Re: general protection fault in blkcipher_walk_done

[Eric Biggers <ebiggers3@gmail.com>]

On Mon, Nov 27, 2017 at 10:56:47AM -0800, syzbot wrote:
> Hello,
> 
> syzkaller hit the following crash on
> 1ea8d039f9edcfefb20d8ddfe136930f6e551529
> git://git.cmpxchg.org/linux-mmots.git/master
> compiler: gcc (GCC) 7.1.1 20170620
> .config is attached
> Raw console output is attached.
> C reproducer is attached
> syzkaller reproducer is attached. See https://goo.gl/kgGztJ
> for information about syzkaller reproducers

Still happens on latest Linus tree (v4.15-rc1) with crypto/master merged in.  It
seems that _aead_recvmsg() is being confused by the operation mode being changed
from encryption to decryption while it has dropped the socket lock in
af_alg_wait_for_data().  Here's a simplified reproducer:

#include <linux/if_alg.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <unistd.h>

int main()
{
        for (;;) {
                int algfd, reqfd;
                struct sockaddr_alg addr = {
                        .salg_type = "aead",
                        .salg_name = "gcm(aes)",
                };
                struct {
                        struct cmsghdr hdr;
                        __u32 op;
                } set_op = {
                        .hdr = {
                                .cmsg_len = sizeof(set_op),
                                .cmsg_level = SOL_ALG,
                                .cmsg_type = ALG_SET_OP,
                        },
                };
                struct msghdr set_op_msg = {
                        .msg_control = &set_op,
                        .msg_controllen = sizeof(set_op),
                };
                char key[16] = { 0 };
                char buf[4096];

                algfd = socket(AF_ALG, SOCK_SEQPACKET, 0);

                bind(algfd, (void *)&addr, sizeof(addr));

                setsockopt(algfd, SOL_ALG, ALG_SET_KEY, key, sizeof(key));

                reqfd = accept(algfd, NULL, NULL);

                set_op.op = ALG_OP_ENCRYPT;
                sendmsg(reqfd, &set_op_msg, 0);

                if (fork() == 0) {
                        set_op.op = ALG_OP_DECRYPT;
                        sendmsg(reqfd, &set_op_msg, 0);
                        break;
                }

                read(reqfd, buf, sizeof(buf));
                wait(NULL);
        }
}