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

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

[syzbot]

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 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@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the bug is already fixed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite bug's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the bug is a duplicate of another bug, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

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

[Yonghong Song]

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.

> 
> 
> ---
> 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@googlegroups.com.
> 
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> 
> If the bug is already fixed, let syzbot know by replying with:
> #syz fix: exact-commit-title
> 
> If you want to overwrite bug's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
> 
> If the bug is a duplicate of another bug, reply with:
> #syz dup: exact-subject-of-another-report
> 
> If you want to undo deduplication, reply with:
> #syz undup
> 

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@linux.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@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.

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.

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

[Daniel Borkmann]

On 8/29/23 8:53 PM, Marco Elver wrote:
> On Tue, 29 Aug 2023 at 20:30, Yonghong Song <yonghong.song@linux.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@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.
> 
> 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. */

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

[Marco Elver]

On Tue, 29 Aug 2023 at 22:04, Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 8/29/23 8:53 PM, Marco Elver wrote:
> > On Tue, 29 Aug 2023 at 20:30, Yonghong Song <yonghong.song@linux.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@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.
> >
> > 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.

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

[Daniel Borkmann]

On 8/29/23 10:36 PM, Marco Elver wrote:
[...]
> 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.

I wasn't aware of data_race(), thanks for the suggestion! Just flushed out
a small patch.

Thanks,
Daniel