Re: [syzbot] KASAN: use-after-free Read in put_pmu_ctx

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Dec 19, 2022 at 12:04:43AM -0800, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    13e3c7793e2f Merge tag 'for-netdev' of https://git.kernel...
> git tree:       bpf
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=177df7e0480000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=b0e91ad4b5f69c47
> dashboard link: https://syzkaller.appspot.com/bug?extid=b8e8c01c8ade4fe6e48f
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15e87100480000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16ceeb13880000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/373a99daa295/disk-13e3c779.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/7fa71ed0fe17/vmlinux-13e3c779.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/2842ad5c698b/bzImage-13e3c779.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+b8e8c01c8ade4fe6e48f@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KASAN: use-after-free in __lock_acquire+0x3ee7/0x56d0 kernel/locking/lockdep.c:4925
> Read of size 8 at addr ffff8880237d6018 by task syz-executor287/8300

OK, lemme try this.. still think having to repeat the tree it already
has is daft..

#syz test: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git 13e3c7793e2f


diff --git a/kernel/events/core.c b/kernel/events/core.c
index eacc3702654d..7da593504c5b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -12689,7 +12689,8 @@ SYSCALL_DEFINE5(perf_event_open,
 	return event_fd;
 
 err_context:
-	/* event->pmu_ctx freed by free_event() */
+	put_pmu_ctx(event->pmu_ctx);
+	event->pmu_ctx = NULL; /* _free_event() */
 err_locked:
 	mutex_unlock(&ctx->mutex);
 	perf_unpin_context(ctx);