Subject: [PATCH bpf 4/9] bpf: fix 32-bit ALU op verification

From: Alexei Starovoitov <[email-masked]>
Date: Mon, 18 Dec 2017 20:11:56 -0800

From: Jann Horn <[email-masked]>

32-bit ALU ops operate on 32-bit values and have 32-bit outputs.
Adjust the verifier accordingly.

Fixes: f1174f77b50c ("bpf/verifier: rework value tracking")
Signed-off-by: Jann Horn <[email-masked]>
Signed-off-by: Alexei Starovoitov <[email-masked]>
---
 kernel/bpf/verifier.c | 28 +++++++++++++++++-----------
 1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index f716bdf29dd0..ecdc265244ca 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2017,6 +2017,10 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 	return 0;
 }
 
+/* WARNING: This function does calculations on 64-bit values, but the actual
+ * execution may occur on 32-bit values. Therefore, things like bitshifts
+ * need extra checks in the 32-bit case.
+ */
 static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 				      struct bpf_insn *insn,
 				      struct bpf_reg_state *dst_reg,
@@ -2027,12 +2031,8 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 	bool src_known, dst_known;
 	s64 smin_val, smax_val;
 	u64 umin_val, umax_val;
+	u64 insn_bitness = (BPF_CLASS(insn->code) == BPF_ALU64) ? 64 : 32;
 
-	if (BPF_CLASS(insn->code) != BPF_ALU64) {
-		/* 32-bit ALU ops are (32,32)->64 */
-		coerce_reg_to_size(dst_reg, 4);
-		coerce_reg_to_size(&src_reg, 4);
-	}
 	smin_val = src_reg.smin_value;
 	smax_val = src_reg.smax_value;
 	umin_val = src_reg.umin_value;
@@ -2168,9 +2168,9 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 		__update_reg_bounds(dst_reg);
 		break;
 	case BPF_LSH:
-		if (umax_val > 63) {
-			/* Shifts greater than 63 are undefined.  This includes
-			 * shifts by a negative number.
+		if (umax_val >= insn_bitness) {
+			/* Shifts greater than 31 or 63 are undefined.
+			 * This includes shifts by a negative number.
 			 */
 			mark_reg_unknown(env, regs, insn->dst_reg);
 			break;
@@ -2196,9 +2196,9 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 		__update_reg_bounds(dst_reg);
 		break;
 	case BPF_RSH:
-		if (umax_val > 63) {
-			/* Shifts greater than 63 are undefined.  This includes
-			 * shifts by a negative number.
+		if (umax_val >= insn_bitness) {
+			/* Shifts greater than 31 or 63 are undefined.
+			 * This includes shifts by a negative number.
 			 */
 			mark_reg_unknown(env, regs, insn->dst_reg);
 			break;
@@ -2234,6 +2234,12 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 		break;
 	}
 
+	if (BPF_CLASS(insn->code) != BPF_ALU64) {
+		/* 32-bit ALU ops are (32,32)->32 */
+		coerce_reg_to_size(dst_reg, 4);
+		coerce_reg_to_size(&src_reg, 4);
+	}
+
 	__reg_deduce_bounds(dst_reg);
 	__reg_bound_offset(dst_reg);
 	return 0;
-- 
2.9.5