linux-kernel - Re: [PATCH] crypto: algif_hash, avoid zero-sized array

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Date:   Thu, 15 Dec 2016 11:08:24 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     herbert@...dor.apana.org.au
Cc:     linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] crypto: algif_hash, avoid zero-sized array

Ping.

On 11/03/2016, 07:32 PM, Jiri Slaby wrote:
> With this reproducer:
>   struct sockaddr_alg alg = {
>           .salg_family = 0x26,
>           .salg_type = "hash",
>           .salg_feat = 0xf,
>           .salg_mask = 0x5,
>           .salg_name = "digest_null",
>   };
>   int sock, sock2;
> 
>   sock = socket(AF_ALG, SOCK_SEQPACKET, 0);
>   bind(sock, (struct sockaddr *)&alg, sizeof(alg));
>   sock2 = accept(sock, NULL, NULL);
>   setsockopt(sock, SOL_ALG, ALG_SET_KEY, "\x9b\xca", 2);
>   accept(sock2, NULL, NULL);
> 
> ==== 8< ======== 8< ======== 8< ======== 8< ====
> 
> one can immediatelly see an UBSAN warning:
> UBSAN: Undefined behaviour in crypto/algif_hash.c:187:7
> variable length array bound value 0 <= 0
> CPU: 0 PID: 15949 Comm: syz-executor Tainted: G            E      4.4.30-0-default #1
> ...
> Call Trace:
> ...
>  [<ffffffff81d598fd>] ? __ubsan_handle_vla_bound_not_positive+0x13d/0x188
>  [<ffffffff81d597c0>] ? __ubsan_handle_out_of_bounds+0x1bc/0x1bc
>  [<ffffffffa0e2204d>] ? hash_accept+0x5bd/0x7d0 [algif_hash]
>  [<ffffffffa0e2293f>] ? hash_accept_nokey+0x3f/0x51 [algif_hash]
>  [<ffffffffa0e206b0>] ? hash_accept_parent_nokey+0x4a0/0x4a0 [algif_hash]
>  [<ffffffff8235c42b>] ? SyS_accept+0x2b/0x40
> 
> It is a correct warning, as hash state is propagated to accept as zero,
> but creating a zero-length variable array is not allowed in C.
> 
> Fix this as proposed by Herbert -- do "?: 1" on that site. No sizeof or
> similar happens in the code there, so we just allocate one byte even
> though we do not use the array.
> 
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> Cc: Herbert Xu <herbert@...dor.apana.org.au>
> Cc: "David S. Miller" <davem@...emloft.net> (maintainer:CRYPTO API)
> Reported-by: Sasha Levin <sasha.levin@...cle.com>
> ---
>  crypto/algif_hash.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/crypto/algif_hash.c b/crypto/algif_hash.c
> index 2d8466f9e49b..32e8a8d797be 100644
> --- a/crypto/algif_hash.c
> +++ b/crypto/algif_hash.c
> @@ -242,7 +242,7 @@ static int hash_accept(struct socket *sock, struct socket *newsock, int flags)
>  	struct alg_sock *ask = alg_sk(sk);
>  	struct hash_ctx *ctx = ask->private;
>  	struct ahash_request *req = &ctx->req;
> -	char state[crypto_ahash_statesize(crypto_ahash_reqtfm(req))];
> +	char state[crypto_ahash_statesize(crypto_ahash_reqtfm(req)) ? : 1];
>  	struct sock *sk2;
>  	struct alg_sock *ask2;
>  	struct hash_ctx *ctx2;
> 


-- 
js
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