seccomp: fix UAF in user-trap code

On the failure path, we do an fput() of the listener fd if the filter fails
to install (e.g. because of a TSYNC race that's lost, or if the thread is
killed, etc.). fput() doesn't actually release the fd, it just ads it to a
work queue. Then the thread proceeds to free the filter, even though the
listener struct file has a reference to it.

To fix this, on the failure path let's set the private data to null, so we
know in ->release() to ignore the filter.

Reported-by: syzbot+981c26489b2d1c6316ba@syzkaller.appspotmail.com
Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index d7f5388..e815781 100644 (file)
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -976,6 +976,9 @@ static int seccomp_notify_release(struct inode *inode, struct file *file)
        struct seccomp_filter *filter = file->private_data;
        struct seccomp_knotif *knotif;
 
+       if (!filter)
+               return 0;
+
        mutex_lock(&filter->notify_lock);
 
        /*
@@ -1300,6 +1303,7 @@ out:
 out_put_fd:
        if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
                if (ret < 0) {
+                       listener_f->private_data = NULL;
                        fput(listener_f);
                        put_unused_fd(listener);
                } else {