Vulnerability: kernel/bpf/verifier.c in the Linux kernel through 4.14.8 allows local users to cause a denial of service (memory corruption) or possibly have unspecified other impact by leveraging mishandling of 32-bit ALU ops.
Vulnerability ID: CVE-2017-17852
CWE: CWE-119
Severity score: 10
Project: https://github.com/torvalds/linux
Published: 2017-12-27
File: kernel/bpf/verifier.c
Commit message: bpf: fix 32-bit ALU op verification

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 <jannh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Commit author: Jann Horn

Vulnerable version (before fix):
 static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env, 	return 0; }  static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, 				      struct bpf_insn *insn, 				      struct bpf_reg_state *dst_reg, 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;  	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; 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. 			 */ 			mark_reg_unknown(env, regs, insn->dst_reg); 			break; 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. 			 */ 			mark_reg_unknown(env, regs, insn->dst_reg); 			break; static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, 		break; 	}  	__reg_deduce_bounds(dst_reg); 	__reg_bound_offset(dst_reg); 	return 0;

Fixed version (after fix):
 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, 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;  	smin_val = src_reg.smin_value; 	smax_val = src_reg.smax_value; 	umin_val = src_reg.umin_value; static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, 		__update_reg_bounds(dst_reg); 		break; 	case BPF_LSH: 		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; static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env, 		__update_reg_bounds(dst_reg); 		break; 	case BPF_RSH: 		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; 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;

Patch:
patch: @@ -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;
path: kernel/bpf/verifier.c