Re: [PATCH] bpf: fix probe read error in ___bpf_prog_run()

[Daniel Borkmann]

On 5/23/22 9:37 AM, menglong8.dong@xxxxxxxxx wrote:
> From: Menglong Dong <imagedong@xxxxxxxxxxx>
>
> 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@xxxxxxxxxxx>
> Reviewed-by: Hao Peng <flyingpeng@xxxxxxxxxxx>
> Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> ---
>   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)

Completely uncompiled, but maybe just fold it into LDST instead:

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 9cc91f0f3115..fc5c29243739 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1948,6 +1948,11 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
                CONT;                                                   \
        LDX_MEM_##SIZEOP:                                               \
                DST = *(SIZE *)(unsigned long) (SRC + insn->off);       \
+               CONT;                                                   \
+       LDX_PROBE_MEM_##SIZEOP:                                         \
+               bpf_probe_read_kernel(&DST, sizeof(SIZE),               \
+                                     (const void *)(long)(SRC + insn->off)); \
+               DST = *((SIZE *)&DST);                                  \
                CONT;

        LDST(B,   u8)
@@ -1955,15 +1960,6 @@ 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)                                                        \
-       LDX_PROBE_MEM_##SIZEOP:                                                 \
-               bpf_probe_read_kernel(&DST, SIZE, (const void *)(long) (SRC + insn->off));      \
-               CONT;
-       LDX_PROBE(B,  1)
-       LDX_PROBE(H,  2)
-       LDX_PROBE(W,  4)
-       LDX_PROBE(DW, 8)
-#undef LDX_PROBE

Thanks,
Daniel