x86/kprobes: Fix crash when probing CS CALL instructionsgrafted

When using eBPF to probe CS CALL instructions within a function,
a crash can be triggered.

The eBPF tool probes offset 257 of the __hrtimer_run_queues()
function:

<__hrtimer_run_queues+249>:  nopl   0x0(%rax,%rax,1)
<__hrtimer_run_queues+254>:  mov    %r14,%rdi
<__hrtimer_run_queues+257>:  cs call <__x86_indirect_thunk_r12>
<__hrtimer_run_queues+263>:  mov    %eax,%r12d
<__hrtimer_run_queues+266>:  xchg   %ax,%ax
<__hrtimer_run_queues+268>:  mov    %r13,%rdi

Which triggers this crash:

  BUG: unable to handle page fault for address: 00000000000f41c9
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP NOPTI
  CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P
  RIP: 0010:__hrtimer_run_queues+0x106/0x230

Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is
at the 6th byte of the above CS CALL instruction. Since the CS CALL
instruction occupies 6 bytes, the exception occurred in the middle of that
call instruction.

The root cause is that when using eBPF tools to probe in the middle of a
function, a kprobe with INT3 is used as the underlying implementation.

During single-step emulation of the original CALL instruction,
int3_emulate_call() assumes that the probed CALL instruction is 5 bytes
long. However, the actual CS-prefixed CALL instruction occupies 6 bytes,
so it constructs an incorrect exception return address. When the CPU
returns from the kprobe handler, the next instruction to be executed is at
the address of the last byte of that CS CALL instruction. Coincidentally,
starting from that address, the CPU fetches and decodes a completely
different instruction, which ultimately triggers a kernel crash.

Fix the issue by using the actual instruction length obtained from
the instruction decoder when constructing the exception return
address, rather than relying on the hardcoded CALL_INSN_SIZE macro.

[ mingo: Refined the changelog ]

Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Jinke Han <jinkehan@didiglobal.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000

