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    Subject[PATCH 4.9 078/212] Smack: fix use-after-free in smk_write_relabel_self()
    Date
    From: Eric Biggers <ebiggers@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+e6416dabb497a650da40@syzkaller.appspotmail.com
    Fixes: 38416e53936e ("Smack: limited capability for changing process label")
    Cc: <stable@vger.kernel.org> # v4.4+
    Signed-off-by: Eric Biggers <ebiggers@google.com>
    Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

    --- a/security/smack/smackfs.c
    +++ b/security/smack/smackfs.c
    @@ -2741,7 +2741,6 @@ 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);
    @@ -2766,11 +2765,21 @@ 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;
    }

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