[PATCH v2] riscv: signal: protect regs->status RMW from concurrent preemption

[Paul Walmsley]

On Tue, 20 Oct 2026, Guobin Zhang wrote:

> __fstate_clean() performs a non-atomic read-modify-write (RMW) on
> task_pt_regs(current)->status to clear the FS bits. This RMW is
> unprotected - it runs with preemption enabled and IRQs on during
> signal delivery (setup_rt_frame -> fstate_save) and sigreturn
> (restore_fp_state -> fstate_restore).
> 
> If a reschedule IPI or timer interrupt triggers preemption between
> the load and store of this RMW, __switch_to_vector() calls
> riscv_v_vstate_set_restore() which modifies the VS bits of the
> same regs->status. When the preempted task resumes, it stores back
> the stale value captured before preemption, overwriting the VS
> update and restoring VS=DIRTY while TIF_RISCV_V_DEFER_RESTORE
> remains set - a combination that should never occur and leads to
> vector state corruption.
> 
> The race window:
> 
>   __fstate_clean (signal path)      set_restore (schedule path)
>   -----------------------------     -----------------------------
>   ld  a0, regs->status  // VS=DIRTY, FS=DIRTY
>                                      <- preempted (reschedule IPI)
>                                      regs->status VS = INITIAL
>                                      set TIF_RISCV_V_DEFER_RESTORE
>                                      <- resumed
>   andi a0, ~FS
>   ori  a0, FS_CLEAN     // stale a0 still has VS=DIRTY
>   sd   a0, regs->status // overwrites VS=INITIAL with VS=DIRTY
>   -> result: VS=DIRTY + DEFER -> ANOMALY
> 
> Fix by adding preempt_disable/enable in fstate_save() and
> fstate_restore(), which covers the RMW in __fstate_clean(), and
> around the RMW in fstate_off(). Add fstate_{save,restore}_preempt_off()
> for code that already runs with preemption disabled (the scheduler and
> kernel_mode_fpu).
> 
> riscv_v_vstate_set_restore() has the identical hazard: it is called
> from __restore_v_state() (sigreturn path) with preemption enabled, and
> it can race the same way against the scheduler's own call to
> riscv_v_vstate_set_restore() for the same task in __switch_to_vector().
> Wrap that call site with preempt_disable/enable as well.
> 
> Add WARN_ON_ONCE(preemptible()) to riscv_v_vstate_restore()
> to catch any future unprotected callers at development time.
> 
> Suggested-by: Andy Chiu <tchiu at tenstorrent.com>
> Signed-off-by: Guobin Zhang <guobin.zhang at intel.com>

Thanks, queued for v7.4-rc.


- Paul