100644
> >> --- a/net/bpf/test_run.c
> >> +++ b/net/bpf/test_run.c
> >> @@ -774,6 +774,7 @@ static void *bpf_test_init(const union bpf_attr 
> >> *kattr, u32 user_size,
> >>       if (user_size > size)
> >>           return ERR_PTR(-EMSGSIZE);
> >> +    size = kmalloc_size_roundup(size);
> >>       data = kzalloc(size + headroom + tailroom, GFP_USER);  
> > 
> > The fact that you need to do this roundup on call sites feels broken, no?
> > Was there some discussion / consensus that now all k*alloc() call sites
> > would need to be fixed up? Couldn't this be done transparently in k*alloc()
> > when KFENCE is enabled? I presume there may be lots of other such occasions
> > in the kernel where similar issue triggers, fixing up all call-sites feels
> > like ton of churn compared to api-internal, generic fix.
> >   
> >>       if (!data)
> >>           return ERR_PTR(-ENOMEM);
> >>  
> > 
> > Thanks,
> > Daniel
> >  
> 
>

Re: [PATCH -next] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

Posted by Kees Cook 3 years, 5 months ago

On Tue, Nov 01, 2022 at 09:05:42PM -0700, Jakub Kicinski wrote:
> On Wed, 2 Nov 2022 10:59:44 +0800 zhongbaisong wrote:
> > On 2022/11/2 0:45, Daniel Borkmann wrote:
> > > [ +kfence folks ]  
> > 
> > + cc: Alexander Potapenko, Marco Elver, Dmitry Vyukov
> > 
> > Do you have any suggestions about this problem?
> 
> + Kees who has been sending similar patches for drivers
> 
> > > On 11/1/22 5:04 AM, Baisong Zhong wrote:  
> > >> Recently, we got a syzkaller problem because of aarch64
> > >> alignment fault if KFENCE enabled.
> > >>
> > >> When the size from user bpf program is an odd number, like
> > >> 399, 407, etc, it will cause skb shard info's alignment access,
> > >> as seen below:
> > >>
> > >> BUG: KFENCE: use-after-free read in __skb_clone+0x23c/0x2a0 
> > >> net/core/skbuff.c:1032
> > >>
> > >> Use-after-free read at 0xffff6254fffac077 (in kfence-#213):
> > >>   __lse_atomic_add arch/arm64/include/asm/atomic_lse.h:26 [inline]
> > >>   arch_atomic_add arch/arm64/include/asm/atomic.h:28 [inline]
> > >>   arch_atomic_inc include/linux/atomic-arch-fallback.h:270 [inline]
> > >>   atomic_inc include/asm-generic/atomic-instrumented.h:241 [inline]
> > >>   __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
> > >>   skb_clone+0xf4/0x214 net/core/skbuff.c:1481
> > >>   ____bpf_clone_redirect net/core/filter.c:2433 [inline]
> > >>   bpf_clone_redirect+0x78/0x1c0 net/core/filter.c:2420
> > >>   bpf_prog_d3839dd9068ceb51+0x80/0x330
> > >>   bpf_dispatcher_nop_func include/linux/bpf.h:728 [inline]
> > >>   bpf_test_run+0x3c0/0x6c0 net/bpf/test_run.c:53
> > >>   bpf_prog_test_run_skb+0x638/0xa7c net/bpf/test_run.c:594
> > >>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
> > >>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
> > >>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
> > >>
> > >> kfence-#213: 0xffff6254fffac000-0xffff6254fffac196, size=407, 
> > >> cache=kmalloc-512
> > >>
> > >> allocated by task 15074 on cpu 0 at 1342.585390s:
> > >>   kmalloc include/linux/slab.h:568 [inline]
> > >>   kzalloc include/linux/slab.h:675 [inline]
> > >>   bpf_test_init.isra.0+0xac/0x290 net/bpf/test_run.c:191
> > >>   bpf_prog_test_run_skb+0x11c/0xa7c net/bpf/test_run.c:512
> > >>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
> > >>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
> > >>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
> > >>   __arm64_sys_bpf+0x50/0x60 kernel/bpf/syscall.c:4381
> > >>
> > >> To fix the problem, we round up allocations with kmalloc_size_roundup()
> > >> so that build_skb()'s use of kize() is always alignment and no special
> > >> handling of the memory is needed by KFENCE.
> > >>
> > >> Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
> > >> Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
> > >> ---
> > >>   net/bpf/test_run.c | 1 +
> > >>   1 file changed, 1 insertion(+)
> > >>
> > >> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> > >> index 13d578ce2a09..058b67108873 100644
> > >> --- a/net/bpf/test_run.c
> > >> +++ b/net/bpf/test_run.c
> > >> @@ -774,6 +774,7 @@ static void *bpf_test_init(const union bpf_attr 
> > >> *kattr, u32 user_size,
> > >>       if (user_size > size)
> > >>           return ERR_PTR(-EMSGSIZE);
> > >> +    size = kmalloc_size_roundup(size);
> > >>       data = kzalloc(size + headroom + tailroom, GFP_USER);  
> > > 
> > > The fact that you need to do this roundup on call sites feels broken, no?
> > > Was there some discussion / consensus that now all k*alloc() call sites
> > > would need to be fixed up? Couldn't this be done transparently in k*alloc()
> > > when KFENCE is enabled? I presume there may be lots of other such occasions
> > > in the kernel where similar issue triggers, fixing up all call-sites feels
> > > like ton of churn compared to api-internal, generic fix.

