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, 556 insertions, 0 deletions

diff --git a/net/lapb/lapb_in.c b/net/lapb/lapb_in.c
new file mode 100644
index 000000000..38ae23c09
--- /dev/null
+++ b/net/lapb/lapb_in.c
@@ -0,0 +1,556 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *	LAPB release 002
+ *
+ *	This code REQUIRES 2.1.15 or higher/ NET3.038
+ *
+ *	History
+ *	LAPB 001	Jonathan Naulor	Started Coding
+ *	LAPB 002	Jonathan Naylor	New timer architecture.
+ *	2000-10-29	Henner Eisen	lapb_data_indication() return status.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/errno.h>
+#include <linux/types.h>
+#include <linux/socket.h>
+#include <linux/in.h>
+#include <linux/kernel.h>
+#include <linux/timer.h>
+#include <linux/string.h>
+#include <linux/sockios.h>
+#include <linux/net.h>
+#include <linux/inet.h>
+#include <linux/netdevice.h>
+#include <linux/skbuff.h>
+#include <linux/slab.h>
+#include <net/sock.h>
+#include <linux/uaccess.h>
+#include <linux/fcntl.h>
+#include <linux/mm.h>
+#include <linux/interrupt.h>
+#include <net/lapb.h>
+
+/*
+ *	State machine for state 0, Disconnected State.
+ *	The handling of the timer(s) is in file lapb_timer.c.
+ */
+static void lapb_state0_machine(struct lapb_cb *lapb, struct sk_buff *skb,
+				struct lapb_frame *frame)
+{
+	switch (frame->type) {
+	case LAPB_SABM:
+		lapb_dbg(1, "(%p) S0 RX SABM(%d)\n", lapb->dev, frame->pf);
+		if (lapb->mode & LAPB_EXTENDED) {
+			lapb_dbg(1, "(%p) S0 TX DM(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_DM, frame->pf,
+					  LAPB_RESPONSE);
+		} else {
+			lapb_dbg(1, "(%p) S0 TX UA(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_dbg(0, "(%p) S0 -> S3\n", lapb->dev);
+			lapb_send_control(lapb, LAPB_UA, frame->pf,
+					  LAPB_RESPONSE);
+			lapb_stop_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb->state     = LAPB_STATE_3;
+			lapb->condition = 0x00;
+			lapb->n2count   = 0;
+			lapb->vs        = 0;
+			lapb->vr        = 0;
+			lapb->va        = 0;
+			lapb_connect_indication(lapb, LAPB_OK);
+		}
+		break;
+
+	case LAPB_SABME:
+		lapb_dbg(1, "(%p) S0 RX SABME(%d)\n", lapb->dev, frame->pf);
+		if (lapb->mode & LAPB_EXTENDED) {
+			lapb_dbg(1, "(%p) S0 TX UA(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_dbg(0, "(%p) S0 -> S3\n", lapb->dev);
+			lapb_send_control(lapb, LAPB_UA, frame->pf,
+					  LAPB_RESPONSE);
+			lapb_stop_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb->state     = LAPB_STATE_3;
+			lapb->condition = 0x00;
+			lapb->n2count   = 0;
+			lapb->vs        = 0;
+			lapb->vr        = 0;
+			lapb->va        = 0;
+			lapb_connect_indication(lapb, LAPB_OK);
+		} else {
+			lapb_dbg(1, "(%p) S0 TX DM(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_DM, frame->pf,
+					  LAPB_RESPONSE);
+		}
+		break;
+
+	case LAPB_DISC:
+		lapb_dbg(1, "(%p) S0 RX DISC(%d)\n", lapb->dev, frame->pf);
+		lapb_dbg(1, "(%p) S0 TX UA(%d)\n", lapb->dev, frame->pf);
+		lapb_send_control(lapb, LAPB_UA, frame->pf, LAPB_RESPONSE);
+		break;
+
+	default:
+		break;
+	}
+
+	kfree_skb(skb);
+}
+
+/*
+ *	State machine for state 1, Awaiting Connection State.
+ *	The handling of the timer(s) is in file lapb_timer.c.
