[PATCH 2/2] arm64: efi: kaslr: Fix boot failure if efi_random_alloc() fails

[Benjamin Herrenschmidt]

On Tue, 2021-07-20 at 14:57 +0200, Ard Biesheuvel wrote:
> On Tue, 20 Jul 2021 at 13:14, Benjamin Herrenschmidt
> <benh at kernel.crashing.org> wrote:
> > If efi_random_alloc() fails, we still try to use EFI_KIMG_ALIGN
> > instead of MIN_KIMG_ALIGN to check the kernel image alignment,
> > which is incorrect, we need to fallback to MIN_KIMG_ALIGN (2M).
> > 
> 
> Why? Relocatable kernels can happily execute from any 64k aligned
> address, and the PE/COFF header carries this value of 64k as the
> minimum alignment.

This is not what I'm changing. If you look at the code before the
patch, it was *already* only allowing 64k alignment with kaslr enabled
(commit 7c116db24d94). There's even a big fat comment explaining why
though it could use more details.

In any case, the code isn't even testing for CONFIG_RELOCATABLE today,
it makes its decisions entirely based on kaslr and doesn't properly
handle the case where efi_random_alloc() fails.

This fixes it.

Cheers,
Ben.


> > This removes the not-that-useful min_kimg_align helper and instead
> > uses the appropriate aligment in the respective call sites:
> > 
> > efi_random_alloc() always wants EFI_KIMG_ALIGN as this is only
> > used when kaslr is on, and all other cases go into alignment
> > check code which always need to check (and enforce) MIN_KIMG_ALIGN
> > 
> > Signed-off-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>
> > Fixes: 7c116db24d94 (efi/libstub/arm64: Retain 2MB kernel Image alignment if !KASLR)
> > ---
> >  drivers/firmware/efi/libstub/arm64-stub.c | 27 ++++++++++-------------
> >  1 file changed, 12 insertions(+), 15 deletions(-)
> > 
> > diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
> > index 7bf0a7acae5e..e264ff90ba03 100644
> > --- a/drivers/firmware/efi/libstub/arm64-stub.c
> > +++ b/drivers/firmware/efi/libstub/arm64-stub.c
> > @@ -34,18 +34,6 @@ efi_status_t check_platform_features(void)
> >         return EFI_SUCCESS;
> >  }
> > 
> > -/*
> > - * Although relocatable kernels can fix up the misalignment with respect to
> > - * MIN_KIMG_ALIGN, the resulting virtual text addresses are subtly out of
> > - * sync with those recorded in the vmlinux when kaslr is disabled but the
> > - * image required relocation anyway. Therefore retain 2M alignment unless
> > - * KASLR is in use.
> > - */
> > -static u64 min_kimg_align(void)
> > -{
> > -       return efi_nokaslr ? MIN_KIMG_ALIGN : EFI_KIMG_ALIGN;
> > -}
> > -
> >  efi_status_t handle_kernel_image(unsigned long *image_addr,
> >                                  unsigned long *image_size,
> >                                  unsigned long *reserve_addr,
> > @@ -84,15 +72,24 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
> >                 /*
> >                  * If KASLR is enabled, and we have some randomness available,
> >                  * locate the kernel at a randomized offset in physical memory.
> > +                *
> > +                * In that case, we don't need to preserve the 2M alignment
> >                  */
> > -               status = efi_random_alloc(*reserve_size, min_kimg_align(),
> > +               status = efi_random_alloc(*reserve_size, EFI_KIMG_ALIGN,
> >                                           reserve_addr, phys_seed);
> >         } else {
> >                 status = EFI_OUT_OF_RESOURCES;
> >         }
> > 
> >         if (status != EFI_SUCCESS) {
> > -               if (IS_ALIGNED((u64)_text, min_kimg_align())) {
> > +               /*
> > +                * Although relocatable kernels can fix up the misalignment with respect to
> > +                * MIN_KIMG_ALIGN, the resulting virtual text addresses are subtly out of
> > +                * sync with those recorded in the vmlinux when kaslr is disabled but the
> > +                * image required relocation anyway. Therefore retain 2M alignment unless
> > +                * KASLR is in use.
> > +                */
> > +               if (IS_ALIGNED((u64)_text, MIN_KIMG_ALIGN)) {
> >                         /*
> >                          * Just execute from wherever we were loaded by the
> >                          * UEFI PE/COFF loader if the alignment is suitable.
> > @@ -103,7 +100,7 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
> >                 }
> > 
> >                 status = efi_allocate_pages_aligned(*reserve_size, reserve_addr,
> > -                                                   ULONG_MAX, min_kimg_align());
> > +                                                   ULONG_MAX, MIN_KIMG_ALIGN);
> > 
> >                 if (status != EFI_SUCCESS) {
> >                         efi_err("Failed to relocate kernel\n");
> > 
> > 
> > 
> 
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