Re: [PATCH v3] Fixes: null pointer dereference in pfnmap_lockdep_assert

[Matthew Wilcox <willy@infradead.org>]

On Fri, Oct 04, 2024 at 07:15:48PM +0530, Manas via B4 Relay wrote:
> +++ b/mm/memory.c
> @@ -6346,10 +6346,10 @@ 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;

Overly long and complex line.  Much simpler to write:

	struct address_space *mapping = NULL;

	if (vma->vm_file)
		mapping = vma->vm_file->f_mapping;

>  	if (mapping)
> -		lockdep_assert(lockdep_is_held(&vma->vm_file->f_mapping->i_mmap_rwsem) ||
> +		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));

This one should have been lockdep_assert_held(&vma->vm_mm->mmap_lock).

I'm not sure that the previous one is correct.  The
lockdep_assert_held() macro is pretty careful about checking
LOCK_STATE_NOT_HELD to avoid the LOCK_STATE_UNKNOWN possibility.
But I'll leave that for Peter to fix.