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

Summary

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. ]

Timeline

1. Jul 24, 2026, 3:27 PM UTCCVE published
2. Oct 16, 2026, 3:37 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: total

3. Oct 19, 2026, 8:28 PM UTCRecord last updated

Affected versions

• Linux Linux

  From 59bec00ace28d565ae0a68b23063ef3b961d82d5 · fixed in 8093442a2d1d4b42b9340a86023ccb2afb30b93a; From 59bec00ace28d565ae0a68b23063ef3b961d82d5 · fixed in d59cc66b702757e3c5a711e78a38583eac0c2738; From 59bec00ace28d565ae0a68b23063ef3b961d82d5 · fixed in 9edff632ca216169846f8a63a5a3dc467e239c7a; From 59bec00ace28d565ae0a68b23063ef3b961d82d5 · fixed in a17dc12bfed8868e6a86f3b45c16065a70641acb

• Linux Linux

  6.9

• KernelLinux

  From 6.9.0 · fixed in 6.12.93; From 6.13.0 · fixed in 6.18.35; From 6.19.0 · fixed in 7.0.12

References 4