linux-kernel - [PATCH] ovl: explicitly initialize error in ovl_copy

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Message-Id: <20200604084245.161480-1-glider@google.com>
Date:   Thu,  4 Jun 2020 10:42:45 +0200
From:   glider@...gle.com
To:     miklos@...redi.hu, vgoyal@...hat.com
Cc:     linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        keescook@...omium.org, royyang@...gle.com, stable@...r.kernel.org,
        Alexander Potapenko <glider@...gle.com>
Subject: [PATCH] ovl: explicitly initialize error in ovl_copy_xattr()

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@...gle.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Roy Yang <royyang@...gle.com>
Cc: <stable@...r.kernel.org> # 4.1

---

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
---
 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..07e0d1961e96 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 = 0;
 	size_t slen;

 	if (!(old->d_inode->i_opflags & IOP_XATTR) ||
-- 
2.27.0.278.ge193c7cf3a9-goog