Re: [PATCH] x86/efi: Restore IRQ state in EFI page fault handler

[Eric Biggers <ebiggers@kernel.org>]

On Fri, May 01, 2026 at 11:03:12AM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> The kernel's softirq API does not permit re-enabling softirqs while IRQs
> are disabled. The reason for this is that local_bh_enable() will not
> only re-enable delivery of softirqs over the back of IRQs, it will also
> handle any pending softirqs immediately, regardless of whether IRQs are
> enabled at that point.
> 
> For this reason, commit
> 
>   d02198550423 ("x86/fpu: Improve crypto performance by making kernel-mode FPU reliably usable in softirqs")
> 
> disables softirqs only when IRQs are enabled, as it is not permitted
> otherwise, but also unnecessary, given that asynchronous softirq
> delivery never happens to begin with while IRQs are disabled.
> 
> However, this does mean that entering a kernel mode FPU section with
> IRQs enabled and leaving it with IRQs disabled leads to problems, as
> identified by Sashiko [0]: the EFI page fault handler is called from
> page_fault_oops() with IRQs disabled, and thus ends the kernel mode FPU
> section with IRQs disabled as well, regardless of whether IRQs were
> enabled when it was started. This may result in schedule() being called
> with a non-zero preempt_count, causing a BUG().
> 
> So take care to re-enable IRQs when handling any EFI page faults if they
> were taken with IRQs enabled.
> 
> [0] https://sashiko.dev/#/patchset/20260430074107.27051-1-ivan.hu%40canonical.com
> 
> Cc: Eric Biggers <ebiggers@kernel.org>
> Cc: Ivan Hu <ivan.hu@canonical.com>
> Fixes: d02198550423 ("x86/fpu: Improve crypto performance by making kernel-mode FPU reliably usable in softirqs")
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/x86/include/asm/efi.h     |  3 ++-
>  arch/x86/mm/fault.c            |  2 +-
>  arch/x86/platform/efi/quirks.c | 11 ++++++++++-
>  3 files changed, 13 insertions(+), 3 deletions(-)

Reviewed-by: Eric Biggers <ebiggers@kernel.org>

- Eric