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

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: cefa929c034eb5d ("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

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index cdaab30880b91c969344e42ba3392f98a055def3..cd6be6f143e85540399d106465c28e0657237f52 100644 (file)
--- 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);