Commit beb4ee67 authored Jul 08, 2020 by Eric Biggers Committed by Casey Schaufler Jul 14, 2020

Smack: fix use-after-free in smk_write_relabel_self()



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: 38416e53 ("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>

parent b3a9e3b9

security/smack/smackfs.c

+11 −2

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

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