[PATCH 2/2] arm64/efi: efistub: get text offset and image size from the Image header

[Ard Biesheuvel]

On 14 July 2014 20:29, Mark Rutland <mark.rutland at arm.com> wrote:
> On Mon, Jul 14, 2014 at 06:35:32PM +0100, Ard Biesheuvel wrote:
>> On 14 July 2014 18:54, Mark Rutland <mark.rutland at arm.com> wrote:
>> > Hi Ard,
>> >
>> > On Mon, Jul 14, 2014 at 05:17:51PM +0100, Ard Biesheuvel wrote:
>> >> The EFI stub for arm64 needs to behave like an ordinary bootloader in the sense
>> >> that it needs to use the EFI environment and the Image header at runtime and
>> >> not rely on the linker or preprocessor to produce values for text offset,
>> >> image size and kernel size.
>> >
>> > Could you elaborate on _why_ we can't do that, given it's linked into
>> > the kernel Image?
>> >
>> > Are we splitting the stub from the kernel? What's going on?
>> >
>>
>> Perhaps Leif can chime in here? I was under the impression that you
>> were aligned in this regard.
>
> Perhaps we were, and I can see reasons for doing this (e.g. as a step
> towards making it possible to boot a BE kernel using UEFI), but the
> commit message doesn't mention any such reason. For the sake of my poor
> recollection and that of others, some words in the commit message would
> be helpful.
>

OK

>> >> This patch also fixes the corner case where Image happens to be loaded
>> >> at exactly the right offset, but the allocation is actually too small
>> >> to satisfy the requirement imposed by image_size as set in the header.
>> >
>> > It's not really imposed by image_size. While it's described by
>> > image_size it's imposed by the existence of the BSS section and the
>> > initial page tables.
>> >
>>
>> Yes, that is true. But from stub POV, it doesn't matter /why/
>> image_size has a particular value.
>
> Sure, the stub is a piece of software with no particular knowledge of
> anything. However, those of us reading the commit message are not, and
> for our sake it would be nice to have a description in case we care as
> to /why/ image_size has a particular value and why that value happens to
> be useful here.
>
> As far as I can see, we needed to allocate memory for the BSS even
> before the image_size field was introduced, so this isn't a new
> requirement.
>
> All that's needed is a change to the wording of the commit message to
> explain that we're trying to allocate space for (runtime-initialised)
> data between _edata and _end rather than how we find out how much space
> we need for that (reading the image_size field).
>

OK

>> >>
>> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> >> ---
>> >>  arch/arm64/kernel/Makefile   |  2 --
>> >>  arch/arm64/kernel/efi-stub.c | 29 ++++++++++++++++-------------
>> >>  2 files changed, 16 insertions(+), 15 deletions(-)
>> >>
>> >> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
>> >> index cdaedad3afe5..99b676eeeb0f 100644
>> >> --- a/arch/arm64/kernel/Makefile
>> >> +++ b/arch/arm64/kernel/Makefile
>> >> @@ -4,8 +4,6 @@
>> >>
>> >>  CPPFLAGS_vmlinux.lds := -DTEXT_OFFSET=$(TEXT_OFFSET)
>> >>  AFLAGS_head.o                := -DTEXT_OFFSET=$(TEXT_OFFSET)
>> >> -CFLAGS_efi-stub.o    := -DTEXT_OFFSET=$(TEXT_OFFSET) \
>> >> -                        -I$(src)/../../../scripts/dtc/libfdt
>> >>
>> >>  CFLAGS_REMOVE_ftrace.o = -pg
>> >>  CFLAGS_REMOVE_insn.o = -pg
>> >> diff --git a/arch/arm64/kernel/efi-stub.c b/arch/arm64/kernel/efi-stub.c
>> >> index 9b61d66e2d20..4ba90b2ef677 100644
>> >> --- a/arch/arm64/kernel/efi-stub.c
>> >> +++ b/arch/arm64/kernel/efi-stub.c
>> >> @@ -11,8 +11,7 @@
>> >>   */
>> >>  #include <linux/efi.h>
>> >>  #include <asm/efi.h>
>> >> -#include <asm/sections.h>
>> >> -
>> >> +#include <asm/image_hdr.h>
>> >>
>> >>  efi_status_t handle_kernel_image(efi_system_table_t *sys_table,
>> >>                                unsigned long *image_addr,
>> >> @@ -23,24 +22,28 @@ efi_status_t handle_kernel_image(efi_system_table_t *sys_table,
>> >>                                efi_loaded_image_t *image)
>> >>  {
>> >>       efi_status_t status;
>> >> -     unsigned long kernel_size, kernel_memsize = 0;
>> >> +     struct arm64_image_hdr *hdr = (struct arm64_image_hdr *)*image_addr;
>> >> +
>> >> +     /* make sure image_addr points to an arm64 kernel Image */
>> >> +     if (!arm64_image_hdr_check(hdr)) {
>> >> +             pr_efi_err(sys_table, "Kernel Image header check failed\n");
>> >> +             return EFI_LOAD_ERROR;
>> >> +     }
>> >
>> > Surely this should always be the case if the stub is linked into the
>> > kernel?
>> >
>> > It would be nice to know the rationale for this.
>> >
>>
>> Well, there is an actual issue addressed by this: the PE/COFF .text
>> section covers everything from stext to _edata, so it does not cover
>> the header itself. In fact, PE/COFF does not allow the headers
>> themselves to be covered by a section (or at least, the Tianocore/EDK2
>> UEFI implementation does not). So the pointer points *outside* of the
>> .text section, and we are assuming that whatever is at that [negative]
>> offset in the file was also copied to the same offset in memory.(For
>> instance, there is no reason to assume that all implementations of EFI
>> will copy data that is not part of any section to RAM when booting an
>> image located in NOR flash)
>
> If we are making assumptions which aren't always valid, then why the
> hell are we making them in the first place, and then trying to validate
> them? How does reading an address that we have absolutely no guarantee
> as to the contents of solve that problem?
>

