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Message-ID: <20201026212148.GA26823@embeddedor>
Date: Mon, 26 Oct 2020 16:21:48 -0500
From: "Gustavo A. R. Silva" <gustavoars@...nel.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: linux-crypto@...r.kernel.org,
Herbert Xu <herbert@...dor.apana.org.au>,
syzkaller-bugs@...glegroups.com, linux-hardening@...r.kernel.org,
linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
Jann Horn <jannh@...gle.com>,
Kees Cook <keescook@...omium.org>,
Elena Petrova <lenaptr@...gle.com>,
Vegard Nossum <vegard.nossum@...cle.com>,
stable@...r.kernel.org,
syzbot+92ead4eb8e26a26d465e@...kaller.appspotmail.com
Subject: Re: [PATCH] crypto: af_alg - avoid undefined behavior accessing
salg_name
Hi,
On Mon, Oct 26, 2020 at 01:07:15PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.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@...kaller.appspotmail.com
> Fixes: 3f69cc60768b ("crypto: af_alg - Allow arbitrarily long algorithm names")
> Cc: <stable@...r.kernel.org> # v4.12+
> Signed-off-by: Eric Biggers <ebiggers@...gle.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
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