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

From: Kees Cook
Date: Thu Jun 04 2020 - 11:57:40 EST

On Thu, Jun 04, 2020 at 10:42:45AM +0200, glider@xxxxxxxxxx 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@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Roy Yang <royyang@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 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!

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

> 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

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 ||
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@xxxxxxxxxxxxxxx

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

Kees Cook