dm crypt: fix access beyond the end of allocated space

commit d49ec52ff6ddcda178fc2476a109cf1bd1fa19ed upstream.

The DM crypt target accesses memory beyond allocated space resulting in
a crash on 32 bit x86 systems.

This bug is very old (it dates back to 2.6.25 commit 3a7f6c990ad04 "dm
crypt: use async crypto").  However, this bug was masked by the fact
that kmalloc rounds the size up to the next power of two.  This bug
wasn't exposed until 3.17-rc1 commit 298a9fa08a ("dm crypt: use per-bio
data").  By switching to using per-bio data there was no longer any
padding beyond the end of a dm-crypt allocated memory block.

To minimize allocation overhead dm-crypt puts several structures into one
block allocated with kmalloc.  The block holds struct ablkcipher_request,
cipher-specific scratch pad (crypto_ablkcipher_reqsize(any_tfm(cc))),
struct dm_crypt_request and an initialization vector.

The variable dmreq_start is set to offset of struct dm_crypt_request
within this memory block.  dm-crypt allocates the block with this size:
cc->dmreq_start + sizeof(struct dm_crypt_request) + cc->iv_size.

When accessing the initialization vector, dm-crypt uses the function
iv_of_dmreq, which performs this calculation: ALIGN((unsigned long)(dmreq
+ 1), crypto_ablkcipher_alignmask(any_tfm(cc)) + 1).

dm-crypt allocated "cc->iv_size" bytes beyond the end of dm_crypt_request
structure.  However, when dm-crypt accesses the initialization vector, it
takes a pointer to the end of dm_crypt_request, aligns it, and then uses
it as the initialization vector.  If the end of dm_crypt_request is not
aligned on a crypto_ablkcipher_alignmask(any_tfm(cc)) boundary the
alignment causes the initialization vector to point beyond the allocated
space.

Fix this bug by calculating the variable iv_size_padding and adding it
to the allocated size.

Also correct the alignment of dm_crypt_request.  struct dm_crypt_request
is specific to dm-crypt (it isn't used by the crypto subsystem at all),
so it is aligned on __alignof__(struct dm_crypt_request).

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index aa142f9..4878d91 100644 (file)
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -1565,6 +1565,7 @@ static int crypt_ctr(struct dm_target *ti, unsigned int argc, char **argv)
        unsigned int key_size, opt_params;
        unsigned long long tmpll;
        int ret;
+       size_t iv_size_padding;
        struct dm_arg_set as;
        const char *opt_string;
 
@@ -1600,12 +1601,23 @@ static int crypt_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 
        cc->dmreq_start = sizeof(struct ablkcipher_request);
        cc->dmreq_start += crypto_ablkcipher_reqsize(any_tfm(cc));
-       cc->dmreq_start = ALIGN(cc->dmreq_start, crypto_tfm_ctx_alignment());
-       cc->dmreq_start += crypto_ablkcipher_alignmask(any_tfm(cc)) &
-                          ~(crypto_tfm_ctx_alignment() - 1);
+       cc->dmreq_start = ALIGN(cc->dmreq_start, __alignof__(struct dm_crypt_request));
+
+       if (crypto_ablkcipher_alignmask(any_tfm(cc)) < CRYPTO_MINALIGN) {
+               /* Allocate the padding exactly */
+               iv_size_padding = -(cc->dmreq_start + sizeof(struct dm_crypt_request))
+                               & crypto_ablkcipher_alignmask(any_tfm(cc));
+       } else {
+               /*
+                * If the cipher requires greater alignment than kmalloc
+                * alignment, we don't know the exact position of the
+                * initialization vector. We must assume worst case.
+                */
+               iv_size_padding = crypto_ablkcipher_alignmask(any_tfm(cc));
+       }
 
        cc->req_pool = mempool_create_kmalloc_pool(MIN_IOS, cc->dmreq_start +
-                       sizeof(struct dm_crypt_request) + cc->iv_size);
+                       sizeof(struct dm_crypt_request) + iv_size_padding + cc->iv_size);
        if (!cc->req_pool) {
                ti->error = "Cannot allocate crypt request mempool";
                goto bad;