CVE-2026-64235

In the Linux kernel, the following vulnerability has been resolved:

x86/ftrace: Relocate %rip-relative percpu refs in dynamic trampolines

With CONFIG_CALL_DEPTH_TRACKING enabled on an x86 retbleed-affected platform

(eg: Skylake), with retbleed=stuff, registering a dynamic ftrace trampoline

crashes on the first call into the traced function:

BUG: unable to handle page fault for address: ffff88817ae18880

#PF: supervisor write access in kernel mode

#PF: error_code(0x0002) - not-present page

PGD 4b53067 P4D 4b53067 PUD 0

Oops: Oops: 0002 [#1] SMP PTI

CPU: 3 UID: 0 PID: 187 Comm: usleep Not tainted 7.0.10 #243 PREEMPT(full)

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.17.0-2-2 04/01/2014

Code: 24 78 00 00 00 00 48 89 ea 48 89 54 24 20 48 8b b4 24 b8 00 00 00 48 8b bc 24 b0 00 00 00 48 89 bc 24 80 00 00 00 48 83 ef 05 <65> 48 c1 3d 1f a8 b6 02 05 48 8b 15 f6 00 00 00 4c 89 3c 24 4c 89

Call Trace:

<TASK>

? find_held_lock

? exc_page_fault

? lock_release

? __x64_sys_clock_nanosleep

? lockdep_hardirqs_on_prepare

? trace_hardirqs_on

__x64_sys_clock_nanosleep

do_syscall_64

? exc_page_fault

? call_depth_return_thunk

entry_SYSCALL_64_after_hwframe

...

Kernel panic - not syncing: Fatal exception

This small reproducer allows to easily trigger the crash:

# echo 'p __x64_sys_clock_nanosleep' > /sys/kernel/tracing/kprobe_events

# echo 1 > /sys/kernel/tracing/events/kprobes/p___x64_sys_clock_nanosleep_0/enable

# usleep 1

Monitoring the crash under GDB points to the exact instruction in charge of

incrementing the call depth:

sarq $5, %gs:__x86_call_depth(%rip)

This instruction matches the one inserted by the ftrace_regs_caller from

ftrace_64.S. This emitted code was likely working fine until the introduction

of

59bec00ace28 ("x86/percpu: Introduce %rip-relative addressing to PER_CPU_VAR()"):

it has made the call depth accounting addressing relative to $rip, instead of

being based on an absolute address.

As this code exact location depends on where the trampoline lives in memory,

the corresponding displacement needs to be adjusted at runtime to actually

correctly find the per-cpu __x86_call_depth value, otherwise the targeted

address is wrong, leading to the page fault seen above.

Fix the %rip-relative displacement of the copied CALL_DEPTH_ACCOUNT

instruction (from ftrace_regs_caller) by calling text_poke_apply_relocation(),

as it is done for example by the x86 BPF JIT compiler through

x86_call_depth_emit_accounting(). This corrects both CALL_DEPTH_ACCOUNT slots,

in ftrace_caller and ftrace_regs_caller.

[ bp: Massage. ]