netdev - [PATCH v3] bpf: core: fix shift-out-of-bounds in ___bpf_prog

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Date:   Wed,  2 Jun 2021 21:27:26 +0000
From:   Kurt Manucredo <fuzzybritches0@...il.com>
To:     syzbot+bed360704c521841c85d@...kaller.appspotmail.com
Cc:     andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
        daniel@...earbox.net, davem@...emloft.net, hawk@...nel.org,
        john.fastabend@...il.com, kafai@...com, kpsingh@...nel.org,
        kuba@...nel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, songliubraving@...com,
        syzkaller-bugs@...glegroups.com, yhs@...com, nathan@...nel.org,
        ndesaulniers@...gle.com, clang-built-linux@...glegroups.com,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        skhan@...uxfoundation.org, gregkh@...uxfoundation.org,
        Kurt Manucredo <fuzzybritches0@...il.com>
Subject: [PATCH v3] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run

UBSAN: shift-out-of-bounds in kernel/bpf/core.c:1414:2
shift exponent 248 is too large for 32-bit type 'unsigned int'

Reported-and-tested-by: syzbot+bed360704c521841c85d@...kaller.appspotmail.com
Signed-off-by: Kurt Manucredo <fuzzybritches0@...il.com>
---

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

Changelog:
----------
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().

Hi everyone,

I hope this fixes it!

kind regards

 kernel/bpf/verifier.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 94ba5163d4c5..04e3bf344ecd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7880,13 +7880,25 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 			return -EINVAL;
 		}
 
-		if ((opcode == BPF_LSH || opcode == BPF_RSH ||
-		     opcode == BPF_ARSH) && BPF_SRC(insn->code) == BPF_K) {
+		if (opcode == BPF_LSH || opcode == BPF_RSH ||
+		     opcode == BPF_ARSH) {
 			int size = BPF_CLASS(insn->code) == BPF_ALU64 ? 64 : 32;
 
-			if (insn->imm < 0 || insn->imm >= size) {
-				verbose(env, "invalid shift %d\n", insn->imm);
-				return -EINVAL;
+			if (BPF_SRC(insn->code) == BPF_K) {
+				if (insn->imm < 0 || insn->imm >= size) {
+					verbose(env, "invalid shift %d\n", insn->imm);
+					return -EINVAL;
+				}
+			}
+			if (BPF_SRC(insn->code) == BPF_X) {
+				struct bpf_reg_state *src_reg;
+
+				src_reg = &regs[insn->src_reg];
+				if (src_reg->umax_value >= size) {
+					verbose(env, "invalid shift %lld\n",
+							src_reg->umax_value);
+					return -EINVAL;
+				}
 			}
 		}
 
-- 
2.30.2