Re: [syzbot] inconsistent lock state in kmem_cache_alloc

[Jens Axboe]

On 9/29/22 8:07 AM, Jens Axboe wrote:
> On 9/29/22 7:56 AM, Jan Kara wrote:
>> On Thu 29-09-22 15:24:22, Vlastimil Babka wrote:
>>> On 9/26/22 18:33, syzbot wrote:
>>>> Hello,
>>>>
>>>> syzbot found the following issue on:
>>>>
>>>> HEAD commit:    105a36f3694e Merge tag 'kbuild-fixes-v6.0-3' of git://git...
>>>> git tree:       upstream
>>>> console+strace: https://syzkaller.appspot.com/x/log.txt?x=152bf540880000
>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=7db7ad17eb14cb7
>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=dfcc5f4da15868df7d4d
>>>> 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=1020566c880000
>>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=104819e4880000
>>>>
>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>>> Reported-by: syzbot+dfcc5f4da15868df7d4d@xxxxxxxxxxxxxxxxxxxxxxxxx
>>>
>>> +CC more folks
>>>
>>> I'm not fully sure what this report means but I assume it's because there's
>>> a GFP_KERNEL kmalloc() allocation from softirq context? Should it perhaps
>>> use memalloc_nofs_save() at some well defined point?
>>
>> Thanks for the CC. The problem really is that io_uring is calling into
>> fsnotify_access() from softirq context. That isn't going to work. The
>> allocation is just a tip of the iceberg. Fsnotify simply does not expect to
>> be called from softirq context. All the dcache locks are not IRQ safe, it
>> can even obtain some sleeping locks and call to userspace if there are
>> suitable watches set up.
>>
>> So either io_uring needs to postpone fsnotify calls to a workqueue or we
>> need a way for io_uring code to tell iomap dio code that the completion
>> needs to always happen from a workqueue (as it currently does for writes).
>> Jens?
> 
> Something like this should probably work - I'll write a test case and
> vet it.

Ran that with the attached test case, triggers it before but not with
the patch. Side note - I do wish that the syzbot reproducers were not
x86 specific, I always have to go and edit them for arm64. For this
particular one, I just gave up and wrote one myself.

Thanks for the heads-up Jan, I'll queue up this fix and mark for stable
with the right attributions.

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/fanotify.h>
#include <sys/wait.h>
#include <liburing.h>

int main(int argc, char *argv[])
{
	struct io_uring_sqe *sqe;
	struct io_uring_cqe *cqe;
	struct io_uring ring;
	int fan, ret, fd;
	void *buf;

	fan = fanotify_init(FAN_CLASS_NOTIF|FAN_CLASS_CONTENT, 0);
	if (fan < 0) {
		if (errno == ENOSYS)
			return 0;
		perror("fanotify_init");
		return 1;
	}

	if (argc > 1) {
		fd = open(argv[1], O_RDONLY | O_DIRECT);
		if (fd < 0) {
			perror("open");
			return 1;
		}
	} else {
		fd = open("file0", O_RDONLY | O_DIRECT);
		if (fd < 0) {
			perror("open");
			return 1;
		}
	}

	ret = fanotify_mark(fan, FAN_MARK_ADD, FAN_ACCESS|FAN_MODIFY, fd, NULL);
	if (ret < 0) {
		perror("fanotify_mark");
		return 1;
	}

	ret = 0;
	if (fork()) {
		int wstat;

		io_uring_queue_init(4, &ring, 0);
		if (posix_memalign(&buf, 4096, 4096))
			return 0;
		sqe = io_uring_get_sqe(&ring);
		io_uring_prep_read(sqe, fd, buf, 4096, 0);
		io_uring_submit(&ring);
		ret = io_uring_wait_cqe(&ring, &cqe);
		if (ret) {
			fprintf(stderr, "wait_ret=%d\n", ret);
			return 1;
		}
		wait(&wstat);
		ret = WEXITSTATUS(wstat);
	} else {
		struct fanotify_event_metadata m;
		int fret;

		fret = read(fan, &m, sizeof(m));
		if (fret < 0)
			perror("fanotify read");
		/* fail if mask isn't right or pid indicates non-task context */
		else if (!(m.mask & 1) || !m.pid)
			exit(1);
		exit(0);
	}

	return ret;
}

-- 
Jens Axboe