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Date: Tue, 29 Aug 2023 22:04:02 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Marco Elver <elver@...gle.com>, yonghong.song@...ux.dev
Cc: syzbot <syzbot+97522333291430dd277f@...kaller.appspotmail.com>,
andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
haoluo@...gle.com, john.fastabend@...il.com, jolsa@...nel.org,
kpsingh@...nel.org, linux-kernel@...r.kernel.org,
martin.lau@...ux.dev, sdf@...gle.com, song@...nel.org,
syzkaller-bugs@...glegroups.com, yhs@...com,
"Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [syzbot] [bpf?] KCSAN: data-race in bpf_percpu_array_update /
bpf_percpu_array_update (2)
On 8/29/23 8:53 PM, Marco Elver wrote:
> On Tue, 29 Aug 2023 at 20:30, Yonghong Song <yonghong.song@...ux.dev> 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@...kaller.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.
>
> 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++;
+ }
}
/* copy everything but bpf_spin_lock, bpf_timer, and kptrs. There could be one of each. */
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