Subject: [linux-next PATCH] arm64: fix kernel crash with 48-bit VA and 64KB granule

From: [email-masked] (Dennis Chen)
Date: Wed, 6 Jan 2016 16:52:19 +0800

On Wed, Jan 06, 2016 at 08:42:20AM +0100, Ard Biesheuvel wrote:
> On 6 January 2016 at 08:38, Dennis Chen <[email-masked]> wrote:
> > On Wed, Jan 06, 2016 at 08:13:24AM +0100, Ard Biesheuvel wrote:
> >> On 6 January 2016 at 07:14, Dennis Chen <[email-masked]> wrote:
> >> > On Tue, Jan 05, 2016 at 09:56:03AM +0000, Catalin Marinas wrote:
> >> >> On Tue, Jan 05, 2016 at 04:40:44PM +0800, Dennis Chen wrote:
> >> >> > On Tue, Jan 05, 2016 at 09:38:11AM +0100, Ard Biesheuvel wrote:
> >> >> > > >> On 5 January 2016 at 03:18, Dennis Chen <[email-masked]> wrote:
> >> >> > > >> > The commit 3400749b5a22 ("arm64/efi: refactor EFI init and runtime
> >> >> > > >> > code for reuse by 32-bit ARM") uses pgd_alloc() to allocate space for
> >> >> > > >> > efi_mm.pgd while not the static efi_pgd[], since this function will be
> >> >> > > >> > called with early_initcall, which results in the pgd_cache used by
> >> >> > > >> > pgd_alloc() has not been initialized yet, kernel will hang in this
> >> >> > > >> > case. This patch is trying to make the pgd_cache_init() called before
> >> >> > > >> > arm_enable_runtime_services() by changing its core_initcall to
> >> >> > > >> > early_initcall.
> >> >> > > >> >
> >> >> > > >> > Signed-off-by: Dennis Chen <[email-masked]>
> >> >> > > >> > Tested-by: Sudeep Holla <[email-masked]>
> >> >> > > >> >
> >> >> > > >> > Cc: Will Deacon <[email-masked]>
> >> >> > > >> > Cc: Catalin Marinas <[email-masked]>
> >> >> > > >> > Cc: Ard Biesheuvel <[email-masked]>
> >> >> > > >> > Cc: Sudeep Holla <[email-masked]>
> >> >> > > >> > ---
> >> >> > > >> >  arch/arm64/mm/pgd.c | 2 +-
> >> >> > > >> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >> >> > > >> >
> >> >> > > >> > diff --git a/arch/arm64/mm/pgd.c b/arch/arm64/mm/pgd.c
> >> >> > > >> > index cb3ba1b..859a788 100644
> >> >> > > >> > --- a/arch/arm64/mm/pgd.c
> >> >> > > >> > +++ b/arch/arm64/mm/pgd.c
> >> >> > > >> > @@ -56,4 +56,4 @@ static int __init pgd_cache_init(void)
> >> >> > > >> >                                               SLAB_PANIC, NULL);
> >> >> > > >> >         return 0;
> >> >> > > >> >  }
> >> >> > > >> > -core_initcall(pgd_cache_init);
> >> >> > > >> > +early_initcall(pgd_cache_init);
> >> >> [...]
> >> >> > > Well, since arm_enable_runtime_services() is an early_initcall()
> >> >> > > itself, how are you guaranteeing the ordering between the two? Link
> >> >> > > order?
> >> >> >
> >> >> > Link order.
> >> >>
> >> >> And can you explain how this works, what guarantees it gives?
> >> >>
> >> > You can take a look at include/asm-generic/vmlinux.ldx.h: INIT_CALLS macro,
> >> > for the init call section, early_initcall is the first chuck in the section,
> >> > followed by LEVEL[0-7]. For the same level, the layout order is determined
> >> > by the link order, ARCH is always precedence over the drivers. Catalin, are
> >> > you giving me a kernel examination? :)
> >> >
> >>
> >> We all know how initcalls are implemented. The question is how you are
> >> going to ensure that the early_initcall() that initializes the PGD
> >> cache is always invoked before the early_initcall() that creates the
> >> UEFI runtime page tables.
> >>
> >> And saying that the currently observed link order happens to be
> >> correct is not good enough. We need to be sure that, even if we change
> >> the link order, or switch to LTO at some point, things don't suddenly
> >> break again.
> >>
> > Well, if the build system changes the link order, it can't make sure it will break something
> > unexpectedly, needless to say the pgd_cache_init here at all.
>
> That is no excuse to introduce yet another failure mode.
>
> > Do you think the kernel
> > will change its currently link order policy? If the answer is yes, what's the benefit?
> >
>
> It does not matter what I think. You seem to think that the link order
> is set in stone, so it is you who should argue why that is a
> reasonable assumption.
>
OK, If you think the link order is volatile, how can you guarantee the arm_enable_runtime_services with early_initcall
always work in a volatile link order environment?

> --
> Ard.
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