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

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Message-ID: <20130517185317.GM1008@kvack.org>
Date:	Fri, 17 May 2013 14:53:17 -0400
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Sasha Levin <sasha.levin@...cle.com>
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 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.

> 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.

		-ben

>  	}
> -- 
> 1.8.2.1

-- 
"Thought is the essence of where you are now."
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