CVE-2026-64235 in Linux
Summary
by MITRE • 07/24/2026
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. ]
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Analysis
by VulDB Data Team • 07/24/2026
The vulnerability described represents a critical issue within the Linux kernel's x86 architecture implementation, specifically affecting systems with CONFIG_CALL_DEPTH_TRACKING enabled and retbleed mitigation activated. This flaw manifests as a kernel panic during dynamic ftrace trampoline registration when attempting to trace functions that utilize call depth tracking mechanisms. The crash occurs due to improper handling of %rip-relative addressing in per-cpu variables, creating a scenario where memory access attempts target invalid virtual addresses leading to page fault exceptions.
The technical root cause stems from changes introduced in commit 59bec00ace28 which implemented %rip-relative addressing for PER_CPU_VAR() macros. This modification altered how the call depth accounting mechanism operates by using relative addressing instead of absolute addresses, fundamentally changing the memory layout expectations. When dynamic ftrace trampolines are created, they contain instructions that reference the per-cpu __x86_call_depth variable through %rip-relative addressing, but these displacements are not properly adjusted based on where the actual trampoline code is loaded in memory.
The operational impact of this vulnerability is severe as it renders systems unstable when using dynamic tracing capabilities with call depth tracking enabled. The crash specifically occurs during the first function call into a traced function because the problematic instruction sarq $5, %gs:__x86_call_depth(%rip) attempts to access memory at an incorrect offset due to the unadjusted displacement. This vulnerability affects systems running kernel versions where retbleed mitigation is active, particularly those using Skylake processors or similar affected architectures.
The fix implements proper relocation of the %rip-relative displacement by invoking text_poke_apply_relocation() function, similar to patterns used by x86 BPF JIT compiler through x86_call_depth_emit_accounting. This approach ensures that both CALL_DEPTH_ACCOUNT slots within ftrace_caller and ftrace_regs_caller are correctly adjusted at runtime according to their actual memory locations. The solution addresses the core issue identified in the context of CWE-129, which deals with improper validation of array indices or buffer boundaries, and aligns with ATT&CK technique T1059.006 for execution through system commands while maintaining kernel integrity.
The vulnerability represents a memory corruption issue that bypasses normal kernel protection mechanisms, making it particularly dangerous as it can lead to complete system crashes or potential privilege escalation scenarios. The fix ensures proper runtime relocation of the instruction displacements while maintaining the security properties of the original implementation, preventing attackers from exploiting this flaw to gain unauthorized access to kernel memory spaces or execute arbitrary code with elevated privileges.