[PATCH] Smack: fix use-after-free in smk_write_relabel_self()

[Eric Biggers]

On Wed, Jul 08, 2020 at 01:15:20PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers at google.com>
> 
> 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 at syzkaller.appspotmail.com
> Fixes: 38416e53936e ("Smack: limited capability for changing process label")
> Cc: <stable at vger.kernel.org> # v4.4+
> Signed-off-by: Eric Biggers <ebiggers at google.com>

Ping.