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x86/alternative: Don't call text_poke() in lazy TLB mode

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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.

Fixes: cefa929c03 ("x86/mm: Introduce temporary mm structs")
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201009144225.12019-1-jgross@suse.com
This commit is contained in:
Juergen Gross 2020-10-09 16:42:25 +02:00 committed by Peter Zijlstra
parent c3b484c439
commit abee7c494d
1 changed files with 9 additions and 0 deletions
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9
arch/x86/kernel/alternative.c
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@ -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);