netdev - Re: [PATCH] xfrm: Don't increase scratch users if allocation fails

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Date:   Wed, 31 Aug 2022 18:01:54 +0600
From:   Khalid Masum <khalid.masum.92@...il.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com,
        syzbot+5ec9bb042ddfe9644773@...kaller.appspotmail.com,
        Steffen Klassert <steffen.klassert@...unet.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH] xfrm: Don't increase scratch users if allocation fails

On 8/31/22 15:13, Herbert Xu wrote:
> On Wed, Aug 31, 2022 at 07:41:26AM +0600, Khalid Masum wrote:
>> ipcomp_alloc_scratches() routine increases ipcomp_scratch_users count
>> even if it fails to allocate memory. Therefore, ipcomp_free_scratches()
>> routine, when triggered, tries to vfree() non existent percpu
>> ipcomp_scratches.
>>
>> To fix this breakage, do not increase scratch users count if
>> ipcomp_alloc_scratches() fails to allocate scratches.
>>
>> Reported-and-tested-by: syzbot+5ec9bb042ddfe9644773@...kaller.appspotmail.com
>> Signed-off-by: Khalid Masum <khalid.masum.92@...il.com>
>> ---
>>   net/xfrm/xfrm_ipcomp.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/xfrm/xfrm_ipcomp.c b/net/xfrm/xfrm_ipcomp.c
>> index cb40ff0ff28d..af9097983139 100644
>> --- a/net/xfrm/xfrm_ipcomp.c
>> +++ b/net/xfrm/xfrm_ipcomp.c
>> @@ -210,13 +210,15 @@ static void * __percpu *ipcomp_alloc_scratches(void)
>>   	void * __percpu *scratches;
>>   	int i;
>>   
>> -	if (ipcomp_scratch_users++)
>> +	if (ipcomp_scratch_users) {
>> +		ipcomp_scratch_users++;
>>   		return ipcomp_scratches;
>> -
>> +	}
>>   	scratches = alloc_percpu(void *);
>>   	if (!scratches)
>>   		return NULL;
>>   
>> +	ipcomp_scratch_users++;
>>   	ipcomp_scratches = scratches;
> 
> This patch is broken because on error we will always call
> ipcomp_free_scratches which frees any partially allocated memory
> and restores ipcomp_scratch_users to zero.
> 
> With this patch ipcomp_scratch_users will turn negative on error.
> 
> Cheers,

Thanks for the review. I think it can be fixed by assigning NULL in 
ipcomp_scratches when the allocation fails as ipcomp_free_scratches
checks for it. I shall follow this email with a v2 shortly.

thanks,
   -- Khalid Masum