linux-kernel - Re: [PATCH] mm/pgtable: return null if no ptl in __pte_offset_map

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Date:   Wed, 15 Nov 2023 18:05:30 +0200
From:   José Pekkarinen <jose.pekkarinen@...hound.fi>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, skhan@...uxfoundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linux.dev,
        syzbot+89edd67979b52675ddec@...kaller.appspotmail.com,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] mm/pgtable: return null if no ptl in
 __pte_offset_map_lock

On 2023-11-15 16:19, Matthew Wilcox wrote:
> On Wed, Nov 15, 2023 at 08:55:05AM +0200, José Pekkarinen wrote:
>> Documentation of __pte_offset_map_lock suggest there is situations 
>> where
> 
> You should have cc'd Hugh who changed all this code recently.

     Hi,

     Sorry, he seems to be missing if I run get_maintainer.pl:

$ ./scripts/get_maintainer.pl include/linux/mm.h
Andrew Morton <akpm@...ux-foundation.org> (maintainer:MEMORY MANAGEMENT)
linux-mm@...ck.org (open list:MEMORY MANAGEMENT)
linux-kernel@...r.kernel.org (open list)

>> a pmd may not have a corresponding page table, in which case it should
>> return NULL without changing ptlp. Syzbot found its ways to produce a
>> NULL dereference in the function showing this case. This patch will
>> provide the exit path suggested if this unlikely situation turns up. 
>> The
>> output of the kasan null-ptr-report follows:
> 
> There's no need to include all this nonsense in the changelog.

     No problem, we can clean the patch if we find there is something
worth upstreaming.

>>  spin_lock include/linux/spinlock.h:351 [inline]
>>  __pte_offset_map_lock+0x154/0x360 mm/pgtable-generic.c:373
>>  pte_offset_map_lock include/linux/mm.h:2939 [inline]
>>  filemap_map_pages+0x698/0x11f0 mm/filemap.c:3582
> 
> This was the only interesting part.
> 
>> +++ b/include/linux/mm.h
>> @@ -2854,7 +2854,7 @@ void ptlock_free(struct ptdesc *ptdesc);
>> 
>>  static inline spinlock_t *ptlock_ptr(struct ptdesc *ptdesc)
>>  {
>> -	return ptdesc->ptl;
>> +	return (likely(ptdesc)) ? ptdesc->ptl : NULL;
>>  }
> 
> I don't think we should be changing ptlock_ptr().

     This is where the null ptr dereference originates, so the only
alternative I can think of is to protect the life cycle of the ptdesc
to prevent it to die between the pte check and the spin_unlock of
__pte_offset_map_lock. Would that work for you?

>> +++ b/mm/pgtable-generic.c
>> @@ -370,6 +370,8 @@ pte_t *__pte_offset_map_lock(struct mm_struct *mm, 
>> pmd_t *pmd,
>>  	if (unlikely(!pte))
>>  		return pte;
>>  	ptl = pte_lockptr(mm, &pmdval);
>> +	if (unlikely(!ptl))
>> +		return NULL;
>>  	spin_lock(ptl);
> 
> I don't understand how this could possibly solve the problem.  If 
> there's
> no PTE level, then __pte_offset_map() should return NULL and we'd 
> already
> return due to the check for !pte.

     I tested the syzbot reproducer in x86 and it doesn't produce this 
kasan
report anymore.

     José.