A KVM guest could store tags in a page even if the VMM hasn't mapped
the page with PROT_MTE. So when restoring pages from swap we will
need to check to see if there are any saved tags even if !pte_tagged().
However don't check pages which are !pte_valid_user() as these will
not have been swapped out.
Signed-off-by: Steven Price <steven.price@arm.com>
---
arch/arm64/include/asm/pgtable.h | 2 +-
arch/arm64/kernel/mte.c | 16 ++++++++++++----
2 files changed, 13 insertions(+), 5 deletions(-)
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index e17b96d0e4b5..cf4b52a33b3c 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -312,7 +312,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
__sync_icache_dcache(pte);
if (system_supports_mte() &&
- pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
+ pte_present(pte) && (pte_val(pte) & PTE_USER) && !pte_special(pte))
mte_sync_tags(ptep, pte);
__check_racy_pte_update(mm, ptep, pte);
diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index b3c70a612c7a..e016ab57ea36 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -26,17 +26,23 @@ u64 gcr_kernel_excl __ro_after_init;
static bool report_fault_once = true;
-static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
+static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap,
+ bool pte_is_tagged)
{
pte_t old_pte = READ_ONCE(*ptep);
if (check_swap && is_swap_pte(old_pte)) {
swp_entry_t entry = pte_to_swp_entry(old_pte);
- if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
+ if (!non_swap_entry(entry) && mte_restore_tags(entry, page)) {
+ set_bit(PG_mte_tagged, &page->flags);
return;
+ }
}
+ if (!pte_is_tagged || test_and_set_bit(PG_mte_tagged, &page->flags))
+ return;
+
page_kasan_tag_reset(page);
/*
* We need smp_wmb() in between setting the flags and clearing the
@@ -54,11 +60,13 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
struct page *page = pte_page(pte);
long i, nr_pages = compound_nr(page);
bool check_swap = nr_pages == 1;
+ bool pte_is_tagged = pte_tagged(pte);
/* if PG_mte_tagged is set, tags have already been initialised */
for (i = 0; i < nr_pages; i++, page++) {
- if (!test_and_set_bit(PG_mte_tagged, &page->flags))
- mte_sync_page_tags(page, ptep, check_swap);
+ if (!test_bit(PG_mte_tagged, &page->flags))
+ mte_sync_page_tags(page, ptep, check_swap,
+ pte_is_tagged);
}
}
--
2.20.1
On Fri, Apr 16, 2021 at 04:43:04PM +0100, Steven Price wrote:
> A KVM guest could store tags in a page even if the VMM hasn't mapped
> the page with PROT_MTE. So when restoring pages from swap we will
> need to check to see if there are any saved tags even if !pte_tagged().
>
> However don't check pages which are !pte_valid_user() as these will
> not have been swapped out.
You should remove the pte_valid_user() mention from the commit log as
well.
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index e17b96d0e4b5..cf4b52a33b3c 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -312,7 +312,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
> __sync_icache_dcache(pte);
>
> if (system_supports_mte() &&
> - pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
> + pte_present(pte) && (pte_val(pte) & PTE_USER) && !pte_special(pte))
I would add a pte_user() macro here or, if we restore the tags only when
the page is readable, use pte_access_permitted(pte, false). Also add a
comment why we do this.
There's also the pte_user_exec() case which may not have the PTE_USER
set (exec-only permission) but I don't think it matters. We don't do tag
checking on instruction fetches, so if the user adds a PROT_READ to it,
it would go through set_pte_at() again. I'm not sure KVM does anything
special with exec-only mappings at stage 2, I suspect they won't be
accessible by the guest (but needs checking).
> mte_sync_tags(ptep, pte);
>
> __check_racy_pte_update(mm, ptep, pte);
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index b3c70a612c7a..e016ab57ea36 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -26,17 +26,23 @@ u64 gcr_kernel_excl __ro_after_init;
>
> static bool report_fault_once = true;
>
> -static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
> +static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap,
> + bool pte_is_tagged)
> {
> pte_t old_pte = READ_ONCE(*ptep);
>
> if (check_swap && is_swap_pte(old_pte)) {
> swp_entry_t entry = pte_to_swp_entry(old_pte);
>
> - if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
> + if (!non_swap_entry(entry) && mte_restore_tags(entry, page)) {
> + set_bit(PG_mte_tagged, &page->flags);
> return;
> + }
> }
>
> + if (!pte_is_tagged || test_and_set_bit(PG_mte_tagged, &page->flags))
> + return;
I don't think we need another test_bit() here, it was done in the
caller (bar potential races which need more thought).
