Re: [tip: x86/alternatives] x86/efi: Make efi_enter/leave_mm() use the use_/unuse_temporary_mm() machinery

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 17, 2025 at 04:17:51PM +0200, Borislav Petkov wrote:
> On Sat, Apr 12, 2025 at 06:46:48PM -0000, tip-bot2 for Andy Lutomirski wrote:
> > The following commit has been merged into the x86/alternatives branch of tip:
> > 
> > Commit-ID:     e7021e2fe0b4335523d3f6e2221000bdfc633b62
> > Gitweb:        https://git.kernel.org/tip/e7021e2fe0b4335523d3f6e2221000bdfc633b62
> > Author:        Andy Lutomirski <luto@kernel.org>
> > AuthorDate:    Wed, 02 Apr 2025 11:45:39 +02:00
> > Committer:     Ingo Molnar <mingo@kernel.org>
> > CommitterDate: Sat, 12 Apr 2025 10:06:04 +02:00
> > 
> > x86/efi: Make efi_enter/leave_mm() use the use_/unuse_temporary_mm() machinery
> > 
> > This should be considerably more robust.  It's also necessary for optimized
> > for_each_possible_lazymm_cpu() on x86 -- without this patch, EFI calls in
> > lazy context would remove the lazy mm from mm_cpumask().
> > 
> > [ mingo: Merged it on top of x86/alternatives ]
> > 
> > Signed-off-by: Andy Lutomirski <luto@kernel.org>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > Cc: Rik van Riel <riel@surriel.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Link: https://lore.kernel.org/r/20250402094540.3586683-7-mingo@kernel.org
> > ---
> >  arch/x86/platform/efi/efi_64.c | 7 ++-----
> >  1 file changed, 2 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> > index ac57259..a5d3496 100644
> > --- a/arch/x86/platform/efi/efi_64.c
> > +++ b/arch/x86/platform/efi/efi_64.c
> > @@ -434,15 +434,12 @@ void __init efi_dump_pagetable(void)
> >   */
> >  static void efi_enter_mm(void)
> >  {
> > -	efi_prev_mm = current->active_mm;
> > -	current->active_mm = &efi_mm;
> > -	switch_mm(efi_prev_mm, &efi_mm, NULL);
> > +	efi_prev_mm = use_temporary_mm(&efi_mm);
> >  }
> >  
> >  static void efi_leave_mm(void)
> >  {
> > -	current->active_mm = efi_prev_mm;
> > -	switch_mm(&efi_mm, efi_prev_mm, NULL);
> > +	unuse_temporary_mm(efi_prev_mm);
> >  }
> >  
> >  void arch_efi_call_virt_setup(void)
> 
> mingo thinks this one causes this:
> 
> [    0.119491] x86/fpu: xstate_offset[2]:  576, xstate_sizes[2]:  256
> [    0.119498] x86/fpu: Enabled xstate features 0x7, context size is 832 bytes, using 'compacted' format.
> [    0.137368] Freeing SMP alternatives memory: 40K
> [    0.137381] pid_max: default: 32768 minimum: 301
> [    0.137496] ------------[ cut here ]------------
> [    0.137502] WARNING: CPU: 0 PID: 0 at arch/x86/mm/tlb.c:795 switch_mm_irqs_off+0x3d3/0x460
> [    0.137516] Modules linked in:

Ah yes :-( Something like so perhaps..

---
Subject: x86/mm: Fix {,un}use_temporary_mm() IRQ state

As the function switch_mm_irqs_off() implies, it ought to be called with
IRQs *off*. Commit 58f8ffa91766 ("x86/mm: Allow temporary MMs when IRQs
are on") caused this to not be the case for EFI.

Ensure IRQs are off where it matters.

Fixes: 58f8ffa91766 ("x86/mm: Allow temporary MMs when IRQs are on")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 92bde0d6205a..1451e022129a 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -991,6 +991,7 @@ struct mm_struct *use_temporary_mm(struct mm_struct *temp_mm)
 	struct mm_struct *prev_mm;
 
 	lockdep_assert_preemption_disabled();
+	guard(irqsave)();
 
 	/*
 	 * Make sure not to be in TLB lazy mode, as otherwise we'll end up
@@ -1023,6 +1024,7 @@ struct mm_struct *use_temporary_mm(struct mm_struct *temp_mm)
 void unuse_temporary_mm(struct mm_struct *prev_mm)
 {
 	lockdep_assert_preemption_disabled();
+	guard(irqsave)();
 
 	/* Clear the cpumask, to indicate no TLB flushing is needed anywhere */
 	cpumask_clear_cpu(smp_processor_id(), mm_cpumask(this_cpu_read(cpu_tlbstate.loaded_mm)));