arm64: efi: don't restore TTBR0 if active_mm points at init_mm

init_mm isn't a normal mm: it has swapper_pg_dir as its pgd (which
contains kernel mappings) and is used as the active_mm for the idle
thread.

When restoring the pgd after an EFI call, we write current->active_mm
into TTBR0. If the current task is actually the idle thread (e.g. when
initialising the EFI RTC before entering userspace), then the TLB can
erroneously populate itself with junk global entries as a result of
speculative table walks.

When we do eventually return to userspace, the task can end up hitting
these junk mappings leading to lockups, corruption or crashes.

This patch fixes the problem in the same way as the CPU suspend code by
ensuring that we never switch to the init_mm in efi_set_pgd and instead
point TTBR0 at the zero page. A check is also added to cpu_switch_mm to
BUG if we get passed swapper_pg_dir.

Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Fixes: f3cdfd239da5 ("arm64/efi: move SetVirtualAddressMap() to UEFI stub")
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/include/asm/proc-fns.h b/arch/arm64/include/asm/proc-fns.h
index 9a8fd84f8fb2b67fe4bc2d845e7f6b584cbf0799..941c375616e2072d3b9fbf6442426c3491f00c01 100644 (file)
--- a/arch/arm64/include/asm/proc-fns.h
+++ b/arch/arm64/include/asm/proc-fns.h
@@ -39,7 +39,11 @@ extern u64 cpu_do_resume(phys_addr_t ptr, u64 idmap_ttbr);
 
 #include <asm/memory.h>
 
-#define cpu_switch_mm(pgd,mm) cpu_do_switch_mm(virt_to_phys(pgd),mm)
+#define cpu_switch_mm(pgd,mm)                          \
+do {                                                   \
+       BUG_ON(pgd == swapper_pg_dir);                  \
+       cpu_do_switch_mm(virt_to_phys(pgd),mm);         \
+} while (0)
 
 #define cpu_get_pgd()                                  \
 ({                                                     \

diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
index 2b8d70164428010911cff2b66791c5d6c4b1dad4..ab21e0d58278825e5dfc0199cd18b853f07232c1 100644 (file)
--- a/arch/arm64/kernel/efi.c
+++ b/arch/arm64/kernel/efi.c
@@ -337,7 +337,11 @@ core_initcall(arm64_dmi_init);
 
 static void efi_set_pgd(struct mm_struct *mm)
 {
-       cpu_switch_mm(mm->pgd, mm);
+       if (mm == &init_mm)
+               cpu_set_reserved_ttbr0();
+       else
+               cpu_switch_mm(mm->pgd, mm);
+
        flush_tlb_all();
        if (icache_is_aivivt())
                __flush_icache_all();