On Tue, 29 Aug 2023 at 22:04, Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote: > > On 8/29/23 8:53 PM, Marco Elver wrote: > > 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. > > I presume KCSAN could be silenced here via READ_ONCE/WRITE_ONCE conversion? > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h > index f58895830ada..32c4a37045f2 100644 > --- a/include/linux/bpf.h > +++ b/include/linux/bpf.h > @@ -424,8 +424,11 @@ static inline void bpf_long_memcpy(void *dst, const void *src, u32 size) > long *ldst = dst; > > size /= sizeof(long); > - while (size--) > - *ldst++ = *lsrc++; > + while (size--) { > + WRITE_ONCE(*ldst, READ_ONCE(*lsrc)); > + ldst++; > + lsrc++; > + } In this case, maybe data_race(*ldst++ = *lsrc++) would be more appropriate and efficient. Unlikely that READ_ONCE()/WRITE_ONCE() here helps make this any safer, i.e. the memcpy is still not atomic and if it's a bug on the user's side, it'll corrupt data either way.
