Re: [PATCH] ARM: Disallow kernel mode NEON when IRQs are disabled

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Mar 24, 2025 at 07:59:28PM +0100, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Commit
> 
>   c79f81631142 ("ARM: 9283/1: permit non-nested kernel mode NEON in softirq context")
> 
> relaxed the rules around the use of SIMD instructions in kernel mode on
> ARM, to allow such use when serving a softirq. To avoid having to
> preserve/restore kernel mode NEON state when such a softirq is taken,
> softirqs are now disabled when using the NEON from task context.
> 
> However, the fact that the softirq API does not allow unmasking of
> softirqs with interrupts disabled was overlooked, resulting in a WARN()
> in some cases, as reported by Guenter:
> 
>   WARNING: CPU: 0 PID: 1145 at kernel/softirq.c:369 __local_bh_enable_ip+0x118/0x194
>   Call trace:
>    unwind_backtrace from show_stack+0x10/0x14
>    show_stack from dump_stack_lvl+0x7c/0xac
>    dump_stack_lvl from __warn+0x7c/0x1b8
>    __warn from warn_slowpath_fmt+0x19c/0x1a4
>    warn_slowpath_fmt from __local_bh_enable_ip+0x118/0x194
>    __local_bh_enable_ip from crc_t10dif_arch+0xd4/0xe8
>    crc_t10dif_arch from crc_t10dif_wrapper+0x14/0x1c
>    crc_t10dif_wrapper from crc_main_test+0x178/0x360
>    crc_main_test from kunit_try_run_case+0x78/0x1e0
>    kunit_try_run_case from kunit_generic_run_threadfn_adapter+0x1c/0x34
>    kunit_generic_run_threadfn_adapter from kthread+0x118/0x254
>    kthread from ret_from_fork+0x14/0x28
> 
> While disabling softirqs is not really needed when running with IRQs
> disabled (given that the only way a softirq can be delivered
> asynchrously is over the back of an IRQ), let's not complicate this
> logic more than needed, and simply disallow use of the NEON in kernel
> mode when IRQs are disabled.
> 
> Another approach might be to only disable and re-enable softirqs if IRQs
> are enabled, but other than the test case above, there are no clear use
> cases for doing non-trivial arithmetic processing (hence using an
> accelerated SIMD implementation) with IRQs disabled.
> 
> Reported-by: Guenter Roeck <linux@roeck-us.net>
> Tested-by: Guenter Roeck <linux@roeck-us.net>
> Link: https://lore.kernel.org/all/389b899f-893c-4855-9e30-d8920a5d6f91@roeck-us.net
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm/include/asm/simd.h | 3 ++-
>  arch/arm/vfp/vfpmodule.c    | 1 +
>  2 files changed, 3 insertions(+), 1 deletion(-)

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

- Eric