+ */
+static void lapb_state1_machine(struct lapb_cb *lapb, struct sk_buff *skb,
+				struct lapb_frame *frame)
+{
+	switch (frame->type) {
+	case LAPB_SABM:
+		lapb_dbg(1, "(%p) S1 RX SABM(%d)\n", lapb->dev, frame->pf);
+		if (lapb->mode & LAPB_EXTENDED) {
+			lapb_dbg(1, "(%p) S1 TX DM(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_DM, frame->pf,
+					  LAPB_RESPONSE);
+		} else {
+			lapb_dbg(1, "(%p) S1 TX UA(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_UA, frame->pf,
+					  LAPB_RESPONSE);
+		}
+		break;
+
+	case LAPB_SABME:
+		lapb_dbg(1, "(%p) S1 RX SABME(%d)\n", lapb->dev, frame->pf);
+		if (lapb->mode & LAPB_EXTENDED) {
+			lapb_dbg(1, "(%p) S1 TX UA(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_UA, frame->pf,
+					  LAPB_RESPONSE);
+		} else {
+			lapb_dbg(1, "(%p) S1 TX DM(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_DM, frame->pf,
+					  LAPB_RESPONSE);
+		}
+		break;
+
+	case LAPB_DISC:
+		lapb_dbg(1, "(%p) S1 RX DISC(%d)\n", lapb->dev, frame->pf);
+		lapb_dbg(1, "(%p) S1 TX DM(%d)\n", lapb->dev, frame->pf);
+		lapb_send_control(lapb, LAPB_DM, frame->pf, LAPB_RESPONSE);
+		break;
+
+	case LAPB_UA:
+		lapb_dbg(1, "(%p) S1 RX UA(%d)\n", lapb->dev, frame->pf);
+		if (frame->pf) {
+			lapb_dbg(0, "(%p) S1 -> S3\n", lapb->dev);
+			lapb_stop_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb->state     = LAPB_STATE_3;
+			lapb->condition = 0x00;
+			lapb->n2count   = 0;
+			lapb->vs        = 0;
+			lapb->vr        = 0;
+			lapb->va        = 0;
+			lapb_connect_confirmation(lapb, LAPB_OK);
+		}
+		break;
+
+	case LAPB_DM:
+		lapb_dbg(1, "(%p) S1 RX DM(%d)\n", lapb->dev, frame->pf);
+		if (frame->pf) {
+			lapb_dbg(0, "(%p) S1 -> S0\n", lapb->dev);
+			lapb_clear_queues(lapb);
+			lapb->state = LAPB_STATE_0;
+			lapb_start_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb_disconnect_indication(lapb, LAPB_REFUSED);
+		}
+		break;
+	}
+
+	kfree_skb(skb);
+}
+
+/*
+ *	State machine for state 2, Awaiting Release State.
+ *	The handling of the timer(s) is in file lapb_timer.c
+ */
+static void lapb_state2_machine(struct lapb_cb *lapb, struct sk_buff *skb,
+				struct lapb_frame *frame)
+{
+	switch (frame->type) {
+	case LAPB_SABM:
+	case LAPB_SABME:
+		lapb_dbg(1, "(%p) S2 RX {SABM,SABME}(%d)\n",
+			 lapb->dev, frame->pf);
+		lapb_dbg(1, "(%p) S2 TX DM(%d)\n", lapb->dev, frame->pf);
+		lapb_send_control(lapb, LAPB_DM, frame->pf, LAPB_RESPONSE);
+		break;
+
+	case LAPB_DISC:
+		lapb_dbg(1, "(%p) S2 RX DISC(%d)\n", lapb->dev, frame->pf);
+		lapb_dbg(1, "(%p) S2 TX UA(%d)\n", lapb->dev, frame->pf);
+		lapb_send_control(lapb, LAPB_UA, frame->pf, LAPB_RESPONSE);
+		break;
+
+	case LAPB_UA:
+		lapb_dbg(1, "(%p) S2 RX UA(%d)\n", lapb->dev, frame->pf);
+		if (frame->pf) {
+			lapb_dbg(0, "(%p) S2 -> S0\n", lapb->dev);
+			lapb->state = LAPB_STATE_0;
+			lapb_start_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb_disconnect_confirmation(lapb, LAPB_OK);
+		}
+		break;
+
+	case LAPB_DM:
+		lapb_dbg(1, "(%p) S2 RX DM(%d)\n", lapb->dev, frame->pf);
+		if (frame->pf) {
+			lapb_dbg(0, "(%p) S2 -> S0\n", lapb->dev);
+			lapb->state = LAPB_STATE_0;
+			lapb_start_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb_disconnect_confirmation(lapb, LAPB_NOTCONNECTED);
+		}
+		break;
+
+	case LAPB_I:
+	case LAPB_REJ:
+	case LAPB_RNR:
+	case LAPB_RR:
+		lapb_dbg(1, "(%p) S2 RX {I,REJ,RNR,RR}(%d)\n",
+		       lapb->dev, frame->pf);
+		lapb_dbg(1, "(%p) S2 RX DM(%d)\n", lapb->dev, frame->pf);
+		if (frame->pf)
+			lapb_send_control(lapb, LAPB_DM, frame->pf,
+					  LAPB_RESPONSE);
+		break;
+	}
+
+	kfree_skb(skb);
+}
+
+/*
+ *	State machine for state 3, Connected State.
