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    Subject[PATCH 5.4 071/453] crypto: af_alg - avoid undefined behavior accessing salg_name
    Date
    From: Eric Biggers <ebiggers@google.com>

    commit 92eb6c3060ebe3adf381fd9899451c5b047bb14d upstream.

    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>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

    --- a/crypto/af_alg.c
    +++ b/crypto/af_alg.c
    @@ -147,7 +147,7 @@ static int alg_bind(struct socket *sock,
    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,
    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,
    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 (IS_ERR(type) && PTR_ERR(type) == -ENOENT) {
    --- 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[];
    +};
    +
    struct af_alg_iv {
    __u32 ivlen;
    __u8 iv[0];

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