Isn't that the whole point of a magic number?

> This sounds amazingly fragile, and as far as I can tell the header check
> doesn't address the issue so much as paper over it with a failure that's
> not even guaranteed to be graceful (e.g. if no memory exists at the
> negative offset we're accessing we will simply explode).
>

The memory is guaranteed to exist, only the header is not guaranteed
to have been copied into it.

> Is this the first step towards splitting the stub form the kernel
> proper such that we can rely on the image header being present?
>
> If that's the case then please put that in the commit message and cover,
> because it's not obvious and there's no point wasting time arguing over
> whether to put a piece of useful context into the commit message.
>

Well, the more I think of it, the more I am leaning towards dropping
this patch and taking the opposite approach.
Even if we build the stub LE and link it into a BE kernel, we should
be able to poke the right values into the right places at build time
rather than rely on a header of which we can't be even sure that it
exists in the expected place.

>>
>> >>
>> >>       /* Relocate the image, if required. */
>> >> -     kernel_size = _edata - _text;
>> >> -     if (*image_addr != (dram_base + TEXT_OFFSET)) {
>> >> -             kernel_memsize = kernel_size + (_end - _edata) + TEXT_OFFSET;
>> >> -             status = efi_low_alloc(sys_table, kernel_memsize, SZ_2M,
>> >> +     if (*image_addr != (dram_base + hdr->text_offset) ||
>> >> +         image->image_size < hdr->image_size) {
>> >
>> > As far as I can tell the size of the Image (image->image_size) is always
>> > going to be less than the effective run time image size
>> > (hdr->image_size).
>> >
>>
>> Currently, yes.
>>
>> > The SizeOfImage field in head.S which I assume image->image_size is
>> > derived from (not having a UEFI spec in front of me) seems to cover
>> > everything up to _edata but skips the BSS, and initial page tables, but
>> > this is covered by the header's image_size field.
>> >
>> > Am I missing something? Surely we _always_ expect image->image_size to
>> > be smaller than hdr->image_size?
>> >
>>
>> At some point, we may extend the virtual size of .text all the way to _end.
>> (Without growing the actual payload, the remainder will be zero
>> initialized by the loader)
>
> Surely either way we know which is going to be the case? Either it's
> smaller (as it is now) or we've padded it (which we have not done).
>
> Why not either assume it's smaller or change the virtual size of .text?
>

Well, the trouble is that we use more memory by padding the .text
section, leaving less room for the reallocation which we need to do in
99% of the cases. This is somewhat of a waste if EFI loaded the Image
close to base of DRAM.

But after seeing Mark Salter's stub patch for APM Mustang, this needs
major rework anyway, but my position is now that we should not touch
the header *at all* from the stub.

-- 
Ard.