[PATCH 5.4 22/67] Smack: fix use-after-free in smk_write_relabel_self()

Greg Kroah-Hartman
2020-08-10T15:36:55Z

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
@@ -2720,7 +2720,6 @@ 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 = smack_cred(current_cred());
char *data;
int rc;
LIST_HEAD(list_tmp);
@@ -2745,11 +2744,21 @@ 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 = smack_cred(new);
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;
}