linux-kernel - Re: [patch] groups: integer underflow in groups

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Message-ID: <87385w1rmd.fsf@x220.int.ebiederm.org>
Date:	Mon, 23 Feb 2015 11:10:02 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Wang YanQing <udknight@...il.com>,
	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [patch] groups: integer underflow in groups_alloc()

Dan Carpenter <dan.carpenter@...cle.com> writes:

> This is called from rsc_parse() with a use controlled value.  Say for
> example that "gidsetsize" is negative, then we could end up allocating
> less than sizeof(struct group_info) leading to memory corruption.

Right now it is the responsibility of the caller of groups_alloc to make
certain that gidsetsize is a valid value, and the callers of
groups_alloc who know what they are doing already validate this value.

Either the pattern of caller validates the messages needs to continue,
or groups_alloc needs to be changed and all of the callers need to be
updated.

Changing groups_alloc for one particular caller is just going to cause
maintenance problems.

Eric


> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> ---
> I copied the NGROUPS_MAX limit from the surrounding code, I'm not
> absolutely that it's the correct limit to use.
>
> diff --git a/kernel/groups.c b/kernel/groups.c
> index 664411f..e9341b3 100644
> --- a/kernel/groups.c
> +++ b/kernel/groups.c
> @@ -18,6 +18,9 @@ struct group_info *groups_alloc(int gidsetsize)
>  	int nblocks;
>  	int i;
>  
> +	if ((unsigned)gidsetsize > NGROUPS_MAX)
> +		return NULL;
> +
>  	nblocks = (gidsetsize + NGROUPS_PER_BLOCK - 1) / NGROUPS_PER_BLOCK;
>  	/* Make sure we always allocate at least one indirect block pointer */
>  	nblocks = nblocks ? : 1;
--
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