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

8 Jul 2020

From: Eric Biggers ebiggers@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@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
---
 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 c21b656b3263..840a192e9337 100644
--- a/security/smack/smackfs.c
+++ b/security/smack/smackfs.c
@@ -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;
 }
-- 
2.27.0


    