linux-kernel - Re: [patch for-3.7] fs, xattr: fix bug when removing a name not in xattr list

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Message-ID: <20121018134353.GM29956@redhat.com>
Date:	Thu, 18 Oct 2012 09:43:53 -0400
From:	Aristeu Rozanski <aris@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Hugh Dickins <hughd@...gle.com>, Li Zefan <lizefan@...wei.com>,
	Tejun Heo <tj@...nel.org>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	James Morris <jmorris@...ei.org>,
	Dmitry Kasatkin <dmitry.kasatkin@...el.com>,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Sasha Levin <levinsasha928@...il.com>
Subject: Re: [patch for-3.7] fs, xattr: fix bug when removing a name not in
 xattr list

On Wed, Oct 17, 2012 at 08:41:15PM -0700, David Rientjes wrote:
> Commit 38f38657444d ("xattr: extract simple_xattr code from tmpfs") moved
> some code from tmpfs but introduced a subtle bug along the way.
> 
> If the name passed to simple_xattr_remove() does not exist in the list of
> xattrs, then it is possible to call kfree(new_xattr) when new_xattr is
> actually initialized to itself on the stack via uninitialized_var().
> 
> This causes a BUG() since the memory was not allocated via the slab
> allocator and was not bypassed through to the page allocator because it
> was too large.
> 
> Initialize the local variable to NULL so the kfree() never takes place.
> 
> Reported-by: Fengguang Wu <fengguang.wu@...el.com>
> Signed-off-by: David Rientjes <rientjes@...gle.com>
> ---
>  fs/xattr.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xattr.c b/fs/xattr.c
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -846,7 +846,7 @@ static int __simple_xattr_set(struct simple_xattrs *xattrs, const char *name,
>  			      const void *value, size_t size, int flags)
>  {
>  	struct simple_xattr *xattr;
> -	struct simple_xattr *uninitialized_var(new_xattr);
> +	struct simple_xattr *new_xattr = NULL;
>  	int err = 0;
>  
>  	/* value == NULL means remove */

I think Sasha Levin was working on that (Cc'd) along with the smatch fix.
Anyway, __simple_xattr_set is too confusing and rewriting it is in my TODO list
which will shut up smatch too.

Acked-by: Aristeu Rozanski <aris@...hat.com>

-- 
Aristeu

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