[PATCH 3.16.y-ckt 195/216] arm64: kernel: fix __cpu_suspend mm switch on warm-boot

Luis Henriques
2015-01-12T18:37:56Z

From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>

commit f43c27188a49111b58e9611afa2f0365b0b55625 upstream.

On arm64 the TTBR0_EL1 register is set to either the reserved TTBR0
page tables on boot or to the active_mm mappings belonging to user space
processes, it must never be set to swapper_pg_dir page tables mappings.

When a CPU is booted its active_mm is set to init_mm even though its
TTBR0_EL1 points at the reserved TTBR0 page mappings. This implies
that when __cpu_suspend is triggered the active_mm can point at
init_mm even if the current TTBR0_EL1 register contains the reserved
TTBR0_EL1 mappings.

Therefore, the mm save and restore executed in __cpu_suspend might
turn out to be erroneous in that, if the current->active_mm corresponds
to init_mm, on resume from low power it ends up restoring in the
TTBR0_EL1 the init_mm mappings that are global and can cause speculation
of TLB entries which end up being propagated to user space.

This patch fixes the issue by checking the active_mm pointer before
restoring the TTBR0 mappings. If the current active_mm == &init_mm,
the code sets the TTBR0_EL1 to the reserved TTBR0 mapping instead of
switching back to the active_mm, which is the expected behaviour
corresponding to the TTBR0_EL1 settings when __cpu_suspend was entered.

Fixes: 95322526ef62 ("arm64: kernel: cpu_{suspend/resume} implementation")
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
arch/arm64/kernel/suspend.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/suspend.c b/arch/arm64/kernel/suspend.c
index 3771b72b6569..2d6b6065fe7f 100644
--- a/arch/arm64/kernel/suspend.c
+++ b/arch/arm64/kernel/suspend.c
@@ -5,6 +5,7 @@
#include <asm/debug-monitors.h>
#include <asm/pgtable.h>
#include <asm/memory.h>
+#include <asm/mmu_context.h>
#include <asm/smp_plat.h>
#include <asm/suspend.h>
#include <asm/tlbflush.h>
@@ -98,7 +99,18 @@ int __cpu_suspend(unsigned long arg, int (*fn)(unsigned long))
*/
ret = __cpu_suspend_enter(arg, fn);
if (ret == 0) {
- cpu_switch_mm(mm->pgd, mm);
+ /*
+ * We are resuming from reset with TTBR0_EL1 set to the
+ * idmap to enable the MMU; restore the active_mm mappings in
+ * TTBR0_EL1 unless the active_mm == &init_mm, in which case
+ * the thread entered __cpu_suspend with TTBR0_EL1 set to
+ * reserved TTBR0 page tables and should be restored as such.
+ */
+ if (mm == &init_mm)
+ cpu_set_reserved_ttbr0();
+ else
+ cpu_switch_mm(mm->pgd, mm);
+
flush_tlb_all();

/*
--
2.1.4