I hope I answer this in more detail here:
https://lore.kernel.org/lkml/202211010937.4631CB1B0E@keescook/

The problem is that ksize() should never have existed in the first
place. :P Every runtime bounds checker has tripped over it, and with
the addition of the __alloc_size attribute, I had to start ripping
ksize() out: it can't be used to pretend an allocation grew in size.
Things need to either preallocate more or go through *realloc() like
everything else. Luckily, ksize() is rare.

FWIW, the above fix doesn't look correct to me -- I would expect this to
be:

	size_t alloc_size;
	...
	alloc_size = kmalloc_size_roundup(size + headroom + tailroom);
	data = kzalloc(alloc_size, GFP_USER);

-- 
Kees Cook

Re: [PATCH -next] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

Posted by Eric Dumazet 3 years, 5 months ago

On Tue, Nov 1, 2022 at 9:27 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Tue, Nov 01, 2022 at 09:05:42PM -0700, Jakub Kicinski wrote:
> > On Wed, 2 Nov 2022 10:59:44 +0800 zhongbaisong wrote:
> > > On 2022/11/2 0:45, Daniel Borkmann wrote:
> > > > [ +kfence folks ]
> > >
> > > + cc: Alexander Potapenko, Marco Elver, Dmitry Vyukov
> > >
> > > Do you have any suggestions about this problem?
> >
> > + Kees who has been sending similar patches for drivers
> >
> > > > On 11/1/22 5:04 AM, Baisong Zhong wrote:
> > > >> Recently, we got a syzkaller problem because of aarch64
> > > >> alignment fault if KFENCE enabled.
> > > >>
> > > >> When the size from user bpf program is an odd number, like
> > > >> 399, 407, etc, it will cause skb shard info's alignment access,
> > > >> as seen below:
> > > >>
> > > >> BUG: KFENCE: use-after-free read in __skb_clone+0x23c/0x2a0
> > > >> net/core/skbuff.c:1032
> > > >>
> > > >> Use-after-free read at 0xffff6254fffac077 (in kfence-#213):
> > > >>   __lse_atomic_add arch/arm64/include/asm/atomic_lse.h:26 [inline]
> > > >>   arch_atomic_add arch/arm64/include/asm/atomic.h:28 [inline]
> > > >>   arch_atomic_inc include/linux/atomic-arch-fallback.h:270 [inline]
> > > >>   atomic_inc include/asm-generic/atomic-instrumented.h:241 [inline]
> > > >>   __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
> > > >>   skb_clone+0xf4/0x214 net/core/skbuff.c:1481
> > > >>   ____bpf_clone_redirect net/core/filter.c:2433 [inline]
> > > >>   bpf_clone_redirect+0x78/0x1c0 net/core/filter.c:2420
> > > >>   bpf_prog_d3839dd9068ceb51+0x80/0x330
> > > >>   bpf_dispatcher_nop_func include/linux/bpf.h:728 [inline]
> > > >>   bpf_test_run+0x3c0/0x6c0 net/bpf/test_run.c:53
> > > >>   bpf_prog_test_run_skb+0x638/0xa7c net/bpf/test_run.c:594
> > > >>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
> > > >>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
> > > >>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
> > > >>
> > > >> kfence-#213: 0xffff6254fffac000-0xffff6254fffac196, size=407,
> > > >> cache=kmalloc-512
> > > >>
> > > >> allocated by task 15074 on cpu 0 at 1342.585390s:
> > > >>   kmalloc include/linux/slab.h:568 [inline]
> > > >>   kzalloc include/linux/slab.h:675 [inline]
> > > >>   bpf_test_init.isra.0+0xac/0x290 net/bpf/test_run.c:191
> > > >>   bpf_prog_test_run_skb+0x11c/0xa7c net/bpf/test_run.c:512
> > > >>   bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
> > > >>   __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
> > > >>   __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
> > > >>   __arm64_sys_bpf+0x50/0x60 kernel/bpf/syscall.c:4381
> > > >>
> > > >> To fix the problem, we round up allocations with kmalloc_size_roundup()
> > > >> so that build_skb()'s use of kize() is always alignment and no special
> > > >> handling of the memory is needed by KFENCE.
> > > >>
> > > >> Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
> > > >> Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
> > > >> ---
> > > >>   net/bpf/test_run.c | 1 +
> > > >>   1 file changed, 1 insertion(+)
> > > >>
> > > >> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> > > >> index 13d578ce2a09..058b67108873 100644
> > > >> --- a/net/bpf/test_run.c
> > > >> +++ b/net/bpf/test_run.c
> > > >> @@ -774,6 +774,7 @@ static void *bpf_test_init(const union bpf_attr
> > > >> *kattr, u32 user_size,
> > > >>       if (user_size > size)
> > > >>           return ERR_PTR(-EMSGSIZE);
> > > >> +    size = kmalloc_size_roundup(size);
> > > >>       data = kzalloc(size + headroom + tailroom, GFP_USER);
> > > >
> > > > The fact that you need to do this roundup on call sites feels broken, no?
> > > > Was there some discussion / consensus that now all k*alloc() call sites
> > > > would need to be fixed up? Couldn't this be done transparently in k*alloc()
> > > > when KFENCE is enabled? I presume there may be lots of other such occasions
> > > > in the kernel where similar issue triggers, fixing up all call-sites feels
> > > > like ton of churn compared to api-internal, generic fix.
>
> I hope I answer this in more detail here:
> https://lore.kernel.org/lkml/202211010937.4631CB1B0E@keescook/
>
> The problem is that ksize() should never have existed in the first
> place. :P Every runtime bounds checker has tripped over it, and with
> the addition of the __alloc_size attribute, I had to start ripping
> ksize() out: it can't be used to pretend an allocation grew in size.
> Things need to either preallocate more or go through *realloc() like
> everything else. Luckily, ksize() is rare.
>
> FWIW, the above fix doesn't look correct to me -- I would expect this to
> be:
>
>         size_t alloc_size;
>         ...
>         alloc_size = kmalloc_size_roundup(size + headroom + tailroom);
>         data = kzalloc(alloc_size, GFP_USER);

