linux-kernel - [PATCH] bpf: fix probe read error in ___bpf_prog

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Date:   Mon, 23 May 2022 15:37:32 +0800
From:   menglong8.dong@...il.com
To:     ast@...nel.org
Cc:     daniel@...earbox.net, andrii@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Menglong Dong <imagedong@...cent.com>,
        Jiang Biao <benbjiang@...cent.com>,
        Hao Peng <flyingpeng@...cent.com>
Subject: [PATCH] bpf: fix probe read error in ___bpf_prog_run()

From: Menglong Dong <imagedong@...cent.com>

I think there is something wrong with BPF_PROBE_MEM in ___bpf_prog_run()
in big-endian machine. Let's make a test and see what will happen if we
want to load a 'u16' with BPF_PROBE_MEM.

Let's make the src value '0x0001', the value of dest register will become
0x0001000000000000, as the value will be loaded to the first 2 byte of
DST with following code:

  bpf_probe_read_kernel(&DST, SIZE, (const void *)(long) (SRC + insn->off));

Obviously, the value in DST is not correct. In fact, we can compare
BPF_PROBE_MEM with LDX_MEM_H:

  DST = *(SIZE *)(unsigned long) (SRC + insn->off);

If the memory load is done by LDX_MEM_H, the value in DST will be 0x1 now.

And I think this error results in the test case 'test_bpf_sk_storage_map'
failing:

  test_bpf_sk_storage_map:PASS:bpf_iter_bpf_sk_storage_map__open_and_load 0 nsec
  test_bpf_sk_storage_map:PASS:socket 0 nsec
  test_bpf_sk_storage_map:PASS:map_update 0 nsec
  test_bpf_sk_storage_map:PASS:socket 0 nsec
  test_bpf_sk_storage_map:PASS:map_update 0 nsec
  test_bpf_sk_storage_map:PASS:socket 0 nsec
  test_bpf_sk_storage_map:PASS:map_update 0 nsec
  test_bpf_sk_storage_map:PASS:attach_iter 0 nsec
  test_bpf_sk_storage_map:PASS:create_iter 0 nsec
  test_bpf_sk_storage_map:PASS:read 0 nsec
  test_bpf_sk_storage_map:FAIL:ipv6_sk_count got 0 expected 3
  $10/26 bpf_iter/bpf_sk_storage_map:FAIL

The code of the test case is simply, it will load sk->sk_family to the
register with BPF_PROBE_MEM and check if it is AF_INET6. With this patch,
now the test case 'bpf_iter' can pass:

  $10  bpf_iter:OK

Reviewed-by: Jiang Biao <benbjiang@...cent.com>
Reviewed-by: Hao Peng <flyingpeng@...cent.com>
Signed-off-by: Menglong Dong <imagedong@...cent.com>
---
 kernel/bpf/core.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 13e9dbeeedf3..09e3f374739a 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1945,14 +1945,15 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
 	LDST(W,  u32)
 	LDST(DW, u64)
 #undef LDST
-#define LDX_PROBE(SIZEOP, SIZE)							\
+#define LDX_PROBE(SIZEOP, SIZE, TYPE)						\
 	LDX_PROBE_MEM_##SIZEOP:							\
 		bpf_probe_read_kernel(&DST, SIZE, (const void *)(long) (SRC + insn->off));	\
+		DST = *((TYPE *)&DST);						\
 		CONT;
-	LDX_PROBE(B,  1)
-	LDX_PROBE(H,  2)
-	LDX_PROBE(W,  4)
-	LDX_PROBE(DW, 8)
+	LDX_PROBE(B,  1, u8)
+	LDX_PROBE(H,  2, u16)
+	LDX_PROBE(W,  4, u32)
+	LDX_PROBE(DW, 8, u64)
 #undef LDX_PROBE
 
 #define ATOMIC_ALU_OP(BOP, KOP)						\
-- 
2.36.1