From: Manas <manas18244@iiitd.ac.in>

syzbot has pointed to a possible null pointer dereference in
pfnmap_lockdep_assert. vm_file member of vm_area_struct is being
dereferenced without any checks.

This fix returns if vm_file member in vm_area_struct is NULL.

Reported-by: syzbot+093d096417e7038a689b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b
---
This bug[1] triggers a general protection fault in follow_pfnmap_start
function. An assertion pfnmap_lockdep_assert inside this function
dereferences vm_file member of vm_area_struct. And panic gets triggered
when vm_file is NULL.

This patch returns from the assertion pfnmap_lockdep_assert if vm_file
is found to be NULL.

[1] https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b

Signed-off-by: Manas <manas18244@iiitd.ac.in>
---
Changes in v2:
- v2: use ternary operator according to feedback
- Link to v1: https://lore.kernel.org/r/20241003-fix-null-deref-v1-1-0a45df9d016a@iiitd.ac.in
---
 mm/memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 2366578015ad..5ed109a8f02e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6346,7 +6346,7 @@ static inline void pfnmap_args_setup(struct follow_pfnmap_args *args,
 static inline void pfnmap_lockdep_assert(struct vm_area_struct *vma)
 {
 #ifdef CONFIG_LOCKDEP
-	struct address_space *mapping = vma->vm_file->f_mapping;
+	struct address_space *mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
 
 	if (mapping)
 		lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||

---
base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
change-id: 20241003-fix-null-deref-6bfa0337efc3

Best regards,
-- 
Manas <manas18244@iiitd.ac.in>

On Fri, Oct 04, 2024 at 06:35:53PM +0530, Manas via B4 Relay wrote:
> From: Manas <manas18244@iiitd.ac.in>
> 
> syzbot has pointed to a possible null pointer dereference in
> pfnmap_lockdep_assert. vm_file member of vm_area_struct is being
> dereferenced without any checks.
> 
> This fix returns if vm_file member in vm_area_struct is NULL.
> 
> Reported-by: syzbot+093d096417e7038a689b@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b
> ---
> This bug[1] triggers a general protection fault in follow_pfnmap_start
> function. An assertion pfnmap_lockdep_assert inside this function
> dereferences vm_file member of vm_area_struct. And panic gets triggered
> when vm_file is NULL.
> 
> This patch returns from the assertion pfnmap_lockdep_assert if vm_file
> is found to be NULL.
> 
> [1] https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b
> 
> Signed-off-by: Manas <manas18244@iiitd.ac.in>

Reviewed-by: Peter Xu <peterx@redhat.com>

One nitpick:

> ---
> Changes in v2:
> - v2: use ternary operator according to feedback
> - Link to v1: https://lore.kernel.org/r/20241003-fix-null-deref-v1-1-0a45df9d016a@iiitd.ac.in
> ---
>  mm/memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 2366578015ad..5ed109a8f02e 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6346,7 +6346,7 @@ static inline void pfnmap_args_setup(struct follow_pfnmap_args *args,
>  static inline void pfnmap_lockdep_assert(struct vm_area_struct *vma)
>  {
>  #ifdef CONFIG_LOCKDEP
> -	struct address_space *mapping = vma->vm_file->f_mapping;
> +	struct address_space *mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
>  
>  	if (mapping)
>  		lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||

This can use "mapping" directly, as I mentioned in previous email (but
probably got overlooked..).

Thanks!

> 
> ---
> base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
> change-id: 20241003-fix-null-deref-6bfa0337efc3
> 
> Best regards,
> -- 
> Manas <manas18244@iiitd.ac.in>
> 
> 

-- 
Peter Xu

On 04.10.2024 09:21, Peter Xu wrote:
>On Fri, Oct 04, 2024 at 06:35:53PM +0530, Manas via B4 Relay wrote:
>> From: Manas <manas18244@iiitd.ac.in>
>>
>> syzbot has pointed to a possible null pointer dereference in
>> pfnmap_lockdep_assert. vm_file member of vm_area_struct is being
>> dereferenced without any checks.
>>
>> This fix returns if vm_file member in vm_area_struct is NULL.
>>
>> Reported-by: syzbot+093d096417e7038a689b@syzkaller.appspotmail.com
>> Closes: https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b
>> ---
>> This bug[1] triggers a general protection fault in follow_pfnmap_start
>> function. An assertion pfnmap_lockdep_assert inside this function
>> dereferences vm_file member of vm_area_struct. And panic gets triggered
>> when vm_file is NULL.
>>
>> This patch returns from the assertion pfnmap_lockdep_assert if vm_file
>> is found to be NULL.
>>
>> [1] https://syzkaller.appspot.com/bug?extid=093d096417e7038a689b
>>
>> Signed-off-by: Manas <manas18244@iiitd.ac.in>
>
>Reviewed-by: Peter Xu <peterx@redhat.com>
>
>One nitpick:
>
>> ---
>> Changes in v2:
>> - v2: use ternary operator according to feedback
>> - Link to v1: https://lore.kernel.org/r/20241003-fix-null-deref-v1-1-0a45df9d016a@iiitd.ac.in
>> ---
>>  mm/memory.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 2366578015ad..5ed109a8f02e 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -6346,7 +6346,7 @@ static inline void pfnmap_args_setup(struct follow_pfnmap_args *args,
>>  static inline void pfnmap_lockdep_assert(struct vm_area_struct *vma)
>>  {
>>  #ifdef CONFIG_LOCKDEP
>> -	struct address_space *mapping = vma->vm_file->f_mapping;
>> +	struct address_space *mapping = vma->vm_file ? vma->vm_file->f_mapping : NULL;
>>
>>  	if (mapping)
>>  		lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||
>
>This can use "mapping" directly, as I mentioned in previous email (but
>probably got overlooked..).
>
>Thanks!
Oh no! I missed it. Sending v3...

>
>>
>> ---
>> base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
>> change-id: 20241003-fix-null-deref-6bfa0337efc3
>>
>> Best regards,
>> --
>> Manas <manas18244@iiitd.ac.in>
>>
>>
>
>-- 
>Peter Xu
>

-- 
Manas