Making sure the struct skb_shared_info is aligned to a cache line does
not need kmalloc_size_roundup().

What is needed is to adjust @size so that (@size + @headroom) is a
multiple of SMP_CACHE_BYTES

Re: [PATCH -next] bpf, test_run: fix alignment problem in bpf_prog_test_run_skb()

Posted by zhongbaisong 3 years, 5 months ago


On 2022/11/2 12:37, Eric Dumazet wrote:
> On Tue, Nov 1, 2022 at 9:27 PM Kees Cook <keescook@chromium.org> wrote:
>>
>> On Tue, Nov 01, 2022 at 09:05:42PM -0700, Jakub Kicinski wrote:
>>> On Wed, 2 Nov 2022 10:59:44 +0800 zhongbaisong wrote:
>>>> On 2022/11/2 0:45, Daniel Borkmann wrote:
>>>>> [ +kfence folks ]
>>>>
>>>> + cc: Alexander Potapenko, Marco Elver, Dmitry Vyukov
>>>>
>>>> Do you have any suggestions about this problem?
>>>
>>> + Kees who has been sending similar patches for drivers
>>>
>>>>> On 11/1/22 5:04 AM, Baisong Zhong wrote:
>>>>>> Recently, we got a syzkaller problem because of aarch64
>>>>>> alignment fault if KFENCE enabled.
>>>>>>
>>>>>> When the size from user bpf program is an odd number, like
>>>>>> 399, 407, etc, it will cause skb shard info's alignment access,
>>>>>> as seen below:
>>>>>>
>>>>>> BUG: KFENCE: use-after-free read in __skb_clone+0x23c/0x2a0
>>>>>> net/core/skbuff.c:1032
>>>>>>
>>>>>> Use-after-free read at 0xffff6254fffac077 (in kfence-#213):
>>>>>>    __lse_atomic_add arch/arm64/include/asm/atomic_lse.h:26 [inline]
>>>>>>    arch_atomic_add arch/arm64/include/asm/atomic.h:28 [inline]
>>>>>>    arch_atomic_inc include/linux/atomic-arch-fallback.h:270 [inline]
>>>>>>    atomic_inc include/asm-generic/atomic-instrumented.h:241 [inline]
>>>>>>    __skb_clone+0x23c/0x2a0 net/core/skbuff.c:1032
>>>>>>    skb_clone+0xf4/0x214 net/core/skbuff.c:1481
>>>>>>    ____bpf_clone_redirect net/core/filter.c:2433 [inline]
>>>>>>    bpf_clone_redirect+0x78/0x1c0 net/core/filter.c:2420
>>>>>>    bpf_prog_d3839dd9068ceb51+0x80/0x330
>>>>>>    bpf_dispatcher_nop_func include/linux/bpf.h:728 [inline]
>>>>>>    bpf_test_run+0x3c0/0x6c0 net/bpf/test_run.c:53
>>>>>>    bpf_prog_test_run_skb+0x638/0xa7c net/bpf/test_run.c:594
>>>>>>    bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
>>>>>>    __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
>>>>>>    __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
>>>>>>
>>>>>> kfence-#213: 0xffff6254fffac000-0xffff6254fffac196, size=407,
>>>>>> cache=kmalloc-512
>>>>>>
>>>>>> allocated by task 15074 on cpu 0 at 1342.585390s:
>>>>>>    kmalloc include/linux/slab.h:568 [inline]
>>>>>>    kzalloc include/linux/slab.h:675 [inline]
>>>>>>    bpf_test_init.isra.0+0xac/0x290 net/bpf/test_run.c:191
>>>>>>    bpf_prog_test_run_skb+0x11c/0xa7c net/bpf/test_run.c:512