+ *	The handling of the timer(s) is in file lapb_timer.c
+ */
+static void lapb_state3_machine(struct lapb_cb *lapb, struct sk_buff *skb,
+				struct lapb_frame *frame)
+{
+	int queued = 0;
+	int modulus = (lapb->mode & LAPB_EXTENDED) ? LAPB_EMODULUS :
+						     LAPB_SMODULUS;
+
+	switch (frame->type) {
+	case LAPB_SABM:
+		lapb_dbg(1, "(%p) S3 RX SABM(%d)\n", lapb->dev, frame->pf);
+		if (lapb->mode & LAPB_EXTENDED) {
+			lapb_dbg(1, "(%p) S3 TX DM(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_DM, frame->pf,
+					  LAPB_RESPONSE);
+		} else {
+			lapb_dbg(1, "(%p) S3 TX UA(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_UA, frame->pf,
+					  LAPB_RESPONSE);
+			lapb_stop_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb->condition = 0x00;
+			lapb->n2count   = 0;
+			lapb->vs        = 0;
+			lapb->vr        = 0;
+			lapb->va        = 0;
+			lapb_requeue_frames(lapb);
+		}
+		break;
+
+	case LAPB_SABME:
+		lapb_dbg(1, "(%p) S3 RX SABME(%d)\n", lapb->dev, frame->pf);
+		if (lapb->mode & LAPB_EXTENDED) {
+			lapb_dbg(1, "(%p) S3 TX UA(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_UA, frame->pf,
+					  LAPB_RESPONSE);
+			lapb_stop_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb->condition = 0x00;
+			lapb->n2count   = 0;
+			lapb->vs        = 0;
+			lapb->vr        = 0;
+			lapb->va        = 0;
+			lapb_requeue_frames(lapb);
+		} else {
+			lapb_dbg(1, "(%p) S3 TX DM(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_DM, frame->pf,
+					  LAPB_RESPONSE);
+		}
+		break;
+
+	case LAPB_DISC:
+		lapb_dbg(1, "(%p) S3 RX DISC(%d)\n", lapb->dev, frame->pf);
+		lapb_dbg(0, "(%p) S3 -> S0\n", lapb->dev);
+		lapb_clear_queues(lapb);
+		lapb_send_control(lapb, LAPB_UA, frame->pf, LAPB_RESPONSE);
+		lapb_start_t1timer(lapb);
+		lapb_stop_t2timer(lapb);
+		lapb->state = LAPB_STATE_0;
+		lapb_disconnect_indication(lapb, LAPB_OK);
+		break;
+
+	case LAPB_DM:
+		lapb_dbg(1, "(%p) S3 RX DM(%d)\n", lapb->dev, frame->pf);
+		lapb_dbg(0, "(%p) S3 -> S0\n", lapb->dev);
+		lapb_clear_queues(lapb);
+		lapb->state = LAPB_STATE_0;
+		lapb_start_t1timer(lapb);
+		lapb_stop_t2timer(lapb);
+		lapb_disconnect_indication(lapb, LAPB_NOTCONNECTED);
+		break;
+
+	case LAPB_RNR:
+		lapb_dbg(1, "(%p) S3 RX RNR(%d) R%d\n",
+			 lapb->dev, frame->pf, frame->nr);
+		lapb->condition |= LAPB_PEER_RX_BUSY_CONDITION;
+		lapb_check_need_response(lapb, frame->cr, frame->pf);
+		if (lapb_validate_nr(lapb, frame->nr)) {
+			lapb_check_iframes_acked(lapb, frame->nr);
+		} else {
+			lapb->frmr_data = *frame;
+			lapb->frmr_type = LAPB_FRMR_Z;
+			lapb_transmit_frmr(lapb);
+			lapb_dbg(0, "(%p) S3 -> S4\n", lapb->dev);
+			lapb_start_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb->state   = LAPB_STATE_4;
+			lapb->n2count = 0;
+		}
+		break;
+
+	case LAPB_RR:
+		lapb_dbg(1, "(%p) S3 RX RR(%d) R%d\n",
+			 lapb->dev, frame->pf, frame->nr);
+		lapb->condition &= ~LAPB_PEER_RX_BUSY_CONDITION;
+		lapb_check_need_response(lapb, frame->cr, frame->pf);
+		if (lapb_validate_nr(lapb, frame->nr)) {
+			lapb_check_iframes_acked(lapb, frame->nr);
+		} else {
+			lapb->frmr_data = *frame;
+			lapb->frmr_type = LAPB_FRMR_Z;
+			lapb_transmit_frmr(lapb);
+			lapb_dbg(0, "(%p) S3 -> S4\n", lapb->dev);
