Backported patch for linux-5.4

["Jürgen Groß" <jgross@suse.com>]

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Hi,

there has been a report of a failure in a 5.4 based kernel, which has
been fixed in kernel 5.10 with commit abee7c494d8c41bb388839bccc47e06247f0d7de.

Please apply the attached backported patch to the stable 5.4 kernel.


Juergen

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From abee7c494d8c41bb388839bccc47e06247f0d7de Mon Sep 17 00:00:00 2001
From: Juergen Gross <jgross@suse.com>
Date: Fri, 9 Oct 2020 16:42:25 +0200
Subject: x86/alternative: Don't call text_poke() in lazy TLB mode

[ Upstream commit  abee7c494d8c41bb388839bccc47e06247f0d7de ]

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
---
 arch/x86/include/asm/mmu_context.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index cdaab30880b91..cd6be6f143e85 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -379,6 +379,15 @@ 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);
 
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