Re: [PATCH v2] Fixes: null pointer dereference in pfnmap_lockdep_assert

[Peter Xu <peterx@redhat.com>]

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