Re: [PATCH v2 2/2] arm64: sdei: Preserve the interrupted context's exception registers

[Breno Leitao <leitao@debian.org>]

On Thu, Nov 05, 2026 at 12:08:34PM +0000, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
> 
> An SDEI event is dispatched by firmware regardless of PSTATE.DAIF, so it
> can interrupt the kernel where the exception registers are live:
> 
>  - exception entry, before kernel_entry has saved ELR_EL1/SPSR_EL1 and
>    before the C handler has read ESR_EL1/FAR_EL1;
>  - exception return, after kernel_exit has loaded ELR_EL1/SPSR_EL1 and
>    before the ERET.
> 
> An IRQ or pseudo-NMI cannot arrive there with PSTATE.I set. Firmware
> saves only x0-x17, PC and PSTATE around an SDEI event, so a synchronous
> exception taken by the handler overwrites the interrupted context's
> ELR, SPSR, ESR and FAR. do_sdei_event() only notices that ELR changed,
> and warns.
> 
> The interrupted context then carries on with the handler's state. A
> task interrupted on its way into the kernel can be sent SIGILL for the
> handler's kernel data abort. A task interrupted on its way back to user
> space ERETs to EL1, at the handler's fixup address, with user registers
> loaded.
> 
> With SDEI delivering NMI backtraces, this happens in normal operation.
> The handler runs show_regs(), which reads through possibly-NULL pointers
> with the _nofault() accessors. print_worker_info() faults this way for a
> kworker that is not running a work item, and a single sysrq-l warned
> once per such kworker.
> 
> Save ELR, SPSR, ESR and FAR in __sdei_handler() before
> irqentry_nmi_enter(), which can take an exception too, and restore them
> after irqentry_nmi_exit().
> 
> Once they are restored, a nested exception is harmless if its handler is
> safe in NMI context. That holds for EL1 data aborts on translation,
> permission and access-flag faults, which never take the mmap lock in NMI
> context, and for BRK. Warn once, with the syndrome, about any other
> exception class.
> 
> Fixes: f5df26961853 ("arm64: kernel: Add arch-specific SDEI entry code and CPU masking")
> Cc: stable@vger.kernel.org
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

Tested-by: Breno Leitao <leitao@debian.org>

Tested on NVIDIA Grace (firmware SDEI) with next-20261104, KASAN + lockdep,
ARM_SDEI_NMI=y: sysrq-l under stress-ng, with a kprobe on show_regs() and
eprobes on irq_disable/irq_enable that fault on purpose.

Without the series the box crashed (a task interrupted in el0t_64_sync
got the handler's BRK, then lockdep/KASAN fallout and a hard lockup)

With it, KPTI off and kpti=1, ~4000 kprobe hits and ~5500 faults in
irqentry_nmi_enter()/exit() produced no warnings or stalls.