bpf: Fix buggy rsh min/max bounds tracking

[ no upstream commit ]

Fix incorrect bounds tracking for RSH opcode. Commit f23cc64 ("bpf: fix
range arithmetic for bpf map access") had a wrong assumption about min/max
bounds. The new dst_reg->min_value needs to be derived by right shifting the
max_val bounds, not min_val, and likewise new dst_reg->max_value needs to be
derived by right shifting the min_val bounds, not max_val. Later stable kernels
than 4.9 are not affected since bounds tracking was overall reworked and they
already track this similarly as in the fix.

Fixes: f23cc64 ("bpf: fix range arithmetic for bpf map access")
Reported-by: Ryota Shiga (Flatt Security)
Signed-off-by: Daniel Borkmann <[email protected]>
Reviewed-by: John Fastabend <[email protected]>
Cc: Josef Bacik <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

kernel/bpf/verifier.c

@@ -1732,12 +1732,11 @@ static void adjust_reg_min_max_vals(struct bpf_verifier_env *env,
 		 * unsigned shift, so make the appropriate casts.
 		 */
 		if (min_val < 0 || dst_reg->min_value < 0)
-			dst_reg->min_value = BPF_REGISTER_MIN_RANGE;
+			reset_reg_range_values(regs, insn->dst_reg);
 		else
-			dst_reg->min_value =
-				(u64)(dst_reg->min_value) >> min_val;
+			dst_reg->min_value = (u64)(dst_reg->min_value) >> max_val;
 		if (dst_reg->max_value != BPF_REGISTER_MAX_RANGE)
-			dst_reg->max_value >>= max_val;
+			dst_reg->max_value >>= min_val;
 		break;
 	default:
 		reset_reg_range_values(regs, insn->dst_reg);