Re: [PATCH] efi: x86: Wipe setup_data on pure EFI boot

Jason A. Donenfeld
2022-09-05T09:57:03Z

On Mon, Sep 5, 2022 at 11:55 AM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Sun, 4 Sept 2022 at 18:53, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >
> > From: Ard Biesheuvel <ardb@kernel.org>
> >
> > When booting the x86 kernel via EFI using the LoadImage/StartImage boot
> > services [as opposed to the deprecated EFI handover protocol], the setup
> > header is taken from the image directly, and given that EFI's LoadImage
> > has no Linux/x86 specific knowledge regarding struct bootparams or
> > struct setup_header, any absolute addresses in the setup header must
> > originate from the file and not from a prior loading stage.
> >
> > Since we cannot generally predict where LoadImage() decides to load an
> > image (*), such absolute addresses must be treated as suspect: even if a
> > prior boot stage intended to make them point somewhere inside the
> > [signed] image, there is no way to validate that, and if they point at
> > an arbitrary location in memory, the setup_data nodes will not be
> > covered by any signatures or TPM measurements either, and could be made
> > to contain an arbitrary sequence of SETUP_xxx nodes, which could
> > interfere quite badly with the early x86 boot sequence.
> >
> > (*) Note that, while LoadImage() does take a buffer/size tuple in
> > addition to a device path, which can be used to provide the image
> > contents directly, it will re-allocate such images, as the memory
> > footprint of an image is generally larger than the PE/COFF file
> > representation.
> >
> > Next, in order to allow hypervisors to still use setup_data in scenarios
> > where it may be useful, bump the x86 boot protocol version, so that
> > hypervisors, e.g. QEMU in the linked patch, can do the right thing
> > automatically depending on whether it is safe.
> >
> > Link: https://lore.kernel.org/qemu-devel/20220904165058.1140503-1-Jason@zx2c4.com/
> > Coauthored-by: Ard Biesheuvel <ardb@kernel.org>
> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> > ---
> > arch/x86/boot/header.S | 2 +-
> > drivers/firmware/efi/libstub/x86-stub.c | 7 +++++++
> > 2 files changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> > index f912d7770130..e4e2d6e33924 100644
> > --- a/arch/x86/boot/header.S
> > +++ b/arch/x86/boot/header.S
> > @@ -307,7 +307,7 @@ _start:
> > # Part 2 of the header, from the old setup.S
> >
> > .ascii "HdrS" # header signature
> > - .word 0x020f # header version number (>= 0x0105)
> > + .word 0x0210 # header version number (>= 0x0105)
> > # or else old loadlin-1.5 will fail)
> > .globl realmode_swtch
> > realmode_swtch: .word 0, 0 # default_switch, SETUPSEG
> > diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> > index 05ae8bcc9d67..9780f32a9f24 100644
> > --- a/drivers/firmware/efi/libstub/x86-stub.c
> > +++ b/drivers/firmware/efi/libstub/x86-stub.c
> > @@ -517,6 +517,13 @@ efi_status_t __efiapi efi_pe_entry(efi_handle_t handle,
> > hdr->ramdisk_image = 0;
> > hdr->ramdisk_size = 0;
> >
> > + /*
> > + * Disregard any setup data that was provided by the bootloader:
> > + * setup_data could be pointing anywhere, and we have no way of
> > + * authenticating or validating the payload.
> > + */
> > + hdr->setup_data = 0;
> > +
> > efi_stub_entry(handle, sys_table_arg, boot_params);
> > /* not reached */
> >
>
> if the x86 folks are ok with this, I would like to send this to
> cc:stable, but I imagine retroactively changing the header version
> number might be something they would prefer to avoid. In that case,
> better to split these up.

Just FYI, the rng seed thing is new in 6.0 anyway. That still leaves
the more obscure dtb one, and whatever else is used, but at least
doing this for only 6.0+ would take care of the rng seed one.

Jason