bpf: Sanity check max value for var_off stack access

As discussed in [1] max value of variable offset has to be checked for
overflow on stack access otherwise verifier would accept code like this:

  0: (b7) r2 = 6
  1: (b7) r3 = 28
  2: (7a) *(u64 *)(r10 -16) = 0
  3: (7a) *(u64 *)(r10 -8) = 0
  4: (79) r4 = *(u64 *)(r1 +168)
  5: (c5) if r4 s< 0x0 goto pc+4
   R1=ctx(id=0,off=0,imm=0) R2=inv6 R3=inv28
   R4=inv(id=0,umax_value=9223372036854775807,var_off=(0x0;
   0x7fffffffffffffff)) R10=fp0,call_-1 fp-8=mmmmmmmm fp-16=mmmmmmmm
  6: (17) r4 -= 16
  7: (0f) r4 += r10
  8: (b7) r5 = 8
  9: (85) call bpf_getsockopt#57
  10: (b7) r0 = 0
  11: (95) exit

, where R4 obviosly has unbounded max value.

Fix it by checking that reg->smax_value is inside (-BPF_MAX_VAR_OFF;
BPF_MAX_VAR_OFF) range.

reg->smax_value is used instead of reg->umax_value because stack
pointers are calculated using negative offset from fp. This is opposite
to e.g. map access where offset must be non-negative and where
umax_value is used.

Also dedicated verbose logs are added for both min and max bound check
failures to have diagnostics consistent with variable offset handling in
check_map_access().

[1] https://marc.info/?l=linux-netdev&m=155433357510597&w=2

Fixes: 2011fccf ("bpf: Support variable offset stack access from helpers")
Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/verifier.c

@@ -2248,16 +2248,28 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
 		if (meta && meta->raw_mode)
 			meta = NULL;
 
+		if (reg->smax_value >= BPF_MAX_VAR_OFF ||
+		    reg->smax_value <= -BPF_MAX_VAR_OFF) {
+			verbose(env, "R%d unbounded indirect variable offset stack access\n",
+				regno);
+			return -EACCES;
+		}
 		min_off = reg->smin_value + reg->off;
-		max_off = reg->umax_value + reg->off;
+		max_off = reg->smax_value + reg->off;
 		err = __check_stack_boundary(env, regno, min_off, access_size,
 					     zero_size_allowed);
-		if (err)
+		if (err) {
+			verbose(env, "R%d min value is outside of stack bound\n",
+				regno);
 			return err;
+		}
 		err = __check_stack_boundary(env, regno, max_off, access_size,
 					     zero_size_allowed);
-		if (err)
+		if (err) {
+			verbose(env, "R%d max value is outside of stack bound\n",
+				regno);
 			return err;
+		}
 	}
 
 	if (meta && meta->raw_mode) {