+			lapb_start_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb->state   = LAPB_STATE_4;
+			lapb->n2count = 0;
+		}
+		break;
+
+	case LAPB_REJ:
+		lapb_dbg(1, "(%p) S3 RX REJ(%d) R%d\n",
+			 lapb->dev, frame->pf, frame->nr);
+		lapb->condition &= ~LAPB_PEER_RX_BUSY_CONDITION;
+		lapb_check_need_response(lapb, frame->cr, frame->pf);
+		if (lapb_validate_nr(lapb, frame->nr)) {
+			lapb_frames_acked(lapb, frame->nr);
+			lapb_stop_t1timer(lapb);
+			lapb->n2count = 0;
+			lapb_requeue_frames(lapb);
+		} else {
+			lapb->frmr_data = *frame;
+			lapb->frmr_type = LAPB_FRMR_Z;
+			lapb_transmit_frmr(lapb);
+			lapb_dbg(0, "(%p) S3 -> S4\n", lapb->dev);
+			lapb_start_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb->state   = LAPB_STATE_4;
+			lapb->n2count = 0;
+		}
+		break;
+
+	case LAPB_I:
+		lapb_dbg(1, "(%p) S3 RX I(%d) S%d R%d\n",
+			 lapb->dev, frame->pf, frame->ns, frame->nr);
+		if (!lapb_validate_nr(lapb, frame->nr)) {
+			lapb->frmr_data = *frame;
+			lapb->frmr_type = LAPB_FRMR_Z;
+			lapb_transmit_frmr(lapb);
+			lapb_dbg(0, "(%p) S3 -> S4\n", lapb->dev);
+			lapb_start_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb->state   = LAPB_STATE_4;
+			lapb->n2count = 0;
+			break;
+		}
+		if (lapb->condition & LAPB_PEER_RX_BUSY_CONDITION)
+			lapb_frames_acked(lapb, frame->nr);
+		else
+			lapb_check_iframes_acked(lapb, frame->nr);
+
+		if (frame->ns == lapb->vr) {
+			int cn;
+			cn = lapb_data_indication(lapb, skb);
+			queued = 1;
+			/*
+			 * If upper layer has dropped the frame, we
+			 * basically ignore any further protocol
+			 * processing. This will cause the peer
+			 * to re-transmit the frame later like
+			 * a frame lost on the wire.
+			 */
+			if (cn == NET_RX_DROP) {
+				pr_debug("rx congestion\n");
+				break;
+			}
+			lapb->vr = (lapb->vr + 1) % modulus;
+			lapb->condition &= ~LAPB_REJECT_CONDITION;
+			if (frame->pf)
+				lapb_enquiry_response(lapb);
+			else {
+				if (!(lapb->condition &
+				      LAPB_ACK_PENDING_CONDITION)) {
+					lapb->condition |= LAPB_ACK_PENDING_CONDITION;
+					lapb_start_t2timer(lapb);
+				}
+			}
+		} else {
+			if (lapb->condition & LAPB_REJECT_CONDITION) {
+				if (frame->pf)
+					lapb_enquiry_response(lapb);
+			} else {
+				lapb_dbg(1, "(%p) S3 TX REJ(%d) R%d\n",
+					 lapb->dev, frame->pf, lapb->vr);
+				lapb->condition |= LAPB_REJECT_CONDITION;
+				lapb_send_control(lapb, LAPB_REJ, frame->pf,
+						  LAPB_RESPONSE);
+				lapb->condition &= ~LAPB_ACK_PENDING_CONDITION;
+			}
+		}
+		break;
+
+	case LAPB_FRMR:
+		lapb_dbg(1, "(%p) S3 RX FRMR(%d) %5ph\n",
+			 lapb->dev, frame->pf,
+			 skb->data);
+		lapb_establish_data_link(lapb);
+		lapb_dbg(0, "(%p) S3 -> S1\n", lapb->dev);
+		lapb_requeue_frames(lapb);
+		lapb->state = LAPB_STATE_1;
+		break;
+
+	case LAPB_ILLEGAL:
+		lapb_dbg(1, "(%p) S3 RX ILLEGAL(%d)\n", lapb->dev, frame->pf);
+		lapb->frmr_data = *frame;
+		lapb->frmr_type = LAPB_FRMR_W;
+		lapb_transmit_frmr(lapb);
+		lapb_dbg(0, "(%p) S3 -> S4\n", lapb->dev);
+		lapb_start_t1timer(lapb);
+		lapb_stop_t2timer(lapb);
+		lapb->state   = LAPB_STATE_4;
+		lapb->n2count = 0;
+		break;
+	}
+
+	if (!queued)
+		kfree_skb(skb);
+}
+
+/*
+ *	State machine for state 4, Frame Reject State.
