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

diff --git a/security/smack/smackfs.c b/security/smack/smackfs.c index 371ae368da35..10ee51d04492 100644 --- a/security/smack/smackfs.c +++ b/security/smack/smackfs.c @@ -2746,7 +2746,6 @@ static int smk_open_relabel_self(struct inode *inode, struct file *file) 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(struct file *file, const char __user *buf, 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; }