CVE-2026-72028 — riscv: probes: save original sp in rethook trampoline

Affected: Linux

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

riscv: probes: save original sp in rethook trampoline

Reading a word from the stack in a kretprobe crashes a risc-v kernel.

$ cd /sys/kernel/tracing/ $ echo 'r n_tty_write $stack0' > dynamic_events $ echo 1 > events/kprobes/enable Unable to handle kernel paging request at virtual address 0000000200000128 ... [<ffffffff80016d16>] regs_get_kernel_stack_nth+0x26/0x38 [<ffffffff80177196>] process_fetch_insn+0x3ee/0x760 [<ffffffff80177836>] kretprobe_trace_func+0x116/0x1f0 [<ffffffff8017795a>] kretprobe_dispatcher+0x4a/0x58 [<ffffffff8013572e>] kretprobe_rethook_handler+0x5e/0x90 [<ffffffff80180838>] rethook_trampoline_handler+0x70/0x108 [<ffffffff8001ba32>] arch_rethook_trampoline_callback+0x12/0x1c [<ffffffff8001ba84>] arch_rethook_trampoline+0x48/0x94 [<ffffffff8067872a>] tty_write+0x1a/0x30

In regs_get_kernel_stack_nth, regs->sp contains an arbitrary value.

arch_rethook_trampoline saves the registers from the probed function in a struct pt_regs. sp is not saved. Instead, sp is decremented for arch_rethook_trampoline's local stack.

Fix this crash and save the original sp along with the other registers. Use a0 as a temporary register, it is overwritten anyway.

[pjw@kernel.org: added Fixes tag; cc'ed stable]

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CVE-2026-72028 — riscv: probes: save original sp in rethook trampoline