bpf: Allow narrow loads with offset > 0

Currently BPF verifier allows narrow loads for a context field only with
offset zero. E.g. if there is a __u32 field then only the following
loads are permitted:
  * off=0, size=1 (narrow);
  * off=0, size=2 (narrow);
  * off=0, size=4 (full).

On the other hand LLVM can generate a load with offset different than
zero that make sense from program logic point of view, but verifier
doesn't accept it.

E.g. tools/testing/selftests/bpf/sendmsg4_prog.c has code:

  #define DST_IP4			0xC0A801FEU /* 192.168.1.254 */
  ...
  	if ((ctx->user_ip4 >> 24) == (bpf_htonl(DST_IP4) >> 24) &&

where ctx is struct bpf_sock_addr.

Some versions of LLVM can produce the following byte code for it:

       8:       71 12 07 00 00 00 00 00         r2 = *(u8 *)(r1 + 7)
       9:       67 02 00 00 18 00 00 00         r2 <<= 24
      10:       18 03 00 00 00 00 00 fe 00 00 00 00 00 00 00 00         r3 = 4261412864 ll
      12:       5d 32 07 00 00 00 00 00         if r2 != r3 goto +7 <LBB0_6>

where `*(u8 *)(r1 + 7)` means narrow load for ctx->user_ip4 with size=1
and offset=3 (7 - sizeof(ctx->user_family) = 3). This load is currently
rejected by verifier.

Verifier code that rejects such loads is in bpf_ctx_narrow_access_ok()
what means any is_valid_access implementation, that uses the function,
works this way, e.g. bpf_skb_is_valid_access() for __sk_buff or
sock_addr_is_valid_access() for bpf_sock_addr.

The patch makes such loads supported. Offset can be in [0; size_default)
but has to be multiple of load size. E.g. for __u32 field the following
loads are supported now:
  * off=0, size=1 (narrow);
  * off=1, size=1 (narrow);
  * off=2, size=1 (narrow);
  * off=3, size=1 (narrow);
  * off=0, size=2 (narrow);
  * off=2, size=2 (narrow);
  * off=0, size=4 (full).
Reported-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/filter.h

@@ -668,24 +668,10 @@ static inline u32 bpf_ctx_off_adjust_machine(u32 size)
 	return size;
 }
 
-static inline bool bpf_ctx_narrow_align_ok(u32 off, u32 size_access,
-					   u32 size_default)
-{
-	size_default = bpf_ctx_off_adjust_machine(size_default);
-	size_access  = bpf_ctx_off_adjust_machine(size_access);
-
-#ifdef __LITTLE_ENDIAN
-	return (off & (size_default - 1)) == 0;
-#else
-	return (off & (size_default - 1)) + size_access == size_default;
-#endif
-}
-
 static inline bool
 bpf_ctx_narrow_access_ok(u32 off, u32 size, u32 size_default)
 {
-	return bpf_ctx_narrow_align_ok(off, size, size_default) &&
-	       size <= size_default && (size & (size - 1)) == 0;
+	return size <= size_default && (size & (size - 1)) == 0;
 }
 
 #define bpf_classic_proglen(fprog) (fprog->len * sizeof(fprog->filter[0]))

kernel/bpf/verifier.c

@@ -5718,10 +5718,10 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 	int i, cnt, size, ctx_field_size, delta = 0;
 	const int insn_cnt = env->prog->len;
 	struct bpf_insn insn_buf[16], *insn;
+	u32 target_size, size_default, off;
 	struct bpf_prog *new_prog;
 	enum bpf_access_type type;
 	bool is_narrower_load;
-	u32 target_size;
 
 	if (ops->gen_prologue || env->seen_direct_write) {
 		if (!ops->gen_prologue) {
@@ -5814,9 +5814,9 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 		 * we will apply proper mask to the result.
 		 */
 		is_narrower_load = size < ctx_field_size;
+		size_default = bpf_ctx_off_adjust_machine(ctx_field_size);
+		off = insn->off;
 		if (is_narrower_load) {
-			u32 size_default = bpf_ctx_off_adjust_machine(ctx_field_size);
-			u32 off = insn->off;
 			u8 size_code;
 
 			if (type == BPF_WRITE) {
@@ -5844,12 +5844,23 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 		}
 
 		if (is_narrower_load && size < target_size) {
-			if (ctx_field_size <= 4)
+			u8 shift = (off & (size_default - 1)) * 8;
+
+			if (ctx_field_size <= 4) {
+				if (shift)
+					insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH,
+									insn->dst_reg,
+									shift);
 				insn_buf[cnt++] = BPF_ALU32_IMM(BPF_AND, insn->dst_reg,
 								(1 << size * 8) - 1);
-			else
+			} else {
+				if (shift)
+					insn_buf[cnt++] = BPF_ALU64_IMM(BPF_RSH,
+									insn->dst_reg,
+									shift);
 				insn_buf[cnt++] = BPF_ALU64_IMM(BPF_AND, insn->dst_reg,
 								(1 << size * 8) - 1);
+			}
 		}
 
 		new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);