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, 134 insertions, 0 deletions

diff --git a/scripts/tracing/ftrace-bisect.sh b/scripts/tracing/ftrace-bisect.sh
new file mode 100755
index 000000000..bb4f59262
--- /dev/null
+++ b/scripts/tracing/ftrace-bisect.sh
@@ -0,0 +1,134 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Here's how to use this:
+#
+# This script is used to help find functions that are being traced by function
+# tracer or function graph tracing that causes the machine to reboot, hang, or
+# crash. Here's the steps to take.
+#
+# First, determine if function tracing is working with a single function:
+#
+#   (note, if this is a problem with function_graph tracing, then simply
+#    replace "function" with "function_graph" in the following steps).
+#
+#  # cd /sys/kernel/tracing
+#  # echo schedule > set_ftrace_filter
+#  # echo function > current_tracer
+#
+# If this works, then we know that something is being traced that shouldn't be.
+#
+#  # echo nop > current_tracer
+#
+# Starting with v5.1 this can be done with numbers, making it much faster:
+#
+# The old (slow) way, for kernels before v5.1.
+#
+# [old-way] # cat available_filter_functions > ~/full-file
+#
+# [old-way] *** Note ***  this process will take several minutes to update the
+# [old-way] filters. Setting multiple functions is an O(n^2) operation, and we
+# [old-way] are dealing with thousands of functions. So go have coffee, talk
+# [old-way] with your coworkers, read facebook. And eventually, this operation
+# [old-way] will end.
+#
+# The new way (using numbers) is an O(n) operation, and usually takes less than a second.
+#
+# seq `wc -l available_filter_functions | cut -d' ' -f1` > ~/full-file
+#
+# This will create a sequence of numbers that match the functions in
+# available_filter_functions, and when echoing in a number into the
+# set_ftrace_filter file, it will enable the corresponding function in
+# O(1) time. Making enabling all functions O(n) where n is the number of
+# functions to enable.
+#
+# For either the new or old way, the rest of the operations remain the same.
+#
+#  # ftrace-bisect ~/full-file ~/test-file ~/non-test-file
+#  # cat ~/test-file > set_ftrace_filter
+#
+#  # echo function > current_tracer
+#
+# If it crashes, we know that ~/test-file has a bad function.
+#
+#   Reboot back to test kernel.
+#
+#     # cd /sys/kernel/tracing
+#     # mv ~/test-file ~/full-file
+#
+# If it didn't crash.
+#
+#     # echo nop > current_tracer
+#     # mv ~/non-test-file ~/full-file
+#
+# Get rid of the other test file from previous run (or save them off somewhere).
+#  # rm -f ~/test-file ~/non-test-file
+#
+# And start again:
+#
+#  # ftrace-bisect ~/full-file ~/test-file ~/non-test-file
+#
+# The good thing is, because this cuts the number of functions in ~/test-file
+# by half, the cat of it into set_ftrace_filter takes half as long each
+# iteration, so don't talk so much at the water cooler the second time.
+#
+# Eventually, if you did this correctly, you will get down to the problem
+# function, and all we need to do is to notrace it.
+#
+# The way to figure out if the problem function is bad, just do:
+#
+#  # echo <problem-function> > set_ftrace_notrace
+#  # echo > set_ftrace_filter
+#  # echo function > current_tracer
+#
+# And if it doesn't crash, we are done.
+#
+# If it does crash, do this again (there's more than one problem function)
+# but you need to echo the problem function(s) into set_ftrace_notrace before
+# enabling function tracing in the above steps. Or if you can compile the
+# kernel, annotate the problem functions with "notrace" and start again.
+#
+
+
+if [ $# -ne 3 ]; then
+  echo 'usage: ftrace-bisect full-file test-file  non-test-file'
+  exit
+fi
+
+full=$1
+test=$2
+nontest=$3
+
+x=`cat $full | wc -l`
+if [ $x -eq 1 ]; then
+	echo "There's only one function left, must be the bad one"
+	cat $full
+	exit 0
+fi
+
+let x=$x/2
+let y=$x+1
+
+if [ ! -f $full ]; then
+	echo "$full does not exist"
+	exit 1
+fi
+
+if [ -f $test ]; then
+	echo -n "$test exists, delete it? [y/N]"
+	read a
+	if [ "$a" != "y" -a "$a" != "Y" ]; then
+		exit 1
+	fi
+fi
+
+if [ -f $nontest ]; then
+	echo -n "$nontest exists, delete it? [y/N]"
+	read a
+	if [ "$a" != "y" -a "$a" != "Y" ]; then
+		exit 1
+	fi
+fi
+
+sed -ne "1,${x}p" $full > $test
+sed -ne "$y,\$p" $full > $nontest