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, 59 insertions, 0 deletions

diff --git a/Documentation/networking/bareudp.rst b/Documentation/networking/bareudp.rst
new file mode 100644
index 000000000..621cb9575
--- /dev/null
+++ b/Documentation/networking/bareudp.rst
@@ -0,0 +1,59 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+========================================
+Bare UDP Tunnelling Module Documentation
+========================================
+
+There are various L3 encapsulation standards using UDP being discussed to
+leverage the UDP based load balancing capability of different networks.
+MPLSoUDP (https://tools.ietf.org/html/rfc7510) is one among them.
+
+The Bareudp tunnel module provides a generic L3 encapsulation support for
+tunnelling different L3 protocols like MPLS, IP, NSH etc. inside a UDP tunnel.
+
+Special Handling
+----------------
+
+The bareudp device supports special handling for MPLS & IP as they can have
+multiple ethertypes.
+The MPLS protocol can have ethertypes ETH_P_MPLS_UC (unicast) & ETH_P_MPLS_MC (multicast).
+IP protocol can have ethertypes ETH_P_IP (v4) & ETH_P_IPV6 (v6).
+This special handling can be enabled only for ethertypes ETH_P_IP & ETH_P_MPLS_UC
+with a flag called multiproto mode.
+
+Usage
+------
+
+1) Device creation & deletion
+
+    a) ip link add dev bareudp0 type bareudp dstport 6635 ethertype mpls_uc
+
+       This creates a bareudp tunnel device which tunnels L3 traffic with ethertype
+       0x8847 (MPLS traffic). The destination port of the UDP header will be set to
+       6635.The device will listen on UDP port 6635 to receive traffic.
+
+    b) ip link delete bareudp0
+
+2) Device creation with multiproto mode enabled
+
+The multiproto mode allows bareudp tunnels to handle several protocols of the
+same family. It is currently only available for IP and MPLS. This mode has to
+be enabled explicitly with the "multiproto" flag.
+
+    a) ip link add dev bareudp0 type bareudp dstport 6635 ethertype ipv4 multiproto
+
+       For an IPv4 tunnel the multiproto mode allows the tunnel to also handle
+       IPv6.
+
+    b) ip link add dev bareudp0 type bareudp dstport 6635 ethertype mpls_uc multiproto
+
+       For MPLS, the multiproto mode allows the tunnel to handle both unicast
+       and multicast MPLS packets.
+
+3) Device Usage
+
+The bareudp device could be used along with OVS or flower filter in TC.
+The OVS or TC flower layer must set the tunnel information in the SKB dst field before
+sending the packet buffer to the bareudp device for transmission. On reception, the
+bareUDP device extracts and stores the tunnel information in the SKB dst field before
+passing the packet buffer to the network stack.