CVE-2026-98273: x86/kprobes: Fix crash when probing CS CALL instructions

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

x86/kprobes: Fix crash when probing CS CALL instructions

When using eBPF to probe CS CALL instructions within a function, a crash can be triggered.

The eBPF tool probes offset 257 of the hrtimerrunqueues() function:

<hrtimerrunqueues+249>: nopl 0x0(%rax,%rax,1) <hrtimerrunqueues+254>: mov %r14,%rdi <hrtimerrunqueues+257>: cs call <x86indirectthunkr12> <hrtimerrunqueues+263>: mov %eax,%r12d <hrtimerrunqueues+266>: xchg %ax,%ax <hrtimerrunqueues+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: errorcode(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:hrtimerrunqueues+0x106/0x230

Note that hrtimerrunqueues+0x106 is hrtimerrunqueues+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, int3emulatecall() 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 CALLINSNSIZE macro.

[ mingo: Refined the changelog ]