CVE-2026-64235

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

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

With CONFIGCALLDEPTH_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: errorcode(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> ? findheldlock ? excpagefault ? lockrelease ? __x64sysclocknanosleep ? lockdephardirqsonprepare ? tracehardirqson __x64sysclocknanosleep dosyscall64 ? excpagefault ? calldepthreturnthunk entrySYSCALL64afterhwframe ... Kernel panic - not syncing: Fatal exception

This small reproducer allows to easily trigger the crash:

# echo 'p __x64sysclock_nanosleep' > /sys/kernel/tracing/kprobeevents # echo 1 > /sys/kernel/tracing/events/kprobes/p__x64sysclocknanosleep0/enable # usleep 1

Monitoring the crash under GDB points to the exact instruction in charge of incrementing the call depth:

sarq $5, %gs:__x86calldepth(%rip)

This instruction matches the one inserted by the ftraceregscaller from ftrace_64.S. This emitted code was likely working fine until the introduction of

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

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 __x86calldepth value, otherwise the targeted address is wrong, leading to the page fault seen above.

Fix the %rip-relative displacement of the copied CALLDEPTHACCOUNT instruction (from ftraceregscaller) by calling textpokeapplyrelocation(), as it is done for example by the x86 BPF JIT compiler through x86calldepthemitaccounting(). This corrects both CALLDEPTHACCOUNT slots, in ftracecaller and ftraceregscaller.

[ bp: Massage. ]