Re: [PATCH] x86/alternative: don't call text_poke() in lazy TLB mode

[Peter Zijlstra]

On Fri, Oct 09, 2020 at 04:42:25PM +0200, Juergen Gross wrote:
> When running in lazy TLB mode the currently active page tables might
> be the ones of a previous process, e.g. when running a kernel thread.
> 
> This can be problematic in case kernel code is being modified via
> text_poke() in a kernel thread, and on another processor exit_mmap()
> is active for the process which was running on the first cpu before
> the kernel thread.
> 
> As text_poke() is using a temporary address space and the former
> address space (obtained via cpu_tlbstate.loaded_mm) is restored
> afterwards, there is a race possible in case the cpu on which
> exit_mmap() is running wants to make sure there are no stale
> references to that address space on any cpu active (this e.g. is
> required when running as a Xen PV guest, where this problem has been
> observed and analyzed).
> 
> In order to avoid that, drop off TLB lazy mode before switching to the
> temporary address space.

Oh man, that must've been 'fun' :/

> Fixes: cefa929c034eb5d ("x86/mm: Introduce temporary mm structs")
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  arch/x86/kernel/alternative.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index cdaab30880b9..cd6be6f143e8 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -807,6 +807,15 @@ static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
>  	temp_mm_state_t temp_state;
>  
>  	lockdep_assert_irqs_disabled();
> +
> +	/*
> +	 * Make sure not to be in TLB lazy mode, as otherwise we'll end up
> +	 * with a stale address space WITHOUT being in lazy mode after
> +	 * restoring the previous mm.
> +	 */
> +	if (this_cpu_read(cpu_tlbstate.is_lazy))
> +		leave_mm(smp_processor_id());
> +
>  	temp_state.mm = this_cpu_read(cpu_tlbstate.loaded_mm);
>  	switch_mm_irqs_off(NULL, mm, current);

Would it make sense to write it like:

	this_state.mm = this_cpu_read(cpu_tlbstate.is_lazy) ?
			&init_mm : this_cpu_read(cpu_tlbstate.loaded_mm);

Possibly with that wrapped in a conveniently named helper function.