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

[Kiryl Shutsemau]

On Thu, Nov 05, 2026 at 07:41:12AM -0800, Breno Leitao wrote:
> On Thu, Nov 05, 2026 at 12:08:34PM +0000, Kiryl Shutsemau wrote:
> > From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> 
> Tested-by: Breno Leitao <leitao@xxxxxxxxxx>
> 
> 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.

Thanks for testing!

Just to clarify, does your Tested-by apply for whole series, right?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov