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

[Dmitry Vyukov]

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.

> ==================================================================
> 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.