>>>>>>    bpf_prog_test_run kernel/bpf/syscall.c:3148 [inline]
>>>>>>    __do_sys_bpf kernel/bpf/syscall.c:4441 [inline]
>>>>>>    __se_sys_bpf+0xad0/0x1634 kernel/bpf/syscall.c:4381
>>>>>>    __arm64_sys_bpf+0x50/0x60 kernel/bpf/syscall.c:4381
>>>>>>
>>>>>> To fix the problem, we round up allocations with kmalloc_size_roundup()
>>>>>> so that build_skb()'s use of kize() is always alignment and no special
>>>>>> handling of the memory is needed by KFENCE.
>>>>>>
>>>>>> Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
>>>>>> Signed-off-by: Baisong Zhong <zhongbaisong@huawei.com>
>>>>>> ---
>>>>>>    net/bpf/test_run.c | 1 +
>>>>>>    1 file changed, 1 insertion(+)
>>>>>>
>>>>>> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
>>>>>> index 13d578ce2a09..058b67108873 100644
>>>>>> --- a/net/bpf/test_run.c
>>>>>> +++ b/net/bpf/test_run.c
>>>>>> @@ -774,6 +774,7 @@ static void *bpf_test_init(const union bpf_attr
>>>>>> *kattr, u32 user_size,
>>>>>>        if (user_size > size)
>>>>>>            return ERR_PTR(-EMSGSIZE);
>>>>>> +    size = kmalloc_size_roundup(size);
>>>>>>        data = kzalloc(size + headroom + tailroom, GFP_USER);
>>>>>
>>>>> The fact that you need to do this roundup on call sites feels broken, no?
>>>>> Was there some discussion / consensus that now all k*alloc() call sites
>>>>> would need to be fixed up? Couldn't this be done transparently in k*alloc()
>>>>> when KFENCE is enabled? I presume there may be lots of other such occasions
>>>>> in the kernel where similar issue triggers, fixing up all call-sites feels
>>>>> like ton of churn compared to api-internal, generic fix.
>>
>> I hope I answer this in more detail here:
>> https://lore.kernel.org/lkml/202211010937.4631CB1B0E@keescook/
>>
>> The problem is that ksize() should never have existed in the first
>> place. :P Every runtime bounds checker has tripped over it, and with
>> the addition of the __alloc_size attribute, I had to start ripping
>> ksize() out: it can't be used to pretend an allocation grew in size.
>> Things need to either preallocate more or go through *realloc() like
>> everything else. Luckily, ksize() is rare.
>>
>> FWIW, the above fix doesn't look correct to me -- I would expect this to
>> be:
>>
>>          size_t alloc_size;
>>          ...
>>          alloc_size = kmalloc_size_roundup(size + headroom + tailroom);
>>          data = kzalloc(alloc_size, GFP_USER);
> 
> Making sure the struct skb_shared_info is aligned to a cache line does
> not need kmalloc_size_roundup().
> 
> What is needed is to adjust @size so that (@size + @headroom) is a
> multiple of SMP_CACHE_BYTES

ok, I'll fix it and send v2.

Thanks

.