Messages in this thread | | | Subject | Re: [PATCH] x86/alternative: don't call text_poke() in lazy TLB mode | From | Jürgen Groß <> | Date | Mon, 12 Oct 2020 12:26:06 +0200 |
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On 12.10.20 12:13, Peter Zijlstra wrote: > 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' :/
Yeah.
> >> 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.
Fine with me, but I don't think it matters that much.
For each batch of text_poke() it will be hit only once, and I'm not sure it is really a good idea to use the knowledge that leave_mm() is just a switch to init_mm here.
In case it is still the preferred way to do it I can send an update of the patch.
Juergen
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