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

[Peter Xu <peterx@redhat.com>]

On Thu, Oct 03, 2024 at 09:31:06PM +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>

Thanks for the patch!

> ---
>  mm/memory.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 2366578015ad..b152a95e543f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6346,6 +6346,9 @@ 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
> +	if (!vma->vm_file)
> +		return;
> +

Hmm I guess I wasn't careful enough here as I was mostly only thinking
about file mappings, but I just notice we have other pfnmaps like the vvar
mappings.. the mapping var can also already be reused later when available.

Logically even if !vm_file we can still check against mmap write lock.  So
would it be better to do this instead:

        struct address_space *mapping = vma->vm_file && vma->vm_file->f_mapping;

        if (mapping)
                lockdep_assert(lockdep_is_held(&mapping->i_mmap_rwsem) ||
                               lockdep_is_held(&vma->vm_mm->mmap_lock));
        else
                lockdep_assert(lockdep_is_held(&vma->vm_mm->mmap_lock));

?

>  	struct address_space *mapping = vma->vm_file->f_mapping;
>  
>  	if (mapping)
> 
> ---
> base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
> change-id: 20241003-fix-null-deref-6bfa0337efc3
> 
> Best regards,
> -- 
> Manas <manas18244@iiitd.ac.in>
> 
> 
> 

-- 
Peter Xu