> +
> page_kasan_tag_reset(page);
> /*
> * We need smp_wmb() in between setting the flags and clearing the
> @@ -54,11 +60,13 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
> struct page *page = pte_page(pte);
> long i, nr_pages = compound_nr(page);
> bool check_swap = nr_pages == 1;
> + bool pte_is_tagged = pte_tagged(pte);
>
> /* if PG_mte_tagged is set, tags have already been initialised */
> for (i = 0; i < nr_pages; i++, page++) {
> - if (!test_and_set_bit(PG_mte_tagged, &page->flags))
> - mte_sync_page_tags(page, ptep, check_swap);
> + if (!test_bit(PG_mte_tagged, &page->flags))
> + mte_sync_page_tags(page, ptep, check_swap,
> + pte_is_tagged);
> }
> }
You were right in the previous thread that if we have a race, it's
already there even without your patches KVM patches.
If it's the same pte in a multithreaded app, we should be ok as the core
code holds the ptl (the arch code also holds the mmap_lock during
exception handling but only as a reader, so you can have multiple
holders).
If there are multiple ptes to the same page, for example mapped with
MAP_ANONYMOUS | MAP_SHARED, metadata recovery is done via
arch_swap_restore() before we even set the pte and with the page locked.
So calling lock_page() again in mte_restore_tags() would deadlock.
I can see that do_swap_page() also holds the page lock around
set_pte_at(), so I think we are covered.
Any other scenario I may have missed? My understanding is that if the
pte is the same, we have the ptl. Otherwise we have the page lock for
shared pages.
--
Catalin
On 27/04/2021 18:43, Catalin Marinas wrote:
> On Fri, Apr 16, 2021 at 04:43:04PM +0100, Steven Price wrote:
>> A KVM guest could store tags in a page even if the VMM hasn't mapped
>> the page with PROT_MTE. So when restoring pages from swap we will
>> need to check to see if there are any saved tags even if !pte_tagged().
>>
>> However don't check pages which are !pte_valid_user() as these will
>> not have been swapped out.
>
> You should remove the pte_valid_user() mention from the commit log as
> well.
Good spot - sorry about that. I really must get better at reading my own
commit messages.
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index e17b96d0e4b5..cf4b52a33b3c 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -312,7 +312,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
>> __sync_icache_dcache(pte);
>>
>> if (system_supports_mte() &&
>> - pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
>> + pte_present(pte) && (pte_val(pte) & PTE_USER) && !pte_special(pte))
>
> I would add a pte_user() macro here or, if we restore the tags only when
> the page is readable, use pte_access_permitted(pte, false). Also add a
> comment why we do this.
pte_access_permitted() looks like it describes what we want (user space
can access the memory). I'll add the following comment:
/*
* If the PTE would provide user space will access to the tags
* associated with it then ensure that the MTE tags are synchronised.
* Exec-only mappings don't expose tags (instruction fetches don't
* check tags).
*/
> There's also the pte_user_exec() case which may not have the PTE_USER
> set (exec-only permission) but I don't think it matters. We don't do tag
> checking on instruction fetches, so if the user adds a PROT_READ to it,
> it would go through set_pte_at() again. I'm not sure KVM does anything
> special with exec-only mappings at stage 2, I suspect they won't be
> accessible by the guest (but needs checking).
It comes down to the behaviour of get_user_pages(). AFAICT that will
fail if the memory is exec-only, so no stage 2 mapping will be created.
Which of course means the guest can't do anything with that memory. That
certainly seems like the only sane behaviour even without MTE.
>> mte_sync_tags(ptep, pte);
>>
>> __check_racy_pte_update(mm, ptep, pte);
>> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
>> index b3c70a612c7a..e016ab57ea36 100644
>> --- a/arch/arm64/kernel/mte.c
>> +++ b/arch/arm64/kernel/mte.c
>> @@ -26,17 +26,23 @@ u64 gcr_kernel_excl __ro_after_init;
>>
>> static bool report_fault_once = true;
>>
>> -static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap)
>> +static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap,
>> + bool pte_is_tagged)
>> {
>> pte_t old_pte = READ_ONCE(*ptep);
>>
>> if (check_swap && is_swap_pte(old_pte)) {
>> swp_entry_t entry = pte_to_swp_entry(old_pte);
>>
>> - if (!non_swap_entry(entry) && mte_restore_tags(entry, page))
>> + if (!non_swap_entry(entry) && mte_restore_tags(entry, page)) {
>> + set_bit(PG_mte_tagged, &page->flags);
>> return;
>> + }
>> }
>>
>> + if (!pte_is_tagged || test_and_set_bit(PG_mte_tagged, &page->flags))
>> + return;
>
> I don't think we need another test_bit() here, it was done in the
> caller (bar potential races which need more thought).
