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, 180 insertions, 0 deletions

diff --git a/Documentation/trace/tracepoints.rst b/Documentation/trace/tracepoints.rst
new file mode 100644
index 000000000..b35c40e3a
--- /dev/null
+++ b/Documentation/trace/tracepoints.rst
@@ -0,0 +1,180 @@
+==================================
+Using the Linux Kernel Tracepoints
+==================================
+
+:Author: Mathieu Desnoyers
+
+
+This document introduces Linux Kernel Tracepoints and their use. It
+provides examples of how to insert tracepoints in the kernel and
+connect probe functions to them and provides some examples of probe
+functions.
+
+
+Purpose of tracepoints
+----------------------
+A tracepoint placed in code provides a hook to call a function (probe)
+that you can provide at runtime. A tracepoint can be "on" (a probe is
+connected to it) or "off" (no probe is attached). When a tracepoint is
+"off" it has no effect, except for adding a tiny time penalty
+(checking a condition for a branch) and space penalty (adding a few
+bytes for the function call at the end of the instrumented function
+and adds a data structure in a separate section).  When a tracepoint
+is "on", the function you provide is called each time the tracepoint
+is executed, in the execution context of the caller. When the function
+provided ends its execution, it returns to the caller (continuing from
+the tracepoint site).
+
+You can put tracepoints at important locations in the code. They are
+lightweight hooks that can pass an arbitrary number of parameters,
+whose prototypes are described in a tracepoint declaration placed in a
+header file.
+
+They can be used for tracing and performance accounting.
+
+
+Usage
+-----
+Two elements are required for tracepoints :
+
+- A tracepoint definition, placed in a header file.
+- The tracepoint statement, in C code.
+
+In order to use tracepoints, you should include linux/tracepoint.h.
+
+In include/trace/events/subsys.h::
+
+	#undef TRACE_SYSTEM
+	#define TRACE_SYSTEM subsys
+
+	#if !defined(_TRACE_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
+	#define _TRACE_SUBSYS_H
+
+	#include <linux/tracepoint.h>
+
+	DECLARE_TRACE(subsys_eventname,
+		TP_PROTO(int firstarg, struct task_struct *p),
+		TP_ARGS(firstarg, p));
+
+	#endif /* _TRACE_SUBSYS_H */
+
+	/* This part must be outside protection */
+	#include <trace/define_trace.h>
+
+In subsys/file.c (where the tracing statement must be added)::
+
+	#include <trace/events/subsys.h>
+
+	#define CREATE_TRACE_POINTS
+	DEFINE_TRACE(subsys_eventname);
+
+	void somefct(void)
+	{
+		...
+		trace_subsys_eventname_tp(arg, task);
+		...
+	}
+
+Where :
+  - subsys_eventname is an identifier unique to your event
+
+    - subsys is the name of your subsystem.
+    - eventname is the name of the event to trace.
+
+  - `TP_PROTO(int firstarg, struct task_struct *p)` is the prototype of the
+    function called by this tracepoint.
+
+  - `TP_ARGS(firstarg, p)` are the parameters names, same as found in the
+    prototype.
+
+  - if you use the header in multiple source files, `#define CREATE_TRACE_POINTS`
+    should appear only in one source file.
+
+Connecting a function (probe) to a tracepoint is done by providing a
+probe (function to call) for the specific tracepoint through
+register_trace_subsys_eventname().  Removing a probe is done through
+unregister_trace_subsys_eventname(); it will remove the probe.
+
+tracepoint_synchronize_unregister() must be called before the end of
+the module exit function to make sure there is no caller left using
+the probe. This, and the fact that preemption is disabled around the
+probe call, make sure that probe removal and module unload are safe.
+
+The tracepoint mechanism supports inserting multiple instances of the
+same tracepoint, but a single definition must be made of a given
+tracepoint name over all the kernel to make sure no type conflict will
+occur. Name mangling of the tracepoints is done using the prototypes
+to make sure typing is correct. Verification of probe type correctness
+is done at the registration site by the compiler. Tracepoints can be
+put in inline functions, inlined static functions, and unrolled loops
+as well as regular functions.
+
+The naming scheme "subsys_event" is suggested here as a convention
+intended to limit collisions. Tracepoint names are global to the
+kernel: they are considered as being the same whether they are in the
+core kernel image or in modules.
+
+If the tracepoint has to be used in kernel modules, an
+EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be
+used to export the defined tracepoints.
+
+If you need to do a bit of work for a tracepoint parameter, and
+that work is only used for the tracepoint, that work can be encapsulated
+within an if statement with the following::
+
+	if (trace_foo_bar_enabled()) {
+		int i;
+		int tot = 0;
+
+		for (i = 0; i < count; i++)
+			tot += calculate_nuggets();
+
+		trace_foo_bar_tp(tot);
+	}
+
+All trace_<tracepoint>_tp() calls have a matching trace_<tracepoint>_enabled()
+function defined that returns true if the tracepoint is enabled and
+false otherwise. The trace_<tracepoint>_tp() should always be within the
+block of the if (trace_<tracepoint>_enabled()) to prevent races between
+the tracepoint being enabled and the check being seen.
+
+The advantage of using the trace_<tracepoint>_enabled() is that it uses
+the static_key of the tracepoint to allow the if statement to be implemented
+with jump labels and avoid conditional branches.
+
+.. note:: The convenience macro TRACE_EVENT provides an alternative way to
+      define tracepoints. Note, DECLARE_TRACE(foo) creates a function
+      "trace_foo_tp()" whereas TRACE_EVENT(foo) creates a function
+      "trace_foo()", and also exposes the tracepoint as a trace event in
+      /sys/kernel/tracing/events directory.  Check http://lwn.net/Articles/379903,
+      http://lwn.net/Articles/381064 and http://lwn.net/Articles/383362
+      for a series of articles with more details.
+
+If you require calling a tracepoint from a header file, it is not
+recommended to call one directly or to use the trace_<tracepoint>_enabled()
+function call, as tracepoints in header files can have side effects if a
+header is included from a file that has CREATE_TRACE_POINTS set, as
+well as the trace_<tracepoint>() is not that small of an inline
+and can bloat the kernel if used by other inlined functions. Instead,
+include tracepoint-defs.h and use tracepoint_enabled().
+
+In a C file::
+
+	void do_trace_foo_bar_wrapper(args)
+	{
+		trace_foo_bar_tp(args); // for tracepoints created via DECLARE_TRACE
+					//   or
+		trace_foo_bar(args);    // for tracepoints created via TRACE_EVENT
+	}
+
+In the header file::
+
+	DECLARE_TRACEPOINT(foo_bar);
+
+	static inline void some_inline_function()
+	{
+		[..]
+		if (tracepoint_enabled(foo_bar))
+			do_trace_foo_bar_wrapper(args);
+		[..]
+	}