[PATCH 6.19 004/342] bpf: Fix constant blinding for PROBE_MEM32 stores

Greg Kroah-Hartman
2026-03-31T16:32:01Z

From: Sachin Kumar <xcyfun@protonmail.com>

[ Upstream commit 2321a9596d2260310267622e0ad8fbfa6f95378f ]

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.")
Reviewed-by: Puranjay Mohan <puranjay@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Sachin Kumar <xcyfun@protonmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/Y6IT5VvNRchPBLI5D7JZHBzZrU9rb0ycRJPJzJSXGj7kJlX8RJwZFSM2YZjcDxoQKABkxt1T8Os2gi23PYyFuQe6KkZGWVyfz8K5afdy9ak=@protonmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/bpf/core.c | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 85c0feaae0d3c..1b32333d8f8c6 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1419,6 +1419,27 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
*to++ = BPF_ALU64_IMM(BPF_XOR, BPF_REG_AX, imm_rnd);
*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() macro here as it
+ * hardcodes BPF_MEM mode, losing PROBE_MEM32
+ * and breaking arena addressing in the 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;
}
out:
return to - to_buff;
--
2.51.0