crypto: skcipher - fix crash flushing dcache in error path

scatterwalk_done() is only meant to be called after a nonzero number of
bytes have been processed, since scatterwalk_pagedone() will flush the
dcache of the *previous* page.  But in the error case of
skcipher_walk_done(), e.g. if the input wasn't an integer number of
blocks, scatterwalk_done() was actually called after advancing 0 bytes.
This caused a crash ("BUG: unable to handle kernel paging request")
during '!PageSlab(page)' on architectures like arm and arm64 that define
ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE, provided that the input was
page-aligned as in that case walk->offset == 0.

Fix it by reorganizing skcipher_walk_done() to skip the
scatterwalk_advance() and scatterwalk_done() if an error has occurred.

This bug was found by syzkaller fuzzing.

Reproducer, assuming ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE:

	#include <linux/if_alg.h>
	#include <sys/socket.h>
	#include <unistd.h>

	int main()
	{
		struct sockaddr_alg addr = {
			.salg_type = "skcipher",
			.salg_name = "cbc(aes-generic)",
		};
		char buffer[4096] __attribute__((aligned(4096))) = { 0 };
		int fd;

		fd = socket(AF_ALG, SOCK_SEQPACKET, 0);
		bind(fd, (void *)&addr, sizeof(addr));
		setsockopt(fd, SOL_ALG, ALG_SET_KEY, buffer, 16);
		fd = accept(fd, NULL, NULL);
		write(fd, buffer, 15);
		read(fd, buffer, 15);
	}

Reported-by: Liu Chao <liuchao741@huawei.com>
Fixes: b286d8b1a690 ("crypto: skcipher - Add skcipher walk interface")
Cc: <stable@vger.kernel.org> # v4.10+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/crypto/skcipher.c b/crypto/skcipher.c
index 835e5d3..0bd8c6c 100644 (file)
--- a/crypto/skcipher.c
+++ b/crypto/skcipher.c
@@ -95,7 +95,7 @@ static inline u8 *skcipher_get_spot(u8 *start, unsigned int len)
        return max(start, end_page);
 }
 
-static int skcipher_done_slow(struct skcipher_walk *walk, unsigned int bsize)
+static void skcipher_done_slow(struct skcipher_walk *walk, unsigned int bsize)
 {
        u8 *addr;
 
@@ -103,23 +103,24 @@ static int skcipher_done_slow(struct skcipher_walk *walk, unsigned int bsize)
        addr = skcipher_get_spot(addr, bsize);
        scatterwalk_copychunks(addr, &walk->out, bsize,
                               (walk->flags & SKCIPHER_WALK_PHYS) ? 2 : 1);
-       return 0;
 }
 
 int skcipher_walk_done(struct skcipher_walk *walk, int err)
 {
-       unsigned int n = walk->nbytes - err;
-       unsigned int nbytes;
-
-       nbytes = walk->total - n;
-
-       if (unlikely(err < 0)) {
-               nbytes = 0;
-               n = 0;
-       } else if (likely(!(walk->flags & (SKCIPHER_WALK_PHYS |
-                                          SKCIPHER_WALK_SLOW |
-                                          SKCIPHER_WALK_COPY |
-                                          SKCIPHER_WALK_DIFF)))) {
+       unsigned int n; /* bytes processed */
+       bool more;
+
+       if (unlikely(err < 0))
+               goto finish;
+
+       n = walk->nbytes - err;
+       walk->total -= n;
+       more = (walk->total != 0);
+
+       if (likely(!(walk->flags & (SKCIPHER_WALK_PHYS |
+                                   SKCIPHER_WALK_SLOW |
+                                   SKCIPHER_WALK_COPY |
+                                   SKCIPHER_WALK_DIFF)))) {
 unmap_src:
                skcipher_unmap_src(walk);
        } else if (walk->flags & SKCIPHER_WALK_DIFF) {
@@ -131,28 +132,28 @@ unmap_src:
                skcipher_unmap_dst(walk);
        } else if (unlikely(walk->flags & SKCIPHER_WALK_SLOW)) {
                if (WARN_ON(err)) {
+                       /* unexpected case; didn't process all bytes */
                        err = -EINVAL;
-                       nbytes = 0;
-               } else
-                       n = skcipher_done_slow(walk, n);
+                       goto finish;
+               }
+               skcipher_done_slow(walk, n);
+               goto already_advanced;
        }
 
-       if (err > 0)
-               err = 0;
-
-       walk->total = nbytes;
-       walk->nbytes = nbytes;
-
        scatterwalk_advance(&walk->in, n);
        scatterwalk_advance(&walk->out, n);
-       scatterwalk_done(&walk->in, 0, nbytes);
-       scatterwalk_done(&walk->out, 1, nbytes);
+already_advanced:
+       scatterwalk_done(&walk->in, 0, more);
+       scatterwalk_done(&walk->out, 1, more);
 
-       if (nbytes) {
+       if (more) {
                crypto_yield(walk->flags & SKCIPHER_WALK_SLEEP ?
                             CRYPTO_TFM_REQ_MAY_SLEEP : 0);
                return skcipher_walk_next(walk);
        }
+       err = 0;
+finish:
+       walk->nbytes = 0;
 
        /* Short-circuit for the common/fast path. */
        if (!((unsigned long)walk->buffer | (unsigned long)walk->page))