On Thu, Jun 4, 2020 at 10:43 AM 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

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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.

v2: -- Per Vivek Goyal's suggestion, changed |error| to be 0

Thanks, applied patch posted here (with your signed-off as well, since the patch is the same...):

https://lore.kernel.org/linux-unionfs/874ks212uj.fsf@m5Zedd9JOGzJrf0/

Miklos

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...)