linux-kernel - [PATCH AUTOSEL 6.1 4/6] bpf: Annotate bpf_long_memcpy with data

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Message-Id: <20230914015523.51894-4-sashal@kernel.org>
Date:   Wed, 13 Sep 2023 21:55:14 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        syzbot+97522333291430dd277f@...kaller.appspotmail.com,
        Marco Elver <elver@...gle.com>,
        Sasha Levin <sashal@...nel.org>, ast@...nel.org,
        andrii@...nel.org, bpf@...r.kernel.org
Subject: [PATCH AUTOSEL 6.1 4/6] bpf: Annotate bpf_long_memcpy with data_race

From: Daniel Borkmann <daniel@...earbox.net>

[ Upstream commit 6a86b5b5cd76d2734304a0173f5f01aa8aa2025e ]

syzbot reported a data race splat between two processes trying to
update the same BPF map value via syscall on different CPUs:

  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

The bpf_long_memcpy is used with 8-byte aligned pointers, power-of-8 size
and forced to use long read/writes to try to atomically copy long counters.
It is best-effort only and no barriers are here since it _will_ race with
concurrent updates from BPF programs. The bpf_long_memcpy() is called from
bpf(2) syscall. Marco suggested that the best way to make this known to
KCSAN would be to use data_race() annotation.

Reported-by: syzbot+97522333291430dd277f@...kaller.appspotmail.com
Suggested-by: Marco Elver <elver@...gle.com>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Acked-by: Marco Elver <elver@...gle.com>
Link: https://lore.kernel.org/bpf/000000000000d87a7f06040c970c@google.com
Link: https://lore.kernel.org/bpf/57628f7a15e20d502247c3b55fceb1cb2b31f266.1693342186.git.daniel@iogearbox.net
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 include/linux/bpf.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 8cef9ec3a89c2..5b53d4c166506 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -301,7 +301,7 @@ static inline void bpf_long_memcpy(void *dst, const void *src, u32 size)
 
 	size /= sizeof(long);
 	while (size--)
-		*ldst++ = *lsrc++;
+		data_race(*ldst++ = *lsrc++);
 }
 
 /* copy everything but bpf_spin_lock, bpf_timer, and kptrs. There could be one of each. */
-- 
2.40.1