Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()

From: Vivek Goyal <vgoyal@redhat.com>
To: glider@google.com
Cc: miklos@szeredi.hu, linux-unionfs@vger.kernel.org,
	linux-kernel@vger.kernel.org, keescook@chromium.org,
	royyang@google.com, stable@vger.kernel.org
Subject: Re: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()
Date: Wed, 3 Jun 2020 17:46:33 -0400	[thread overview]
Message-ID: <20200603214633.GF48122@redhat.com> (raw)
In-Reply-To: <20200603174714.192027-1-glider@google.com>

On Wed, Jun 03, 2020 at 07:47:14PM +0200, glider@google.com wrote:
> Under certain circumstances (we found this out running Docker on a
> Clang-built kernel with CONFIG_INIT_STACK_ALL) ovl_copy_xattr() may
> return uninitialized value of |error| from ovl_copy_xattr().

If we are returning uninitialized value of error, doesn't that mean
that somewhere in the function we are returning without setting error.
And that probably means that's a bug and we should fix it?

I am wondering if this is triggered by loop finishing because all
the xattr on the file are ovl_is_private_xattr(). In that case, we
will come out of the loop without setting error. This is in fact
success and we should return 0 instead of some random error?

Thanks
Vivek

> It is then returned by ovl_create() to lookup_open(), which casts it to
> an invalid dentry pointer, that can be further read or written by the
> lookup_open() callers.
> 
> Signed-off-by: Alexander Potapenko <glider@google.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Roy Yang <royyang@google.com>
> Cc: <stable@vger.kernel.org> # 4.1
> 
> ---
> 
> It's unclear to me whether error should be initially 0 or some error
> code (both seem to work), but I thought returning an error makes sense,
> as the situation wasn't anticipated by the code authors.
> 
> The bug seem to date back to at least v4.1 where the annotation has been
> introduced (i.e. the compilers started noticing error could be used
> before being initialized). I hovever didn't try to prove that the
> problem is actually reproducible on such ancient kernels. We've seen it
> on a real machine running v4.4 as well.
> ---
>  fs/overlayfs/copy_up.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index 9709cf22cab3..428d43e2d016 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -47,7 +47,7 @@ int ovl_copy_xattr(struct dentry *old, struct dentry *new)
>  {
>  	ssize_t list_size, size, value_size = 0;
>  	char *buf, *name, *value = NULL;
> -	int uninitialized_var(error);
> +	int error = -EINVAL;
>  	size_t slen;
>  
>  	if (!(old->d_inode->i_opflags & IOP_XATTR) ||
> -- 
> 2.27.0.rc2.251.g90737beb825-goog
> 