arm64: kernel: fix __cpu_suspend mm switch on warm-boot

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: <stable@vger.kernel.org> # 3.14+: 18ab7db
Cc: <stable@vger.kernel.org> # 3.14+: 714f599
Cc: <stable@vger.kernel.org> # 3.14+: c3684fb
Cc: <stable@vger.kernel.org> # 3.14+
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>

diff --git a/arch/arm64/kernel/suspend.c b/arch/arm64/kernel/suspend.c
index 3771b72..2d6b606 100644 (file)
--- 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();
 
                /*