Subject: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run

From: Kurt Manucredo <[email-masked]>
Date: Sat, 5 Jun 2021 15:01:57 +0000

Syzbot detects a shift-out-of-bounds in ___bpf_prog_run()
kernel/bpf/core.c:1414:2.

I propose: In adjust_scalar_min_max_vals() move boundary check up to avoid
missing them and return with error when detected.

Reported-and-tested-by: [email-masked]
Signed-off-by: Kurt Manucredo <[email-masked]>
---

https://syzkaller.appspot.com/bug?id=edb51be4c9a320186328893287bb30d5eed09231

Changelog:
----------
v4 - Fix shift-out-of-bounds in adjust_scalar_min_max_vals.
     Fix commit message.
v3 - Make it clearer what the fix is for.
v2 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
     check in check_alu_op() in verifier.c.
v1 - Fix shift-out-of-bounds in ___bpf_prog_run() by adding boundary
     check in ___bpf_prog_run().

thanks

kind regards

Kurt

 kernel/bpf/verifier.c | 30 +++++++++---------------------
 1 file changed, 9 insertions(+), 21 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 94ba5163d4c5..ed0eecf20de5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7510,6 +7510,15 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 	u32_min_val = src_reg.u32_min_value;
 	u32_max_val = src_reg.u32_max_value;
 
+	if ((opcode == BPF_LSH || opcode == BPF_RSH || opcode == BPF_ARSH) &&
+			umax_val >= insn_bitness) {
+		/* Shifts greater than 31 or 63 are undefined.
+		 * This includes shifts by a negative number.
+		 */
+		verbose(env, "invalid shift %lld\n", umax_val);
+		return -EINVAL;
+	}
+
 	if (alu32) {
 		src_known = tnum_subreg_is_const(src_reg.var_off);
 		if ((src_known &&
@@ -7592,39 +7601,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 		scalar_min_max_xor(dst_reg, &src_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;
-		}
 		if (alu32)
 			scalar32_min_max_lsh(dst_reg, &src_reg);
 		else
 			scalar_min_max_lsh(dst_reg, &src_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;
-		}
 		if (alu32)
 			scalar32_min_max_rsh(dst_reg, &src_reg);
 		else
 			scalar_min_max_rsh(dst_reg, &src_reg);
 		break;
 	case BPF_ARSH:
-		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;
-		}
 		if (alu32)
 			scalar32_min_max_arsh(dst_reg, &src_reg);
 		else
-- 
2.30.2