Date:   Fri, 31 Mar 2023 10:47:06 +0400
From:   Ivan Orlov <ivan.orlov0322@...il.com>
To:     Zach O'Keefe <zokeefe@...gle.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, himadrispandya@...il.com,
        skhan@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        shy828301@...il.com,
        syzbot+9578faa5475acb35fa50@...kaller.appspotmail.com
Subject: Re: [PATCH v2] mm: khugepaged: Fix kernel BUG in
 hpage_collapse_scan_file

On 3/31/23 05:33, Zach O'Keefe wrote:

> Thanks, Ivan.
> 
> In the process of reviewing this, I starting thinking if the !shmem case was
> also susceptible to a similar race, and I *think* it might be. Unfortunately, my
> time has ran out, and I haven't been able to validate ; I'm less familiar with
> the file-side of things.
> 
> The underlying problem is race with truncation/hole-punch  under OOM condition.
> The nice do-while loop near the top of collapse_file() attempts to avoid this
> scenario by making sure enough slots are available. However, when we drop xarray
> lock, we open ourselves up to concurrent removal + slot deletion. Those slots
> then need to be allocated again -- which under OOM condition is failable.
> 
> The syzbot reproducer picks on shmem, but I think this can occur for file as
> well. If we find a hole, we unlock the xarray and call
> page_cache_sync_readahead(), which if it succeeds, IIUC, will have allocated a
> new slot in our mapping pointing to the new page. We *then* locks the page. Only
> after the page is locked are we protected from concurrent removal (Note: this is
> what provides us protection in many of the xas_store() cases ; we've held the
> slot's contained page-lock since verifying the slot exists, protecting us from
> removal / reallocation races).
> 
> Maybe I'm just low on caffeine at the end of the day, and am missing something,
> but if I had more time, I'd be looking into the file-side some more to verify.
> Apologies that hasn't occurred to me until now ; I was looking at one of your
> comments and double-checked why I *thought* we were safe.
> 
> Anyways, irrespective of that looming issues, some more notes to follow:
> 
>> The 'xas_store' call during page cache scanning can potentially
>> translate 'xas' into the error state (with the reproducer provided
>> by the syzkaller the error code is -ENOMEM). However, there are no
>> further checks after the 'xas_store', and the next call of 'xas_next'
>> at the start of the scanning cycle doesn't increase the xa_index,
>> and the issue occurs.
>>
>> This patch will add the xarray state error checking after the
>> 'xas_store' and the corresponding result error code. It will
>> also add xarray state error checking via WARN_ON_ONCE macros,
>> to be sure that ENOMEM or other possible errors don't occur
>> at the places they shouldn't.
> 
> Thanks for the additions here. I think it's worthwhile providing even more
> details about the specifics of the race we are fixing and/or guarding against to
> help ppl understand how that -ENOMEM comes about if the do-while loop has
> "Ensured" we have slots available (additionally, I think that comment can be
> augmented).
> 
>> Tested via syzbot.
>>
>> Reported-by: syzbot+9578faa5475acb35fa50@...kaller.appspotmail.com
>> Link: https://syzkaller.appspot.com/bug?id=7d6bb3760e026ece7524500fe44fb024a0e959fc
>> Signed-off-by: Ivan Orlov <ivan.orlov0322@...il.com>
>> ---
>> V1 -> V2: Add WARN_ON_ONCE error checking and comments
>>
>>   mm/khugepaged.c | 20 ++++++++++++++++++++
>>   1 file changed, 20 insertions(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 92e6f56a932d..8b6580b13339 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -55,6 +55,7 @@ enum scan_result {
>>   	SCAN_CGROUP_CHARGE_FAIL,
>>   	SCAN_TRUNCATED,
>>   	SCAN_PAGE_HAS_PRIVATE,
>> +	SCAN_STORE_FAILED,
>>   };
> 
> I'm still reluctant to add a new error code for this as this seems like quite a
> rare race that requires OOM to trigger. I'd be happier just reusing SCAN_FAIL,
> or, something we might get some millage out of later: SCAN_OOM.
> 
> Also, a reminder to update include/trace/events/huge_memory.h, if you go that
> route.
> 
>>   
>>   #define CREATE_TRACE_POINTS
>> @@ -1840,6 +1841,15 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>>   					goto xa_locked;
>>   				}
>>   				xas_store(&xas, hpage);
>> +				if (xas_error(&xas)) {
>> +					/* revert shmem_charge performed
>> +					 * in the previous condition
>> +					 */
> 
> Nit: Here, and following, I think standard convention for multiline comment is
> to have an empty first and last line, eg:
> 
>   +					/*
>   +					 * revert shmem_charge performed
>   +					 * in the previous condition
>   +					 */
> 
> Though, checkpatch.pl --strict didn't seem to care.
> 
>> +					mapping->nrpages--;
>> +					shmem_uncharge(mapping->host, 1);
>> +					result = SCAN_STORE_FAILED;
>> +					goto xa_locked;
>> +				}
>>   				nr_none++;
>>   				continue;
>>   			}
>> @@ -1992,6 +2002,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>>   
>>   		/* Finally, replace with the new page. */
>>   		xas_store(&xas, hpage);
>> +		/* We can't get an ENOMEM here (because the allocation happened before)
>> +		 * but let's check for errors (XArray implementation can be
>> +		 * changed in the future)
>> +		 */
>> +		WARN_ON_ONCE(xas_error(&xas));
> 
> Nit: it's not just that allocation happened before -- need some guarantee we've
> been protected from concurrent removal. This is what made me look at the file
> side.
> 
>>   		continue;
>>   out_unlock:
>>   		unlock_page(page);
>> @@ -2029,6 +2044,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>>   	/* Join all the small entries into a single multi-index entry */
>>   	xas_set_order(&xas, start, HPAGE_PMD_ORDER);
>>   	xas_store(&xas, hpage);
>> +	/* Here we can't get an ENOMEM (because entries were
>> +	 * previously allocated) But let's check for errors
>> +	 * (XArray implementation can be changed in the future)
>> +	 */
>> +	WARN_ON_ONCE(xas_error(&xas));
> 
> Ditto.
> 
> Apologies I won't be around to see this change through -- I'm just out of time,
> and will be shutting my computer down tomorrow for 3 months.  Sorry for the poor
> timing, for raising issues, then disappearing. Hopefully I'm wrong and the
> file-side isn't a concern.
> 
> Best,
> Zach
> 
>>   xa_locked:
>>   	xas_unlock_irq(&xas);
>>   xa_unlocked:
>> -- 
>> 2.34.1
>>

Hello, Zach! Thank you very much for the detailed review! I thought that 
locking is our guarantee to not get an -ENOMEM, but I didn't have the 
deep understanding of the problem's cause, due to the fact I'm just 
starting my memory management journey :) In any case, I will do a 
research about this problem, and hopefully after you get out of the 
vacation you will see a new patch fully fixes this problem. Have a nice 
vacation! Thanks again!

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