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

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Message-Id: <20200603174714.192027-1-glider@google.com>
Date:   Wed,  3 Jun 2020 19:47:14 +0200
From:   glider@...gle.com
To:     miklos@...redi.hu
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.

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

---

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