+ *	The handling of the timer(s) is in file lapb_timer.c.
+ */
+static void lapb_state4_machine(struct lapb_cb *lapb, struct sk_buff *skb,
+				struct lapb_frame *frame)
+{
+	switch (frame->type) {
+	case LAPB_SABM:
+		lapb_dbg(1, "(%p) S4 RX SABM(%d)\n", lapb->dev, frame->pf);
+		if (lapb->mode & LAPB_EXTENDED) {
+			lapb_dbg(1, "(%p) S4 TX DM(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_DM, frame->pf,
+					  LAPB_RESPONSE);
+		} else {
+			lapb_dbg(1, "(%p) S4 TX UA(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_dbg(0, "(%p) S4 -> S3\n", lapb->dev);
+			lapb_send_control(lapb, LAPB_UA, frame->pf,
+					  LAPB_RESPONSE);
+			lapb_stop_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb->state     = LAPB_STATE_3;
+			lapb->condition = 0x00;
+			lapb->n2count   = 0;
+			lapb->vs        = 0;
+			lapb->vr        = 0;
+			lapb->va        = 0;
+			lapb_connect_indication(lapb, LAPB_OK);
+		}
+		break;
+
+	case LAPB_SABME:
+		lapb_dbg(1, "(%p) S4 RX SABME(%d)\n", lapb->dev, frame->pf);
+		if (lapb->mode & LAPB_EXTENDED) {
+			lapb_dbg(1, "(%p) S4 TX UA(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_dbg(0, "(%p) S4 -> S3\n", lapb->dev);
+			lapb_send_control(lapb, LAPB_UA, frame->pf,
+					  LAPB_RESPONSE);
+			lapb_stop_t1timer(lapb);
+			lapb_stop_t2timer(lapb);
+			lapb->state     = LAPB_STATE_3;
+			lapb->condition = 0x00;
+			lapb->n2count   = 0;
+			lapb->vs        = 0;
+			lapb->vr        = 0;
+			lapb->va        = 0;
+			lapb_connect_indication(lapb, LAPB_OK);
+		} else {
+			lapb_dbg(1, "(%p) S4 TX DM(%d)\n",
+				 lapb->dev, frame->pf);
+			lapb_send_control(lapb, LAPB_DM, frame->pf,
+					  LAPB_RESPONSE);
+		}
+		break;
+	}
+
+	kfree_skb(skb);
+}
+
+/*
+ *	Process an incoming LAPB frame
+ */
+void lapb_data_input(struct lapb_cb *lapb, struct sk_buff *skb)
+{
+	struct lapb_frame frame;
+
+	if (lapb_decode(lapb, skb, &frame) < 0) {
+		kfree_skb(skb);
+		return;
+	}
+
+	switch (lapb->state) {
+	case LAPB_STATE_0:
+		lapb_state0_machine(lapb, skb, &frame); break;
+	case LAPB_STATE_1:
+		lapb_state1_machine(lapb, skb, &frame); break;
+	case LAPB_STATE_2:
+		lapb_state2_machine(lapb, skb, &frame); break;
+	case LAPB_STATE_3:
+		lapb_state3_machine(lapb, skb, &frame); break;
+	case LAPB_STATE_4:
+		lapb_state4_machine(lapb, skb, &frame); break;
+	}
+
+	lapb_kick(lapb);
+}