On 12/11/22 2:51?AM, gregkh@linuxfoundation.org wrote:

The patch below does not apply to the 5.15-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

It does apply fine, but it's in fs/io_uring.c rather than io_uring/io_uring.c as for the newer kernels. Attached one here that just does the patch change and it applies witha a slight offset:

checking file fs/io_uring.c Hunk #1 succeeded at 9467 (offset 6760 lines).

From 998b30c3948e4d0b1097e639918c5cff332acac5 Mon Sep 17 00:00:00 2001 From: Harshit Mogalapalli harshit.m.mogalapalli@oracle.com Date: Tue, 6 Dec 2022 01:38:32 -0800 Subject: [PATCH] io_uring: Fix a null-ptr-deref in io_tctx_exit_cb()

Syzkaller reports a NULL deref bug as follows:

BUG: KASAN: null-ptr-deref in io_tctx_exit_cb+0x53/0xd3 Read of size 4 at addr 0000000000000138 by task file1/1955

CPU: 1 PID: 1955 Comm: file1 Not tainted 6.1.0-rc7-00103-gef4d3ea40565 #75 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7 04/01/2014 Call Trace: <TASK> dump_stack_lvl+0xcd/0x134 ? io_tctx_exit_cb+0x53/0xd3 kasan_report+0xbb/0x1f0 ? io_tctx_exit_cb+0x53/0xd3 kasan_check_range+0x140/0x190 io_tctx_exit_cb+0x53/0xd3 task_work_run+0x164/0x250 ? task_work_cancel+0x30/0x30 get_signal+0x1c3/0x2440 ? lock_downgrade+0x6e0/0x6e0 ? lock_downgrade+0x6e0/0x6e0 ? exit_signals+0x8b0/0x8b0 ? do_raw_read_unlock+0x3b/0x70 ? do_raw_spin_unlock+0x50/0x230 arch_do_signal_or_restart+0x82/0x2470 ? kmem_cache_free+0x260/0x4b0 ? putname+0xfe/0x140 ? get_sigframe_size+0x10/0x10 ? do_execveat_common.isra.0+0x226/0x710 ? lockdep_hardirqs_on+0x79/0x100 ? putname+0xfe/0x140 ? do_execveat_common.isra.0+0x238/0x710 exit_to_user_mode_prepare+0x15f/0x250 syscall_exit_to_user_mode+0x19/0x50 do_syscall_64+0x42/0xb0 entry_SYSCALL_64_after_hwframe+0x63/0xcd RIP: 0023:0x0 Code: Unable to access opcode bytes at 0xffffffffffffffd6. RSP: 002b:00000000fffb7790 EFLAGS: 00000200 ORIG_RAX: 000000000000000b RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000 RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000 R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000 </TASK> Kernel panic - not syncing: panic_on_warn set ...

This happens because the adding of task_work from io_ring_exit_work() isn't synchronized with canceling all work items from eg exec. The execution of the two are ordered in that they are both run by the task itself, but if io_tctx_exit_cb() is queued while we're canceling all work items off exec AND gets executed when the task exits to userspace rather than in the main loop in io_uring_cancel_generic(), then we can find current->io_uring == NULL and hit the above crash.

It's safe to add this NULL check here, because the execution of the two paths are done by the task itself.

Cc: stable@vger.kernel.org Fixes: d56d938b4bef ("io_uring: do ctx initiated file note removal") Reported-by: syzkaller syzkaller@googlegroups.com Signed-off-by: Harshit Mogalapalli harshit.m.mogalapalli@oracle.com Link: https://lore.kernel.org/r/20221206093833.3812138-1-harshit.m.mogalapalli@ora... [axboe: add code comment and also put an explanation in the commit msg] Signed-off-by: Jens Axboe axboe@kernel.dk

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c index 8840cf3e20f2..61cd7ffd0f6a 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -2707,8 +2707,10 @@ static __cold void io_tctx_exit_cb(struct callback_head *cb) /* * When @in_idle, we're in cancellation and it's racy to remove the * node. It'll be removed by the end of cancellation, just ignore it. + * tctx can be NULL if the queueing of this task_work raced with + * work cancelation off the exec path. */ - if (!atomic_read(&tctx->in_idle)) + if (tctx && !atomic_read(&tctx->in_idle)) io_uring_del_tctx_node((unsigned long)work->ctx); complete(&work->completion); }