arm64: sdei: Preserve the interrupted context's exception registers

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>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

0 files changed, 0 insertions, 0 deletions