Re: [PATCH] arm64: tls: Fix context-switching of tpidrro_el0 when kpti is enabled

[Marc Zyngier <maz@kernel.org>]

On Thu, 14 Nov 2024 09:53:32 +0000,
Will Deacon <will@kernel.org> wrote:
> 
> Commit 18011eac28c7 ("arm64: tls: Avoid unconditional zeroing of
> tpidrro_el0 for native tasks") tried to optimise the context switching
> of tpidrro_el0 by eliding the clearing of the register when switching
> to a native task with kpti enabled, on the erroneous assumption that
> the kpti trampoline entry code would already have taken care of the
> write.
> 
> Although the kpti trampoline does zero the register on entry from a
> native task, the check in tls_thread_switch() is on the *next* task and
> so we can end up leaving a stale, non-zero value in the register if the
> previous task was 32-bit.
> 
> Drop the broken optimisation and zero tpidrro_el0 unconditionally when
> switching to a native 64-bit task.
> 
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: <stable@vger.kernel.org>
> Fixes: 18011eac28c7 ("arm64: tls: Avoid unconditional zeroing of tpidrro_el0 for native tasks")
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> 
> You fix one side-channel and introduce another... :(
>
>  arch/arm64/kernel/process.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 3e7c8c8195c3..2bbcbb11d844 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -442,7 +442,7 @@ static void tls_thread_switch(struct task_struct *next)
>  
>  	if (is_compat_thread(task_thread_info(next)))
>  		write_sysreg(next->thread.uw.tp_value, tpidrro_el0);
> -	else if (!arm64_kernel_unmapped_at_el0())
> +	else
>  		write_sysreg(0, tpidrro_el0);
>  
>  	write_sysreg(*task_user_tls(next), tpidr_el0);

Acked-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.