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, 75 insertions, 0 deletions

diff --git a/Documentation/admin-guide/LSM/Yama.rst b/Documentation/admin-guide/LSM/Yama.rst
new file mode 100644
index 000000000..d9cd937eb
--- /dev/null
+++ b/Documentation/admin-guide/LSM/Yama.rst
@@ -0,0 +1,75 @@
+====
+Yama
+====
+
+Yama is a Linux Security Module that collects system-wide DAC security
+protections that are not handled by the core kernel itself. This is
+selectable at build-time with ``CONFIG_SECURITY_YAMA``, and can be controlled
+at run-time through sysctls in ``/proc/sys/kernel/yama``:
+
+ptrace_scope
+============
+
+As Linux grows in popularity, it will become a larger target for
+malware. One particularly troubling weakness of the Linux process
+interfaces is that a single user is able to examine the memory and
+running state of any of their processes. For example, if one application
+(e.g. Pidgin) was compromised, it would be possible for an attacker to
+attach to other running processes (e.g. Firefox, SSH sessions, GPG agent,
+etc) to extract additional credentials and continue to expand the scope
+of their attack without resorting to user-assisted phishing.
+
+This is not a theoretical problem. `SSH session hijacking
+<https://www.blackhat.com/presentations/bh-usa-05/bh-us-05-boileau.pdf>`_
+and `arbitrary code injection
+<https://c-skills.blogspot.com/2007/05/injectso.html>`_ attacks already
+exist and remain possible if ptrace is allowed to operate as before.
+Since ptrace is not commonly used by non-developers and non-admins, system
+builders should be allowed the option to disable this debugging system.
+
+For a solution, some applications use ``prctl(PR_SET_DUMPABLE, ...)`` to
+specifically disallow such ptrace attachment (e.g. ssh-agent), but many
+do not. A more general solution is to only allow ptrace directly from a
+parent to a child process (i.e. direct "gdb EXE" and "strace EXE" still
+work), or with ``CAP_SYS_PTRACE`` (i.e. "gdb --pid=PID", and "strace -p PID"
+still work as root).
+
+In mode 1, software that has defined application-specific relationships
+between a debugging process and its inferior (crash handlers, etc),
+``prctl(PR_SET_PTRACER, pid, ...)`` can be used. An inferior can declare which
+other process (and its descendants) are allowed to call ``PTRACE_ATTACH``
+against it. Only one such declared debugging process can exists for
+each inferior at a time. For example, this is used by KDE, Chromium, and
+Firefox's crash handlers, and by Wine for allowing only Wine processes
+to ptrace each other. If a process wishes to entirely disable these ptrace
+restrictions, it can call ``prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, ...)``
+so that any otherwise allowed process (even those in external pid namespaces)
+may attach.
+
+The sysctl settings (writable only with ``CAP_SYS_PTRACE``) are:
+
+0 - classic ptrace permissions:
+    a process can ``PTRACE_ATTACH`` to any other
+    process running under the same uid, as long as it is dumpable (i.e.
+    did not transition uids, start privileged, or have called
+    ``prctl(PR_SET_DUMPABLE...)`` already). Similarly, ``PTRACE_TRACEME`` is
+    unchanged.
+
+1 - restricted ptrace:
+    a process must have a predefined relationship
+    with the inferior it wants to call ``PTRACE_ATTACH`` on. By default,
+    this relationship is that of only its descendants when the above
+    classic criteria is also met. To change the relationship, an
+    inferior can call ``prctl(PR_SET_PTRACER, debugger, ...)`` to declare
+    an allowed debugger PID to call ``PTRACE_ATTACH`` on the inferior.
+    Using ``PTRACE_TRACEME`` is unchanged.
+
+2 - admin-only attach:
+    only processes with ``CAP_SYS_PTRACE`` may use ptrace, either with
+    ``PTRACE_ATTACH`` or through children calling ``PTRACE_TRACEME``.
+
+3 - no attach:
+    no processes may use ptrace with ``PTRACE_ATTACH`` nor via
+    ``PTRACE_TRACEME``. Once set, this sysctl value cannot be changed.
+
+The original children-only logic was based on the restrictions in grsecurity.