Re: [PATCH] crypto: af_alg - avoid undefined behavior accessing salg_name

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

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