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

[Kees Cook <keescook@chromium.org>]

On Thu, Jun 04, 2020 at 10:42:45AM +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().
> 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.
> 
> The uninitialized value is returned when all the xattr on the file
> are ovl_is_private_xattr(), which is actually a successful case,
> therefore we initialize |error| with 0.
> 
> 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

Please include a Fixes (more below) and Link tags for details to help
guide backporting, then you don't need to bother with with "# 4.1",
the -stable tools will figure it out with a "Fixes" tag.

Thanks for the v2!

Link: https://bugs.chromium.org/p/chromium/issues/detail?id=1050405
Reviewed-by: Kees Cook <keescook@chromium.org>

> 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.

It seems like it came from this, but that's v4.5:

Fixes: e4ad29fa0d22 ("ovl: use a minimal buffer in ovl_copy_xattr")

What did you find in v4.1? It looks like error isn't uninitialized in
v4.1:

int ovl_copy_xattr(struct dentry *old, struct dentry *new)
{
        ssize_t list_size, size;
        char *buf, *name, *value;
        int error;

        if (!old->d_inode->i_op->getxattr ||
            !new->d_inode->i_op->getxattr)
                return 0;

        list_size = vfs_listxattr(old, NULL, 0);
        if (list_size <= 0) {
                if (list_size == -EOPNOTSUPP)
                        return 0;
                return list_size;
        }

        buf = kzalloc(list_size, GFP_KERNEL);
        if (!buf)
                return -ENOMEM;

        error = -ENOMEM;
...

But v4.1.52 backported the above patch (e4ad29fa0d22), which is why I
don't try to figure these things out manually. Once we find the commit,
the tools will figure it out. I think you just need:

Fixes: e4ad29fa0d22 ("ovl: use a minimal buffer in ovl_copy_xattr")
Cc: stable@vger.kernel.org

and things like v4.1.52 will get fixed (if anyone is actually doing
updates for v4.1.z any more...)

-- 
Kees Cook