Re: [syzbot] [ext4?] [mm?] KCSAN: data-race in strscpy / strscpy (3)

[Matthew Wilcox]

On Fri, Apr 21, 2023 at 04:43:19PM +0200, Dmitry Vyukov wrote:
> On Fri, 21 Apr 2023 at 16:33, syzbot
> <syzbot+c2de99a72baaa06d31f3@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    76f598ba7d8e Merge tag 'for-linus' of git://git.kernel.org..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=133bfbedc80000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=9c5d44636e91081b
> > dashboard link: https://syzkaller.appspot.com/bug?extid=c2de99a72baaa06d31f3
> > compiler:       Debian clang version 15.0.7, GNU ld (GNU Binutils for Debian) 2.35.2
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/a3654f5f77b9/disk-76f598ba.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/abfb4aaa5772/vmlinux-76f598ba.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/789fb5546551/bzImage-76f598ba.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+c2de99a72baaa06d31f3@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> +bpf maintainers
> 
> If I am reading this correctly, this can cause a leak of kernel memory
> and/or crash via bpf_get_current_comm helper.
> 
> strcpy() can temporary leave comm buffer non-0 terminated as it
> terminates it only after the copy:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/string.c?id=76f598ba7d8e2bfb4855b5298caedd5af0c374a8#n184
> 
> If bpf_get_current_comm() observes such non-0-terminated comm, it will
> start reading off bounds.

Just to be clear, this isn't ext4 at all; ext4 happens to be calling
kthread_create(), but it's actually a generic kthread problem, right?

I'm not sure how it is that bpf is able to see the task before comm is
initialised; that seems to be the real race here, that comm is not set
before the kthread is a schedulable entity?  Adding the scheduler people.

> > ==================================================================
> > BUG: KCSAN: data-race in strscpy / strscpy
> >
> > write to 0xffff88812ed8b730 of 8 bytes by task 16157 on cpu 1:
> >  strscpy+0xa9/0x170 lib/string.c:165
> >  strscpy_pad+0x27/0x80 lib/string_helpers.c:835
> >  __set_task_comm+0x46/0x140 fs/exec.c:1232
> >  set_task_comm include/linux/sched.h:1984 [inline]
> >  __kthread_create_on_node+0x2b2/0x320 kernel/kthread.c:474
> >  kthread_create_on_node+0x8a/0xb0 kernel/kthread.c:512
> >  ext4_run_lazyinit_thread fs/ext4/super.c:3848 [inline]
> >  ext4_register_li_request+0x407/0x650 fs/ext4/super.c:3983
> >  __ext4_fill_super fs/ext4/super.c:5480 [inline]
> >  ext4_fill_super+0x3f4a/0x43f0 fs/ext4/super.c:5637
> >  get_tree_bdev+0x2b1/0x3a0 fs/super.c:1303
> >  ext4_get_tree+0x1c/0x20 fs/ext4/super.c:5668
> >  vfs_get_tree+0x51/0x190 fs/super.c:1510
> >  do_new_mount+0x200/0x650 fs/namespace.c:3042
> >  path_mount+0x498/0xb40 fs/namespace.c:3372
> >  do_mount fs/namespace.c:3385 [inline]
> >  __do_sys_mount fs/namespace.c:3594 [inline]
> >  __se_sys_mount+0x27f/0x2d0 fs/namespace.c:3571
> >  __x64_sys_mount+0x67/0x80 fs/namespace.c:3571
> >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >  do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
> >  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> >
> > read to 0xffff88812ed8b733 of 1 bytes by task 16161 on cpu 0:
> >  strscpy+0xde/0x170 lib/string.c:174
> >  ____bpf_get_current_comm kernel/bpf/helpers.c:260 [inline]
> >  bpf_get_current_comm+0x45/0x70 kernel/bpf/helpers.c:252
> >  ___bpf_prog_run+0x281/0x3050 kernel/bpf/core.c:1822
> >  __bpf_prog_run32+0x74/0xa0 kernel/bpf/core.c:2043
> >  bpf_dispatcher_nop_func include/linux/bpf.h:1124 [inline]
> >  __bpf_prog_run include/linux/filter.h:601 [inline]
> >  bpf_prog_run include/linux/filter.h:608 [inline]
> >  __bpf_trace_run kernel/trace/bpf_trace.c:2263 [inline]
> >  bpf_trace_run4+0x9f/0x140 kernel/trace/bpf_trace.c:2304
> >  __traceiter_sched_switch+0x3a/0x50 include/trace/events/sched.h:222
> >  trace_sched_switch include/trace/events/sched.h:222 [inline]
> >  __schedule+0x7e7/0x8e0 kernel/sched/core.c:6622
> >  schedule+0x51/0x80 kernel/sched/core.c:6701
> >  schedule_preempt_disabled+0x10/0x20 kernel/sched/core.c:6760
> >  kthread+0x11c/0x1e0 kernel/kthread.c:369
> >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:308
> >
> > value changed: 0x72 -> 0x34
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 0 PID: 16161 Comm: ext4lazyinit Not tainted 6.3.0-rc5-syzkaller-00022-g76f598ba7d8e #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/30/2023
> > ==================================================================
> >
> >
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxx.
> >
> > syzbot will keep track of this issue. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>