linux-kernel - Re: [PATCH v2] Fixes: null pointer dereference in pfnmap_lockdep

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Message-ID: <i5g3nwyjrrmbdgrecedyzhvjwl2p6b7yuniiqpyphndnkszta4@xt2dukgff3f3>
Date: Fri, 4 Oct 2024 18:57:57 +0530
From: Manas <manas18244@...td.ac.in>
To: Peter Xu <peterx@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	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 v2] Fixes: null pointer dereference in
 pfnmap_lockdep_assert

On 04.10.2024 09:21, Peter Xu wrote:
>On Fri, Oct 04, 2024 at 06:35:53PM +0530, Manas via B4 Relay wrote:
>> From: Manas <manas18244@...td.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@...kaller.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@...td.ac.in>
>
>Reviewed-by: Peter Xu <peterx@...hat.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!
Oh no! I missed it. Sending v3...

>
>>
>> ---
>> base-commit: 9852d85ec9d492ebef56dc5f229416c925758edc
>> change-id: 20241003-fix-null-deref-6bfa0337efc3
>>
>> Best regards,
>> --
>> Manas <manas18244@...td.ac.in>
>>
>>
>
>-- 
>Peter Xu
>

-- 
Manas