linux-kernel - Re: [PATCH] fs: aio: use correct integer overflow checks when creation aio ctx

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Date:	Fri, 17 May 2013 15:05:50 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
CC:	koverstreet@...gle.com, akpm@...ux-foundation.org, tytso@....edu,
	viro@...iv.linux.org.uk, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: aio: use correct integer overflow checks when creation
 aio ctx

On 05/17/2013 02:53 PM, Benjamin LaHaise wrote:
> On Fri, May 17, 2013 at 02:23:54PM -0400, Sasha Levin wrote:
>> Commit "aio: percpu reqs_available" added some math to the nr_requests
>> calculation, but didn't correct the overflow calculations to handle that.
>>
>> This means that this:
>>
>> 	#include <linux/aio_abi.h>
>> 	void main(void)
>> 	{
>> 	        aio_context_t ctx_idp;
>> 	        io_setup(0x80000001, &ctx_idp);
>> 	}
>>
>> Would trigger the newly added BUG() couple of lines after the overflow
>> checks.
> 
> This BUG() isn't in Linus' tree, and probably should be removed before 
> it gets there.

It's not, it's in -next though.

>> Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
>> ---
>>  fs/aio.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/aio.c b/fs/aio.c
>> index 5b7ed78..0ae450a 100644
>> --- a/fs/aio.c
>> +++ b/fs/aio.c
>> @@ -411,7 +411,8 @@ static struct kioctx *ioctx_alloc(unsigned nr_events)
>>  
>>  	/* Prevent overflows */
>>  	if ((nr_events > (0x10000000U / sizeof(struct io_event))) ||
>> -	    (nr_events > (0x10000000U / sizeof(struct kiocb)))) {
>> +	    (nr_events > (0x10000000U / sizeof(struct kiocb))) ||
>> +	    (nr_events < num_possible_cpus() * 4)) {
>>  		pr_debug("ENOMEM: nr_events too high\n");
>>  		return ERR_PTR(-EINVAL);
> 
> This is completely wrong.  Enforcing a minimum needs to be done in a way 
> that doesn't fail for existing users that potentially use a minimum 
> smaller than what is newly required.  That is: an existing userland program 
> that only requests 16 events must not fail because of changes to the kernel 
> that increase the minimum number of requests.  So I have to NACK this patch 
> as it stands.

You didn't look around the context of the patch.

Couple of lines before that check, this happens:

        nr_events = max(nr_events, num_possible_cpus() * 4);
        nr_events *= 2;

The check I've added would only make sense if nr_events wrapped around, not if
nr_events was originally smaller than (num_possible_cpus() * 4).


Thanks,
Sasha

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