Re: [PATCH bpf v2] bpf: fix narrower loads on s390

[Y Song <ys114321@gmail.com>]

On Thu, Jul 18, 2019 at 8:01 AM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> The very first check in test_pkt_md_access is failing on s390, which
> happens because loading a part of a struct __sk_buff field produces
> an incorrect result.
>
> The preprocessed code of the check is:
>
> {
>         __u8 tmp = *((volatile __u8 *)&skb->len +
>                 ((sizeof(skb->len) - sizeof(__u8)) / sizeof(__u8)));
>         if (tmp != ((*(volatile __u32 *)&skb->len) & 0xFF)) return 2;
> };
>
> clang generates the following code for it:
>
>       0:        71 21 00 03 00 00 00 00 r2 = *(u8 *)(r1 + 3)
>       1:        61 31 00 00 00 00 00 00 r3 = *(u32 *)(r1 + 0)
>       2:        57 30 00 00 00 00 00 ff r3 &= 255
>       3:        5d 23 00 1d 00 00 00 00 if r2 != r3 goto +29 <LBB0_10>
>
> Finally, verifier transforms it to:
>
>   0: (61) r2 = *(u32 *)(r1 +104)
>   1: (bc) w2 = w2
>   2: (74) w2 >>= 24
>   3: (bc) w2 = w2
>   4: (54) w2 &= 255
>   5: (bc) w2 = w2
>
> The problem is that when verifier emits the code to replace a partial
> load of a struct __sk_buff field (*(u8 *)(r1 + 3)) with a full load of
> struct sk_buff field (*(u32 *)(r1 + 104)), an optional shift and a
> bitwise AND, it assumes that the machine is little endian and
> incorrectly decides to use a shift.
>
> Adjust shift count calculation to account for endianness.
>
> Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields")
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

Acked-by: Yonghong Song <yhs@fb.com>

> ---
>  include/linux/filter.h | 13 +++++++++++++
>  kernel/bpf/verifier.c  |  4 ++--
>  2 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index ff65d22cf336..4fe88e43f0fe 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -24,6 +24,8 @@
>
>  #include <net/sch_generic.h>
>
> +#include <asm/byteorder.h>
> +
>  #include <uapi/linux/filter.h>
>  #include <uapi/linux/bpf.h>
>
> @@ -1216,4 +1218,15 @@ struct bpf_sockopt_kern {
>         s32             retval;
>  };
>
> +static inline u8 bpf_narrower_load_shift(u32 size_default, u32 size, u32 off)
> +{
> +       u8 load_off = off & (size_default - 1);
> +
> +#ifdef __LITTLE_ENDIAN
> +       return load_off * 8;
> +#else
> +       return (size_default - (load_off + size)) * 8;
> +#endif
> +}
> +
>  #endif /* __LINUX_FILTER_H__ */
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 5900cbb966b1..48edc9c9a879 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -8616,8 +8616,8 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
>                 }
>
>                 if (is_narrower_load && size < target_size) {
> -                       u8 shift = (off & (size_default - 1)) * 8;
> -
> +                       u8 shift = bpf_narrower_load_shift(size_default, size,
> +                                                          off);
>                         if (ctx_field_size <= 4) {
>                                 if (shift)
>                                         insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH,
> --
> 2.21.0
>