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

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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.

[[email protected]: added Fixes tag; cc'ed stable]

The Linux kernel CVE team has assigned CVE-2026-72028 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.12 with commit c22b0bcb1dd024cb9caad9230e3a387d8b061df5 and fixed in 6.6.145 with commit 5da5cf48a432e30ded8d58087854e5383a36eff1
	Issue introduced in 5.12 with commit c22b0bcb1dd024cb9caad9230e3a387d8b061df5 and fixed in 6.12.97 with commit c386e1c591d72eab58ee2e69105c8cbc70928857
	Issue introduced in 5.12 with commit c22b0bcb1dd024cb9caad9230e3a387d8b061df5 and fixed in 6.18.40 with commit 2faf0198168d2017cb528a79f76c560fda3b6e94
	Issue introduced in 5.12 with commit c22b0bcb1dd024cb9caad9230e3a387d8b061df5 and fixed in 7.1.5 with commit 91b4d76dd07f1a1f20f73dfebb42ba04ac911a56
	Issue introduced in 5.12 with commit c22b0bcb1dd024cb9caad9230e3a387d8b061df5 and fixed in 7.2-rc2 with commit bc7b086a45521a986a49045907f017e3e46c763e

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-72028
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	arch/riscv/kernel/probes/rethook_trampoline.S


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/5da5cf48a432e30ded8d58087854e5383a36eff1
	https://git.kernel.org/stable/c/c386e1c591d72eab58ee2e69105c8cbc70928857
	https://git.kernel.org/stable/c/2faf0198168d2017cb528a79f76c560fda3b6e94
	https://git.kernel.org/stable/c/91b4d76dd07f1a1f20f73dfebb42ba04ac911a56
	https://git.kernel.org/stable/c/bc7b086a45521a986a49045907f017e3e46c763e