linux-kernel - Re: [PATCH v3] Fixes: null pointer dereference in pfnmap_lockdep

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Message-ID: <ZwAHFtAmMq9BNuGv@casper.infradead.org>
Date: Fri, 4 Oct 2024 16:17:42 +0100
From: Matthew Wilcox <willy@...radead.org>
To: manas18244@...td.ac.in
Cc: Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.com>,
	Shuah Khan <skhan@...uxfoundation.org>,
	Anup Sharma <anupnewsmail@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	syzbot+093d096417e7038a689b@...kaller.appspotmail.com
Subject: Re: [PATCH v3] Fixes: null pointer dereference in
 pfnmap_lockdep_assert

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.