[PATCH 4.14 042/228] Smack: fix use-after-free in smk_write_relabel_self()

[Greg Kroah-Hartman]

From: Eric Biggers <ebigg...@google.com>

commit beb4ee6770a89646659e6a2178538d2b13e2654e upstream.

smk_write_relabel_self() frees memory from the task's credentials with
no locking, which can easily cause a use-after-free because multiple
tasks can share the same credentials structure.

Fix this by using prepare_creds() and commit_creds() to correctly modify
the task's credentials.

Reproducer for "BUG: KASAN: use-after-free in smk_write_relabel_self":

        #include <fcntl.h>
        #include <pthread.h>
        #include <unistd.h>

        static void *thrproc(void *arg)
        {
                int fd = open("/sys/fs/smackfs/relabel-self", O_WRONLY);
                for (;;) write(fd, "foo", 3);
        }

        int main()
        {
                pthread_t t;
                pthread_create(&t, NULL, thrproc, NULL);
                thrproc(NULL);
        }

Reported-by: syzbot+e6416dabb497a650d...@syzkaller.appspotmail.com
Fixes: 38416e53936e ("Smack: limited capability for changing process label")
Cc: <sta...@vger.kernel.org> # v4.4+
Signed-off-by: Eric Biggers <ebigg...@google.com>
Signed-off-by: Casey Schaufler <ca...@schaufler-ca.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 security/smack/smackfs.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

--- a/security/smack/smackfs.c
+++ b/security/smack/smackfs.c
@@ -2746,7 +2746,6 @@ static int smk_open_relabel_self(struct
 static ssize_t smk_write_relabel_self(struct file *file, const char __user 
*buf,
                                size_t count, loff_t *ppos)
 {
-       struct task_smack *tsp = current_security();
        char *data;
        int rc;
        LIST_HEAD(list_tmp);
@@ -2771,11 +2770,21 @@ static ssize_t smk_write_relabel_self(st
        kfree(data);
 
        if (!rc || (rc == -EINVAL && list_empty(&list_tmp))) {
+               struct cred *new;
+               struct task_smack *tsp;
+
+               new = prepare_creds();
+               if (!new) {
+                       rc = -ENOMEM;
+                       goto out;
+               }
+               tsp = new->security;
                smk_destroy_label_list(&tsp->smk_relabel);
                list_splice(&list_tmp, &tsp->smk_relabel);
+               commit_creds(new);
                return count;
        }
-
+out:
        smk_destroy_label_list(&list_tmp);
        return rc;
 }