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

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Message-ID: <mvf3iorotqyioegxrre67a745ewk2dhqiuttn2vfwemv5kyjh4@m2x22dnxesjr>
Date: Fri, 4 Oct 2024 23:10:37 +0530
From: Manas <manas18244@...td.ac.in>
To: Matthew Wilcox <willy@...radead.org>
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

Hi Matthew,

On 04.10.2024 16:17, Matthew Wilcox wrote:
>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;
>
Thank you for reviewing! I am sending v4.

>>  	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.

-- 
Manas