Subject: [PATCH v5] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run

From: Kurt Manucredo <[email-masked]>
Date: Tue, 15 Jun 2021 16:42:10 +0000

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

The shift-out-of-bounds happens when we have BPF_X. This means we have
to go the same way we go when we want to avoid a divide-by-zero. We do
it in do_misc_fixups().

When we have BPF_K we find divide-by-zero and shift-out-of-bounds guards
next each other in check_alu_op(). It seems only logical to me that the
same should be true for BPF_X in do_misc_fixups() since it is there where
I found the divide-by-zero guard. Or is there a reason I'm not aware of,
that dictates that the checks should be in adjust_scalar_min_max_vals(),
as they are now?

This patch was tested by syzbot.

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

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

Changelog:
----------
v5 - Fix shift-out-of-bounds in do_misc_fixups().
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 | 53 +++++++++++++++++++++++++------------------
 1 file changed, 31 insertions(+), 22 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 94ba5163d4c5..83c7c1ccaf26 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7496,7 +7496,6 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 	u64 umin_val, umax_val;
 	s32 s32_min_val, s32_max_val;
 	u32 u32_min_val, u32_max_val;
-	u64 insn_bitness = (BPF_CLASS(insn->code) == BPF_ALU64) ? 64 : 32;
 	bool alu32 = (BPF_CLASS(insn->code) != BPF_ALU64);
 	int ret;
 
@@ -7592,39 +7591,18 @@ 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
@@ -12353,6 +12331,37 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 			continue;
 		}
 
+		/* Make shift-out-of-bounds exceptions impossible. */
+		if (insn->code == (BPF_ALU64 | BPF_LSH | BPF_X) ||
+		    insn->code == (BPF_ALU64 | BPF_RSH | BPF_X) ||
+		    insn->code == (BPF_ALU64 | BPF_ARSH | BPF_X) ||
+		    insn->code == (BPF_ALU | BPF_LSH | BPF_X) ||
+		    insn->code == (BPF_ALU | BPF_RSH | BPF_X) ||
+		    insn->code == (BPF_ALU | BPF_ARSH | BPF_X)) {
+			bool is64 = BPF_CLASS(insn->code) == BPF_ALU64;
+			u8 insn_bitness = is64 ? 64 : 32;
+			struct bpf_insn chk_and_shift[] = {
+				/* [R,W]x shift >= 32||64 -> 0 */
+				BPF_RAW_INSN((is64 ? BPF_JMP : BPF_JMP32) |
+					     BPF_JLT | BPF_K, insn->src_reg,
+					     insn_bitness, 2, 0),
+				BPF_ALU32_REG(BPF_XOR, insn->dst_reg, insn->dst_reg),
+				BPF_JMP_IMM(BPF_JA, 0, 0, 1),
+				*insn,
+			};
+
+			cnt = ARRAY_SIZE(chk_and_shift);
+
+			new_prog = bpf_patch_insn_data(env, i + delta, chk_and_shift, cnt);
+			if (!new_prog)
+				return -ENOMEM;
+
+			delta    += cnt - 1;
+			env->prog = prog = new_prog;
+			insn      = new_prog->insnsi + i + delta;
+			continue;
+		}
+
 		/* Implement LD_ABS and LD_IND with a rewrite, if supported by the program type. */
 		if (BPF_CLASS(insn->code) == BPF_LD &&
 		    (BPF_MODE(insn->code) == BPF_ABS ||
-- 
2.30.2