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, 76 insertions, 0 deletions

diff --git a/Documentation/filesystems/virtiofs.rst b/Documentation/filesystems/virtiofs.rst
new file mode 100644
index 000000000..fd4d2484e
--- /dev/null
+++ b/Documentation/filesystems/virtiofs.rst
@@ -0,0 +1,76 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _virtiofs_index:
+
+===================================================
+virtiofs: virtio-fs host<->guest shared file system
+===================================================
+
+- Copyright (C) 2019 Red Hat, Inc.
+
+Introduction
+============
+The virtiofs file system for Linux implements a driver for the paravirtualized
+VIRTIO "virtio-fs" device for guest<->host file system sharing.  It allows a
+guest to mount a directory that has been exported on the host.
+
+Guests often require access to files residing on the host or remote systems.
+Use cases include making files available to new guests during installation,
+booting from a root file system located on the host, persistent storage for
+stateless or ephemeral guests, and sharing a directory between guests.
+
+Although it is possible to use existing network file systems for some of these
+tasks, they require configuration steps that are hard to automate and they
+expose the storage network to the guest.  The virtio-fs device was designed to
+solve these problems by providing file system access without networking.
+
+Furthermore the virtio-fs device takes advantage of the co-location of the
+guest and host to increase performance and provide semantics that are not
+possible with network file systems.
+
+Usage
+=====
+Mount file system with tag ``myfs`` on ``/mnt``:
+
+.. code-block:: sh
+
+  guest# mount -t virtiofs myfs /mnt
+
+Please see https://virtio-fs.gitlab.io/ for details on how to configure QEMU
+and the virtiofsd daemon.
+
+Mount options
+-------------
+
+virtiofs supports general VFS mount options, for example, remount,
+ro, rw, context, etc. It also supports FUSE mount options.
+
+atime behavior
+^^^^^^^^^^^^^^
+
+The atime-related mount options, for example, noatime, strictatime,
+are ignored. The atime behavior for virtiofs is the same as the
+underlying filesystem of the directory that has been exported
+on the host.
+
+Internals
+=========
+Since the virtio-fs device uses the FUSE protocol for file system requests, the
+virtiofs file system for Linux is integrated closely with the FUSE file system
+client.  The guest acts as the FUSE client while the host acts as the FUSE
+server.  The /dev/fuse interface between the kernel and userspace is replaced
+with the virtio-fs device interface.
+
+FUSE requests are placed into a virtqueue and processed by the host.  The
+response portion of the buffer is filled in by the host and the guest handles
+the request completion.
+
+Mapping /dev/fuse to virtqueues requires solving differences in semantics
+between /dev/fuse and virtqueues.  Each time the /dev/fuse device is read, the
+FUSE client may choose which request to transfer, making it possible to
+prioritize certain requests over others.  Virtqueues have queue semantics and
+it is not possible to change the order of requests that have been enqueued.
+This is especially important if the virtqueue becomes full since it is then
+impossible to add high priority requests.  In order to address this difference,
+the virtio-fs device uses a "hiprio" virtqueue specifically for requests that
+have priority over normal requests.