commit 4f9087f16651 ("x86/cfi,bpf: Fix BPF JIT call") updated
emit_cfi() to emit FineIBT preambles for JIT-compiled BPF programs, but
did not update emit_bpf_dispatcher(). When prog->bpf_func is not in the
dispatcher table (e.g. an XDP_REDIRECT target program in a CPUMAP or
DEVMAP, or BPF_PROG_TEST_RUN of an unattached prog), the dispatcher
still jumps directly to prog->bpf_func. Without a FineIBT caller
sequence, under FineIBT this raises #CP:
Missing ENDBR: __cfi_bpf_prog_..._xdp_dispatcher_unattached+0x10/0x10
------------[ cut here ]------------
kernel BUG at arch/x86/kernel/cet.c:133!
Oops: invalid opcode: 0000 [#1] SMP NOPTI
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
RIP: 0010:do_kernel_cp_fault+0x126/0x130
Call Trace:
exc_control_protection+0x46/0x80
asm_exc_control_protection+0x2b/0x30
RIP: 0010:__cfi_bpf_prog_..._xdp_dispatcher_unattached+0x10/0x10
bpf_prog_test_run+0xda/0x1b0
__sys_bpf+0x70b/0x990
__x64_sys_bpf+0x2d/0x50
x64_sys_call+0x1079/0x2d40
do_syscall_64+0x12a/0x3c0
entry_SYSCALL_64_after_hwframe+0x76/0x7e
To fix this, we precede the indirect jump in emit_bpf_dispatcher() with
the FineIBT caller sequence: load cfi_bpf_hash into %eax and subtract
cfi_get_offset() from the target so it enters the preamble's real ENDBR.
Only needed for CFI_FINEIBT.
Fixes: 4f9087f16651 ("x86/cfi,bpf: Fix BPF JIT call")
Cc: stable@vger.kernel.org
Signed-off-by: David Windsor
---
arch/x86/net/bpf_jit_comp.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index de7515ea1bea..1c8249d8ca3b 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -3740,6 +3740,15 @@ static int emit_bpf_dispatcher(u8 **pprog, int a, int b, s64 *progs, u8 *image,
if (err)
return err;
+ /* If running under FineIBT, enter the preamble so the following
+ * indirect jump lands on a real ENDBR instead of the poison.
+ */
+ if (cfi_mode == CFI_FINEIBT) {
+ EMIT1_off32(0xb8, cfi_bpf_hash); /* mov $cfi_bpf_hash, %eax */
+ EMIT1(add_1mod(0x48, BPF_REG_3)); /* sub rdx, cfi_get_offset() */
+ EMIT2_off32(0x81, add_1reg(0xE8, BPF_REG_3), cfi_get_offset());
+ }
+
emit_indirect_jump(&prog, BPF_REG_3 /* R3 -> rdx */, image + (prog - buf));
*pprog = prog;
--
2.53.0
Add a regression test for the x86 BPF dispatcher's indirect-jump fallback
under FineIBT.
This test can crash the kernel on an unfixed FineIBT build and so should
be run in a VM.
Signed-off-by: David Windsor
---
.../bpf/prog_tests/xdp_dispatcher_fineibt.c | 47 +++++++++++++++++++
.../bpf/progs/xdp_dispatcher_fineibt.c | 18 +++++++
2 files changed, 65 insertions(+)
create mode 100644 tools/testing/selftests/bpf/prog_tests/xdp_dispatcher_fineibt.c
create mode 100644 tools/testing/selftests/bpf/progs/xdp_dispatcher_fineibt.c
diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_dispatcher_fineibt.c b/tools/testing/selftests/bpf/prog_tests/xdp_dispatcher_fineibt.c
new file mode 100644
index 000000000000..2d3fc4034eb9
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/xdp_dispatcher_fineibt.c
@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 David Windsor */
+
+/*
+ * Regression test for the x86 BPF dispatcher's indirect-jump fallback
+ * under FineIBT.
+ *
+ * WARNING
+ * -------
+ * This test can crash the kernel, thus should be run in a VM.
+ */
+#include
+#include
+#include
+#include "xdp_dispatcher_fineibt.skel.h"
+
+#define IFINDEX_LO 1
+
+void test_xdp_dispatcher_fineibt(void)
+{
+ struct xdp_dispatcher_fineibt *skel;
+ int err, attached_fd, unattached_fd;
+
+ LIBBPF_OPTS(bpf_test_run_opts, topts,
+ .data_in = &pkt_v4,
+ .data_size_in = sizeof(pkt_v4),
+ );
+
+ skel = xdp_dispatcher_fineibt__open_and_load();
+ if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+ return;
+
+ attached_fd = bpf_program__fd(skel->progs.xdp_dispatcher_attached);
+ unattached_fd = bpf_program__fd(skel->progs.xdp_dispatcher_unattached);
+
+ err = bpf_xdp_attach(IFINDEX_LO, attached_fd, XDP_FLAGS_SKB_MODE, NULL);
+ if (!ASSERT_OK(err, "attach_prog"))
+ goto out;
+
+ err = bpf_prog_test_run_opts(unattached_fd, &topts);
+ ASSERT_OK(err, "unattached_test_run");
+ ASSERT_EQ(topts.retval, XDP_PASS, "unattached_retval");
+
+ bpf_xdp_detach(IFINDEX_LO, XDP_FLAGS_SKB_MODE, NULL);
+out:
+ xdp_dispatcher_fineibt__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/xdp_dispatcher_fineibt.c b/tools/testing/selftests/bpf/progs/xdp_dispatcher_fineibt.c
new file mode 100644
index 000000000000..b49b84f01c8e
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/xdp_dispatcher_fineibt.c
@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 David Windsor */
+#include
+#include
+
+SEC("xdp")
+int xdp_dispatcher_attached(struct xdp_md *ctx)
+{
+ return XDP_PASS;
+}
+
+SEC("xdp")
+int xdp_dispatcher_unattached(struct xdp_md *ctx)
+{
+ return XDP_PASS;
+}
+
+char _license[] SEC("license") = "GPL";
--
2.53.0