[RFC/RFT PATCH 04/18] crypto: skcipher - restore default skcipher_walk::iv on error

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

When the user-provided IV buffer is not aligned to the algorithm's
alignmask, skcipher_walk_virt() allocates an aligned buffer and copies
the IV into it.  However, skcipher_walk_virt() can fail after that
point, and in this case the buffer will be freed.

This causes a use-after-free read in callers that read from walk->iv
unconditionally, e.g. the LRW template.  For example, this can be
reproduced by trying to encrypt fewer than 16 bytes using "lrw(aes)".

Fix it by making skcipher_walk_first() reset walk->iv to walk->oiv if
skcipher_walk_next() fails.

This addresses a similar problem as commit 2b4f27c36bcd ("crypto:
skcipher - set walk.iv for zero-length inputs"), but that didn't
consider the case where skcipher_walk_virt() fails after copying the IV.

This bug was detected by my patches that improve testmgr to fuzz
algorithms against their generic implementation, when the extra
self-tests were run on a KASAN-enabled kernel.

Fixes: b286d8b1a690 ("crypto: skcipher - Add skcipher walk interface")
Cc: <stable@vger.kernel.org> # v4.10+
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 crypto/skcipher.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/crypto/skcipher.c b/crypto/skcipher.c
index bcf13d95f54ac..0f639f018047d 100644
--- a/crypto/skcipher.c
+++ b/crypto/skcipher.c
@@ -426,19 +426,27 @@ static int skcipher_copy_iv(struct skcipher_walk *walk)
 
 static int skcipher_walk_first(struct skcipher_walk *walk)
 {
+	int err;
+
 	if (WARN_ON_ONCE(in_irq()))
 		return -EDEADLK;
 
 	walk->buffer = NULL;
 	if (unlikely(((unsigned long)walk->iv & walk->alignmask))) {
-		int err = skcipher_copy_iv(walk);
+		err = skcipher_copy_iv(walk);
 		if (err)
 			return err;
 	}
 
 	walk->page = NULL;
 
-	return skcipher_walk_next(walk);
+	err = skcipher_walk_next(walk);
+
+	/* On error, don't leave 'walk->iv' pointing to freed buffer. */
+	if (unlikely(err))
+		walk->iv = walk->oiv;
+
+	return err;
 }
 
 static int skcipher_walk_skcipher(struct skcipher_walk *walk,
-- 
2.21.0