Commit d9b8aada authored Jul 19, 2019 by Ilya Leoshkevich Committed by Alexei Starovoitov Jul 23, 2019

bpf: fix narrower loads on s390



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: 31fd8581 ("bpf: permits narrower load from bpf program context fields")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

parent c8eee413

include/linux/filter.h

+13 −0

Original line number	Diff line number	Diff line
		@@ -24,6 +24,7 @@

		#include <net/sch_generic.h>

		#include <asm/byteorder.h>
		#include <uapi/linux/filter.h>
		#include <uapi/linux/bpf.h>

		@@ -747,6 +748,18 @@ bpf_ctx_narrow_access_ok(u32 off, u32 size, u32 size_default)
		return size <= size_default && (size & (size - 1)) == 0;
		}

		static inline u8
		bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)
		{
		u8 load_off = off & (size_default - 1);

		#ifdef __LITTLE_ENDIAN
		return load_off * 8;
		#else
		return (size_default - (load_off + size)) * 8;
		#endif
		}

		#define bpf_ctx_wide_access_ok(off, size, type, field) \
		(size == sizeof(__u64) && \
		off >= offsetof(type, field) && \

kernel/bpf/verifier.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -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_ctx_narrow_load_shift(off, size,
		size_default);
		if (ctx_field_size <= 4) {
		if (shift)
		insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH,