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

[Yonghong Song <yonghong.song@linux.dev>]

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@syzkaller.appspotmail.com
> 
> ==================================================================
> 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.

> 
> 
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