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

[Jon Medhurst (Tixy)]

On Thu, 2015-03-19 at 15:43 +0000, Will Deacon wrote:
> 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.

Which seems to happen in idle_task_exit() when you offline a cpu. This
patch is now in 4.0-rc5 and I get ...

# echo 0 > cpu1/online
[   51.750107] BUG: failure at ./arch/arm64/include/asm/mmu_context.h:74/switch_new_context()!
[   51.750111] Kernel panic - not syncing: BUG!
[   51.750116] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.0.0-rc5+ #3
[   51.750118] Hardware name: ARM Juno development board (r0) (DT)
[   51.750120] Call trace:
[   51.750131] [<ffffffc00008a4cc>] dump_backtrace+0x0/0x138
[   51.750136] [<ffffffc00008a620>] show_stack+0x1c/0x28
[   51.750143] [<ffffffc0006f8ed4>] dump_stack+0x80/0xc4
[   51.750146] [<ffffffc0006f58b8>] panic+0xe8/0x220
[   51.750151] [<ffffffc0000c70bc>] idle_task_exit+0x220/0x274
[   51.750155] [<ffffffc0000919bc>] cpu_die+0x20/0x7c
[   51.750159] [<ffffffc0000869fc>] arch_cpu_idle_dead+0x10/0x1c
[   51.750163] [<ffffffc0000d4de4>] cpu_startup_entry+0x2c4/0x36c
[   51.750167] [<ffffffc00009185c>] secondary_start_kernel+0x12c/0x13c

There seems to be quite a number of uses of cpu_switch_mm in the kernel.
I don't know if they have bugs which need fixing, or if it makes more
sense to replace the
	BUG_ON(pgd == swapper_pg_dir);
with 
	if (pgd != swapper_pg_dir)


> 
> Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> Fixes: f3cdfd239da5 ("arm64/efi: move SetVirtualAddressMap() to UEFI stub")
> Signed-off-by: Will Deacon <will.deacon at arm.com>
> ---
> 
> This patch gets armhf Debian booting again on my Juno (I guess 64-bit
> userspace tends to use virtual addresses that are high enough to avoid
> hitting the junk TLB entries!).
> 
>  arch/arm64/include/asm/proc-fns.h | 6 +++++-
>  arch/arm64/kernel/efi.c           | 6 +++++-
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/proc-fns.h b/arch/arm64/include/asm/proc-fns.h
> index 9a8fd84f8fb2..941c375616e2 100644
> --- 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 2b8d70164428..ab21e0d58278 100644
> --- 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();