Good point - I'll change that to a straight set_bit().
>> +
>> page_kasan_tag_reset(page);
>> /*
>> * We need smp_wmb() in between setting the flags and clearing the
>> @@ -54,11 +60,13 @@ void mte_sync_tags(pte_t *ptep, pte_t pte)
>> struct page *page = pte_page(pte);
>> long i, nr_pages = compound_nr(page);
>> bool check_swap = nr_pages == 1;
>> + bool pte_is_tagged = pte_tagged(pte);
>>
>> /* if PG_mte_tagged is set, tags have already been initialised */
>> for (i = 0; i < nr_pages; i++, page++) {
>> - if (!test_and_set_bit(PG_mte_tagged, &page->flags))
>> - mte_sync_page_tags(page, ptep, check_swap);
>> + if (!test_bit(PG_mte_tagged, &page->flags))
>> + mte_sync_page_tags(page, ptep, check_swap,
>> + pte_is_tagged);
>> }
>> }
>
> You were right in the previous thread that if we have a race, it's
> already there even without your patches KVM patches.
>
> If it's the same pte in a multithreaded app, we should be ok as the core
> code holds the ptl (the arch code also holds the mmap_lock during
> exception handling but only as a reader, so you can have multiple
> holders).
>
> If there are multiple ptes to the same page, for example mapped with
> MAP_ANONYMOUS | MAP_SHARED, metadata recovery is done via
> arch_swap_restore() before we even set the pte and with the page locked.
> So calling lock_page() again in mte_restore_tags() would deadlock.
>
> I can see that do_swap_page() also holds the page lock around
> set_pte_at(), so I think we are covered.
>
> Any other scenario I may have missed? My understanding is that if the
> pte is the same, we have the ptl. Otherwise we have the page lock for
> shared pages.
That is my understanding - either the PTL is held or the page is locked.
But I am aware I was partly basing that on an assumption that the
existing code is correct. If there's a way that a new PTE can be created
which races with the arch_swap_restore() path then there is a problem.
I'm not aware of how that would happen though.
Steve
On Thu, Apr 29, 2021 at 05:06:05PM +0100, Steven Price wrote: > On 27/04/2021 18:43, Catalin Marinas wrote: > > On Fri, Apr 16, 2021 at 04:43:04PM +0100, Steven Price wrote: > > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > > > index e17b96d0e4b5..cf4b52a33b3c 100644 > > > --- a/arch/arm64/include/asm/pgtable.h > > > +++ b/arch/arm64/include/asm/pgtable.h > > > @@ -312,7 +312,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr, > > > __sync_icache_dcache(pte); > > > if (system_supports_mte() && > > > - pte_present(pte) && pte_tagged(pte) && !pte_special(pte)) > > > + pte_present(pte) && (pte_val(pte) & PTE_USER) && !pte_special(pte)) > > > > I would add a pte_user() macro here or, if we restore the tags only when > > the page is readable, use pte_access_permitted(pte, false). Also add a > > comment why we do this. > > pte_access_permitted() looks like it describes what we want (user space can > access the memory). I'll add the following comment: > > /* > * If the PTE would provide user space will access to the tags I think drop "will". > * associated with it then ensure that the MTE tags are synchronised. > * Exec-only mappings don't expose tags (instruction fetches don't > * check tags). > */ Sounds fine. > > There's also the pte_user_exec() case which may not have the PTE_USER > > set (exec-only permission) but I don't think it matters. We don't do tag > > checking on instruction fetches, so if the user adds a PROT_READ to it, > > it would go through set_pte_at() again. I'm not sure KVM does anything > > special with exec-only mappings at stage 2, I suspect they won't be > > accessible by the guest (but needs checking). > > It comes down to the behaviour of get_user_pages(). AFAICT that will fail if > the memory is exec-only, so no stage 2 mapping will be created. Which of > course means the guest can't do anything with that memory. That certainly > seems like the only sane behaviour even without MTE. That's my understanding as well. The get_user_pages_fast() path uses pte_access_permitted() and should return false. The slower get_user_pages() relies on checking the vma flags and it checks for VM_READ. -- Catalin
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