Hi,

On Mon, Oct 26, 2020 at 01:07:15PM -0700, Eric Biggers wrote:

From: Eric Biggers ebiggers@google.com

Commit 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm names") made the kernel start accepting arbitrarily long algorithm names in sockaddr_alg. However, the actual length of the salg_name field stayed at the original 64 bytes.

This is broken because the kernel can access indices >= 64 in salg_name, which is undefined behavior -- even though the memory that is accessed is still located within the sockaddr structure. It would only be defined behavior if the array were properly marked as arbitrary-length (either by making it a flexible array, which is the recommended way these days, or by making it an array of length 0 or 1).

We can't simply change salg_name into a flexible array, since that would break source compatibility with userspace programs that embed sockaddr_alg into another struct, or (more commonly) declare a sockaddr_alg like 'struct sockaddr_alg sa = { .salg_name = "foo" };'.

One solution would be to change salg_name into a flexible array only when '#ifdef __KERNEL__'. However, that would keep userspace without an easy way to actually use the longer algorithm names.

Instead, add a new structure 'sockaddr_alg_new' that has the flexible array field, and expose it to both userspace and the kernel. Make the kernel use it correctly in alg_bind().

This addresses the syzbot report "UBSAN: array-index-out-of-bounds in alg_bind" (https://syzkaller.appspot.com/bug?extid=92ead4eb8e26a26d465e).

Reported-by: syzbot+92ead4eb8e26a26d465e@syzkaller.appspotmail.com Fixes: 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm names") Cc: stable@vger.kernel.org # v4.12+ Signed-off-by: Eric Biggers ebiggers@google.com

---

crypto/af_alg.c | 10 +++++++--- include/uapi/linux/if_alg.h | 16 ++++++++++++++++ 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/crypto/af_alg.c b/crypto/af_alg.c index d11db80d24cd1..9acb9d2c4bcf9 100644 --- a/crypto/af_alg.c +++ b/crypto/af_alg.c @@ -147,7 +147,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len) const u32 allowed = CRYPTO_ALG_KERN_DRIVER_ONLY; struct sock *sk = sock->sk; struct alg_sock *ask = alg_sk(sk);

• struct sockaddr_alg *sa = (void *)uaddr;

• struct sockaddr_alg_new *sa = (void *)uaddr; const struct af_alg_type *type; void *private; int err;

@@ -155,7 +155,11 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len) if (sock->state == SS_CONNECTED) return -EINVAL;

• if (addr_len < sizeof(*sa))

• BUILD_BUG_ON(offsetof(struct sockaddr_alg_new, salg_name) !=

•      offsetof(struct sockaddr_alg, salg_name));
  

• BUILD_BUG_ON(offsetof(struct sockaddr_alg, salg_name) != sizeof(*sa));
• if (addr_len < sizeof(*sa) + 1) return -EINVAL;

/* If caller uses non-allowed flag, return error. */ @@ -163,7 +167,7 @@ static int alg_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len) return -EINVAL; sa->salg_type[sizeof(sa->salg_type) - 1] = 0;

• sa->salg_name[sizeof(sa->salg_name) + addr_len - sizeof(*sa) - 1] = 0;

• sa->salg_name[addr_len - sizeof(*sa) - 1] = 0;

type = alg_get_type(sa->salg_type); if (PTR_ERR(type) == -ENOENT) { diff --git a/include/uapi/linux/if_alg.h b/include/uapi/linux/if_alg.h index 60b7c2efd921c..dc52a11ba6d15 100644 --- a/include/uapi/linux/if_alg.h +++ b/include/uapi/linux/if_alg.h @@ -24,6 +24,22 @@ struct sockaddr_alg { __u8 salg_name[64]; }; +/*

• • Linux v4.12 and later removed the 64-byte limit on salg_name[]; it's now an
• • arbitrary-length field. We had to keep the original struct above for source
• • compatibility with existing userspace programs, though. Use the new struct
• • below if support for very long algorithm names is needed. To do this,
• • allocate 'sizeof(struct sockaddr_alg_new) + strlen(algname) + 1' bytes, and
• • copy algname (including the null terminator) into salg_name.
• */

+struct sockaddr_alg_new {

• __u16 salg_family;
• __u8 salg_type[14];
• __u32 salg_feat;
• __u32 salg_mask;
• __u8 salg_name[];

+};

How something like this, instead:

struct sockaddr_alg { - __u16 salg_family; - __u8 salg_type[14]; - __u32 salg_feat; - __u32 salg_mask; - __u8 salg_name[64]; + union { + struct { + __u16 salg_v1_family; + __u8 salg_v1_type[14]; + __u32 salg_v1_feat; + __u32 salg_v1_mask; + __u8 salg_name[64]; + }; + struct { + __u16 salg_family; + __u8 salg_type[14]; + __u32 salg_feat; + __u32 salg_mask; + __u8 salg_name_new[]; + }; + }; };

-- Gustavo