1 files changed, 151 insertions, 0 deletions

diff --git a/net/mptcp/token_test.c b/net/mptcp/token_test.c
new file mode 100644
index 000000000..4fc39fa2e
--- /dev/null
+++ b/net/mptcp/token_test.c
@@ -0,0 +1,151 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <kunit/test.h>
+
+#include "protocol.h"
+
+static struct mptcp_subflow_request_sock *build_req_sock(struct kunit *test)
+{
+	struct mptcp_subflow_request_sock *req;
+
+	req = kunit_kzalloc(test, sizeof(struct mptcp_subflow_request_sock),
+			    GFP_USER);
+	KUNIT_EXPECT_NOT_ERR_OR_NULL(test, req);
+	mptcp_token_init_request((struct request_sock *)req);
+	sock_net_set((struct sock *)req, &init_net);
+	return req;
+}
+
+static void mptcp_token_test_req_basic(struct kunit *test)
+{
+	struct mptcp_subflow_request_sock *req = build_req_sock(test);
+	struct mptcp_sock *null_msk = NULL;
+
+	KUNIT_ASSERT_EQ(test, 0,
+			mptcp_token_new_request((struct request_sock *)req));
+	KUNIT_EXPECT_NE(test, 0, (int)req->token);
+	KUNIT_EXPECT_PTR_EQ(test, null_msk, mptcp_token_get_sock(&init_net, req->token));
+
+	/* cleanup */
+	mptcp_token_destroy_request((struct request_sock *)req);
+}
+
+static struct inet_connection_sock *build_icsk(struct kunit *test)
+{
+	struct inet_connection_sock *icsk;
+
+	icsk = kunit_kzalloc(test, sizeof(struct inet_connection_sock),
+			     GFP_USER);
+	KUNIT_EXPECT_NOT_ERR_OR_NULL(test, icsk);
+	return icsk;
+}
+
+static struct mptcp_subflow_context *build_ctx(struct kunit *test)
+{
+	struct mptcp_subflow_context *ctx;
+
+	ctx = kunit_kzalloc(test, sizeof(struct mptcp_subflow_context),
+			    GFP_USER);
+	KUNIT_EXPECT_NOT_ERR_OR_NULL(test, ctx);
+	return ctx;
+}
+
+static struct mptcp_sock *build_msk(struct kunit *test)
+{
+	struct mptcp_sock *msk;
+	struct sock *sk;
+
+	msk = kunit_kzalloc(test, sizeof(struct mptcp_sock), GFP_USER);
+	KUNIT_EXPECT_NOT_ERR_OR_NULL(test, msk);
+	refcount_set(&((struct sock *)msk)->sk_refcnt, 1);
+	sock_net_set((struct sock *)msk, &init_net);
+
+	sk = (struct sock *)msk;
+
+	/* be sure the token helpers can dereference sk->sk_prot */
+	sk->sk_prot = &tcp_prot;
+	sk->sk_protocol = IPPROTO_MPTCP;
+
+	return msk;
+}
+
+static void mptcp_token_test_msk_basic(struct kunit *test)
+{
+	struct inet_connection_sock *icsk = build_icsk(test);
+	struct mptcp_subflow_context *ctx = build_ctx(test);
+	struct mptcp_sock *msk = build_msk(test);
+	struct mptcp_sock *null_msk = NULL;
+	struct sock *sk;
+
+	rcu_assign_pointer(icsk->icsk_ulp_data, ctx);
+	ctx->conn = (struct sock *)msk;
+	sk = (struct sock *)msk;
+
+	KUNIT_ASSERT_EQ(test, 0,
+			mptcp_token_new_connect((struct sock *)icsk));
+	KUNIT_EXPECT_NE(test, 0, (int)ctx->token);
+	KUNIT_EXPECT_EQ(test, ctx->token, msk->token);
+	KUNIT_EXPECT_PTR_EQ(test, msk, mptcp_token_get_sock(&init_net, ctx->token));
+	KUNIT_EXPECT_EQ(test, 2, (int)refcount_read(&sk->sk_refcnt));
+
+	mptcp_token_destroy(msk);
+	KUNIT_EXPECT_PTR_EQ(test, null_msk, mptcp_token_get_sock(&init_net, ctx->token));
+}
+
+static void mptcp_token_test_accept(struct kunit *test)
+{
+	struct mptcp_subflow_request_sock *req = build_req_sock(test);
+	struct mptcp_sock *msk = build_msk(test);
+
+	KUNIT_ASSERT_EQ(test, 0,
+			mptcp_token_new_request((struct request_sock *)req));
+	msk->token = req->token;
+	mptcp_token_accept(req, msk);
+	KUNIT_EXPECT_PTR_EQ(test, msk, mptcp_token_get_sock(&init_net, msk->token));
+
+	/* this is now a no-op */
+	mptcp_token_destroy_request((struct request_sock *)req);
+	KUNIT_EXPECT_PTR_EQ(test, msk, mptcp_token_get_sock(&init_net, msk->token));
+
+	/* cleanup */
+	mptcp_token_destroy(msk);
+}
+
+static void mptcp_token_test_destroyed(struct kunit *test)
+{
+	struct mptcp_subflow_request_sock *req = build_req_sock(test);
+	struct mptcp_sock *msk = build_msk(test);
+	struct mptcp_sock *null_msk = NULL;
+	struct sock *sk;
+
+	sk = (struct sock *)msk;
+
+	KUNIT_ASSERT_EQ(test, 0,
+			mptcp_token_new_request((struct request_sock *)req));
+	msk->token = req->token;
+	mptcp_token_accept(req, msk);
+
+	/* simulate race on removal */
+	refcount_set(&sk->sk_refcnt, 0);
+	KUNIT_EXPECT_PTR_EQ(test, null_msk, mptcp_token_get_sock(&init_net, msk->token));
+
+	/* cleanup */
+	mptcp_token_destroy(msk);
+}
+
+static struct kunit_case mptcp_token_test_cases[] = {
+	KUNIT_CASE(mptcp_token_test_req_basic),
+	KUNIT_CASE(mptcp_token_test_msk_basic),
+	KUNIT_CASE(mptcp_token_test_accept),
+	KUNIT_CASE(mptcp_token_test_destroyed),
+	{}
+};
+
+static struct kunit_suite mptcp_token_suite = {
+	.name = "mptcp-token",
+	.test_cases = mptcp_token_test_cases,
+};
+
+kunit_test_suite(mptcp_token_suite);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("KUnit tests for MPTCP Token");