[PATCH bpf] bpf: fix constant blinding bypass for PROBE_MEM32 stores

[PATCH bpf] bpf: fix constant blinding bypass for PROBE_MEM32 stores

[CyFun]

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BPF_ST | BPF_PROBE_MEM32 immediate stores are not handled by
bpf_jit_blind_insn(), allowing user-controlled 32-bit immediates to
survive unblinded into JIT-compiled native code when bpf_jit_harden >= 1.

The root cause is that convert_ctx_accesses() rewrites BPF_ST|BPF_MEM
to BPF_ST|BPF_PROBE_MEM32 for arena pointer stores during verification,
before bpf_jit_blind_constants() runs during JIT compilation. The
blinding switch only matches BPF_ST|BPF_MEM (mode 0x60), not
BPF_ST|BPF_PROBE_MEM32 (mode 0xa0). The instruction falls through
unblinded.

Add BPF_ST|BPF_PROBE_MEM32 cases to bpf_jit_blind_insn() alongside the
existing BPF_ST|BPF_MEM cases. The blinding transformation is identical:
load the blinded immediate into BPF_REG_AX via mov+xor, then convert
the immediate store to a register store (BPF_STX).

The rewritten STX instruction must preserve the BPF_PROBE_MEM32 mode so
the architecture JIT emits the correct arena addressing (R12-based on
x86-64). Cannot use the BPF_STX_MEM() macro here because it hardcodes
BPF_MEM mode; construct the instruction directly instead.

Fixes: 6082b6c328b5 ("bpf: Recognize addr_space_cast instruction in the verifier.")
Signed-off-by: s4ch <xcyfun@protonmail.com>
---
 kernel/bpf/core.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 3ece2da55625..000000000000 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2114,6 +2114,22 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
 		*to++ = BPF_STX_MEM(from->code, from->dst_reg,
 				    BPF_REG_AX, from->off);
 		break;
+
+	case BPF_ST | BPF_PROBE_MEM32 | BPF_DW:
+	case BPF_ST | BPF_PROBE_MEM32 | BPF_W:
+	case BPF_ST | BPF_PROBE_MEM32 | BPF_H:
+	case BPF_ST | BPF_PROBE_MEM32 | BPF_B:
+		*to++ = BPF_ALU64_IMM(BPF_MOV, BPF_REG_AX, imm_rnd ^
+				      from->imm);
+		*to++ = BPF_ALU64_IMM(BPF_XOR, BPF_REG_AX, imm_rnd);
+		/* Cannot use BPF_STX_MEM() here: it hardcodes BPF_MEM
+		 * mode which would lose BPF_PROBE_MEM32 and break the
+		 * arena addressing in the architecture JIT. */
+		*to++ = (struct bpf_insn) {
+			.code  = BPF_STX | BPF_PROBE_MEM32 | BPF_SIZE(from->code),
+			.dst_reg = from->dst_reg,
+			.src_reg = BPF_REG_AX,
+			.off   = from->off,
+		};
+		break;
 	}

 	return to - to_buff;
--
2.53.0

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