Re: memory leak in inotify_update_watch

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Jul 07, 2020 at 05:24:11PM +0200, Jan Kara wrote:
> On Mon 06-07-20 08:42:24, syzbot wrote:
> > syzbot found the following crash on:
> > 
> > HEAD commit:    7cc2a8ea Merge tag 'block-5.8-2020-07-01' of git://git.ker..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=17644c05100000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=5ee23b9caef4e07a
> > dashboard link: https://syzkaller.appspot.com/bug?extid=dec34b033b3479b9ef13
> > compiler:       gcc (GCC) 10.1.0-syz 20200507
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1478a67b100000
> > 
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+dec34b033b3479b9ef13@syzkaller.appspotmail.com
> > 
> > BUG: memory leak
> > unreferenced object 0xffff888115db8480 (size 576):
> >   comm "systemd-udevd", pid 11037, jiffies 4295104591 (age 56.960s)
> >   hex dump (first 32 bytes):
> >     00 04 00 00 00 00 00 00 80 fd e8 15 81 88 ff ff  ................
> >     a0 02 dd 20 81 88 ff ff b0 81 d0 09 81 88 ff ff  ... ............
> >   backtrace:
> >     [<00000000288c0066>] radix_tree_node_alloc.constprop.0+0xc1/0x140 lib/radix-tree.c:252
> >     [<00000000f80ba6a7>] idr_get_free+0x231/0x3b0 lib/radix-tree.c:1505
> >     [<00000000ec9ab938>] idr_alloc_u32+0x91/0x120 lib/idr.c:46
> >     [<00000000aea98d29>] idr_alloc_cyclic+0x84/0x110 lib/idr.c:125
> >     [<00000000dbad44a4>] inotify_add_to_idr fs/notify/inotify/inotify_user.c:365 [inline]
> >     [<00000000dbad44a4>] inotify_new_watch fs/notify/inotify/inotify_user.c:578 [inline]
> >     [<00000000dbad44a4>] inotify_update_watch+0x1af/0x2d0 fs/notify/inotify/inotify_user.c:617
> >     [<00000000e141890d>] __do_sys_inotify_add_watch fs/notify/inotify/inotify_user.c:755 [inline]
> >     [<00000000e141890d>] __se_sys_inotify_add_watch fs/notify/inotify/inotify_user.c:698 [inline]
> >     [<00000000e141890d>] __x64_sys_inotify_add_watch+0x12f/0x180 fs/notify/inotify/inotify_user.c:698
> >     [<00000000d872d7cc>] do_syscall_64+0x4c/0xe0 arch/x86/entry/common.c:359
> >     [<000000005c62d8da>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> I've been looking into this for a while and I don't think this is related
> to inotify at all. Firstly the reproducer looks totally benign:
> 
> prlimit64(0x0, 0xe, &(0x7f0000000280)={0x9, 0x8d}, 0x0)
> sched_setattr(0x0, &(0x7f00000000c0)={0x38, 0x2, 0x0, 0x0, 0x9}, 0x0)
> vmsplice(0xffffffffffffffff, 0x0, 0x0, 0x0)
> perf_event_open(0x0, 0x0, 0xffffffffffffffff, 0xffffffffffffffff, 0x0)
> clone(0x20000103, 0x0, 0xfffffffffffffffe, 0x0, 0xffffffffffffffff)
> syz_mount_image$vfat(0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
> 
> So we seem to set SCHED_RR class and prio 9 to itself, the rest of syscalls
> seem to be invalid and should fail. Secondly, the kernel log shows that we
> hit OOM killer frequently and after one of these kills, many leaked objects
> (among them this radix tree node from inotify idr) are reported. I'm not
> sure if it could be the leak detector getting confused (e.g. because it got
> ENOMEM at some point) or something else... Catalin, any idea?

Kmemleak never performs well under heavy load. Normally you'd need to
let the system settle for a bit before checking whether the leaks are
still reported. The issue is caused by the memory scanning not stopping
the whole machine, so pointers may be hidden in registers on different
CPUs (list insertion/deletion for example causes transient kmemleak
confusion).

I think the syzkaller guys tried a year or so ago to run it in parallel
with kmemleak and gave up shortly. The proposal was to add a "stopscan"
command to kmemleak which would do this under stop_machine(). However,
no-one got to implementing it.

So, in this case, does the leak still appear with the reproducer, once
the system went idle?

-- 
Catalin