Messages in this thread | | | Date | Mon, 12 Oct 2020 12:13:30 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86/alternative: don't call text_poke() in lazy TLB mode |
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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@suse.com> > --- > 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.
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