Re: [syzbot] [bpf?] KCSAN: data-race in bpf_percpu_array_update / bpf_percpu_array_update (2)

[Marco Elver]

On Tue, 29 Aug 2023 at 20:30, Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
>
>
> On 8/29/23 5:39 AM, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    727dbda16b83 Merge tag 'hardening-v6.6-rc1' of git://git.k..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=136f39dfa80000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=dea9c2ce3f646a25
> > dashboard link: https://syzkaller.appspot.com/bug?extid=97522333291430dd277f
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/9923a023ab11/disk-727dbda1.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/650dbc695d77/vmlinux-727dbda1.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/361da71276bf/bzImage-727dbda1.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+97522333291430dd277f@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > ==================================================================
> > BUG: KCSAN: data-race in bpf_percpu_array_update / bpf_percpu_array_update
> >
> > write to 0xffffe8fffe7425d8 of 8 bytes by task 8257 on cpu 1:
> >   bpf_long_memcpy include/linux/bpf.h:428 [inline]
> >   bpf_obj_memcpy include/linux/bpf.h:441 [inline]
> >   copy_map_value_long include/linux/bpf.h:464 [inline]
> >   bpf_percpu_array_update+0x3bb/0x500 kernel/bpf/arraymap.c:380
> >   bpf_map_update_value+0x190/0x370 kernel/bpf/syscall.c:175
> >   generic_map_update_batch+0x3ae/0x4f0 kernel/bpf/syscall.c:1749
> >   bpf_map_do_batch+0x2df/0x3d0 kernel/bpf/syscall.c:4648
> >   __sys_bpf+0x28a/0x780
> >   __do_sys_bpf kernel/bpf/syscall.c:5241 [inline]
> >   __se_sys_bpf kernel/bpf/syscall.c:5239 [inline]
> >   __x64_sys_bpf+0x43/0x50 kernel/bpf/syscall.c:5239
> >   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
> >
> > write to 0xffffe8fffe7425d8 of 8 bytes by task 8268 on cpu 0:
> >   bpf_long_memcpy include/linux/bpf.h:428 [inline]
> >   bpf_obj_memcpy include/linux/bpf.h:441 [inline]
> >   copy_map_value_long include/linux/bpf.h:464 [inline]
> >   bpf_percpu_array_update+0x3bb/0x500 kernel/bpf/arraymap.c:380
> >   bpf_map_update_value+0x190/0x370 kernel/bpf/syscall.c:175
> >   generic_map_update_batch+0x3ae/0x4f0 kernel/bpf/syscall.c:1749
> >   bpf_map_do_batch+0x2df/0x3d0 kernel/bpf/syscall.c:4648
> >   __sys_bpf+0x28a/0x780
> >   __do_sys_bpf kernel/bpf/syscall.c:5241 [inline]
> >   __se_sys_bpf kernel/bpf/syscall.c:5239 [inline]
> >   __x64_sys_bpf+0x43/0x50 kernel/bpf/syscall.c:5239
> >   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
> >
> > value changed: 0x0000000000000000 -> 0xfffffff000002788
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 0 PID: 8268 Comm: syz-executor.4 Not tainted 6.5.0-syzkaller-00453-g727dbda16b83 #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/26/2023
> > ==================================================================
>
> This case is with two tasks doing bpf_map batch update together for the
> same map and key.
>    > write to 0xffffe8fffe7425d8 of 8 bytes by task 8257 on cpu 1:
>    > write to 0xffffe8fffe7425d8 of 8 bytes by task 8268 on cpu 0:
>
> So concurrency is introduced by user applications.
> In my opinion, this probably not an issue from kernel perspective.

Perhaps not, but I recall there being a discussion about making KCSAN
aware of memory accesses done by BPF programs (memcpy being a tiny
subset of those). Not sure if the above data race qualifies as
something we might want to still detect, i.e. a kernel dev testing
their kernel might be interested in such a report.

Regardless, in this case we should teach syzkaller to ignore KCSAN
data races that originate from bpf user operations whatever the
origin.