Re: [PATCH efi-next 2/3] efi/arm64: clean EFI stub exit code from cache instead of avoiding it

[Ard Biesheuvel <ardb@kernel.org>]

On Fri, 28 Feb 2020 at 12:19, Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Fri, Feb 28, 2020 at 11:14:50AM +0000, Mark Rutland wrote:
> > On Fri, Feb 28, 2020 at 11:02:43AM +0100, Ard Biesheuvel wrote:
> > > Commit 9f9223778 ("efi/libstub/arm: Make efi_entry() an ordinary PE/COFF
> > > entrypoint") modified the handover code written in assembler, and for
> > > maintainability, aligned the logic with the logic used in the 32-bit ARM
> > > version, which is to avoid cache maintenance on the remaining instructions
> > > in the subroutine that will be executed with the MMU and caches off, and
> > > instead, branch into the relocated copy of the kernel image.
> > >
> > > However, this assumes that this copy is executable, and this means we
> > > expect EFI_LOADER_DATA regions to be executable as well, which is not
> > > a reasonable assumption to make, even if this is true for most UEFI
> > > implementations today.
> > >
> > > So change this back, and add a __flush_dcache_area() call to cover the
> > > remaining code in the subroutine.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > ---
> > >  arch/arm64/kernel/efi-entry.S  | 18 +++++++++---------
> > >  arch/arm64/kernel/image-vars.h |  2 +-
> > >  2 files changed, 10 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/arch/arm64/kernel/efi-entry.S b/arch/arm64/kernel/efi-entry.S
> > > index 4cfd03c35c49..d5dee064975f 100644
> > > --- a/arch/arm64/kernel/efi-entry.S
> > > +++ b/arch/arm64/kernel/efi-entry.S
> > > @@ -19,7 +19,8 @@ ENTRY(efi_enter_kernel)
> > >      * point stored in x0. Save those values in registers which are
> > >      * callee preserved.
> > >      */
> > > -   mov     x19, x0                 // relocated Image address
> > > +   ldr     w2, =stext_offset
> > > +   add     x19, x0, x2             // relocated Image entrypoint
> > >     mov     x20, x1                 // DTB address
> > >
> > >     /*
> > > @@ -29,15 +30,14 @@ ENTRY(efi_enter_kernel)
> > >     ldr     w1, =kernel_size
> > >     bl      __flush_dcache_area
> > >     ic      ialluis
> > > -   dsb     sy
> > >
> > >     /*
> > > -    * Jump across, into the copy of the image that we just cleaned
> > > -    * to the PoC, so that we can safely disable the MMU and caches.
> > > +    * Flush the remainder of this routine to the PoC
> >
> > Minor nit, but could we please say 'Clean' rather than 'Flush' here?
> >
> > Even better, we now have __clean_dcache_area_poc(), and can use that
> > too.
>
> ... or if that's better as a subsequent cleanup for consistency, that'd
> also be fine, and needn't block this patch.
>

Thanks Mark. That sounds like a worthwhile improvement to fold in.
I'll need to add the __efistub_ alias for it, but I'm touching
image-vars.h in this patch anyway.