Re: [syzbot] inconsistent lock state in kmem_cache_alloc

[Jan Kara <jack@suse.cz>]

On Thu 29-09-22 10:54:07, Jens Axboe wrote:
> 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@syzkaller.appspotmail.com
> >>>
> >>> +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.

Thanks for fixing this so quickly! The test looks good to me.

								Honza

> #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
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR