[External] Re: [PATCH 3/3] efistub: fix missed the initialization of gp

[Ard Biesheuvel]

On Wed, 6 Mar 2024 at 16:21, Palmer Dabbelt <palmer at dabbelt.com> wrote:
>
> On Wed, 06 Mar 2024 05:09:07 PST (-0800), Ard Biesheuvel wrote:
> > On Wed, 6 Mar 2024 at 14:02, Ard Biesheuvel <ardb at kernel.org> wrote:
> >>
> >> On Wed, 6 Mar 2024 at 13:34, yunhui cui <cuiyunhui at bytedance.com> wrote:
> >> >
> >> > Hi Ard,
> >> >
> >> > On Wed, Mar 6, 2024 at 5:36 PM Ard Biesheuvel <ardb at kernel.org> wrote:
> >> > >
> >> > > On Wed, 6 Mar 2024 at 09:56, Yunhui Cui <cuiyunhui at bytedance.com> wrote:
> >> > > >
> >> > > > Compared with gcc version 12, gcc version 13 uses the gp
> >> > > > register for compilation optimization, but the efistub module
> >> > > > does not initialize gp.
> >> > > >
> >> > > > Signed-off-by: Yunhui Cui <cuiyunhui at bytedance.com>
> >> > > > Co-Developed-by: Zhipeng Xu <xuzhipeng.1973 at bytedance.com>
> >> > >
> >> > > This needs a sign-off, and your signoff needs to come after.
> >> > >
> >> > > > ---
> >> > > >  arch/riscv/kernel/efi-header.S | 11 ++++++++++-
> >> > > >  1 file changed, 10 insertions(+), 1 deletion(-)
> >> > > >
> >> > > > diff --git a/arch/riscv/kernel/efi-header.S b/arch/riscv/kernel/efi-header.S
> >> > > > index 515b2dfbca75..fa17c08c092a 100644
> >> > > > --- a/arch/riscv/kernel/efi-header.S
> >> > > > +++ b/arch/riscv/kernel/efi-header.S
> >> > > > @@ -40,7 +40,7 @@ optional_header:
> >> > > >         .long   __pecoff_data_virt_end - __pecoff_text_end      // SizeOfInitializedData
> >> > > >  #endif
> >> > > >         .long   0                                       // SizeOfUninitializedData
> >> > > > -       .long   __efistub_efi_pe_entry - _start         // AddressOfEntryPoint
> >> > > > +       .long   _efistub_entry - _start         // AddressOfEntryPoint
> >> > > >         .long   efi_header_end - _start                 // BaseOfCode
> >> > > >  #ifdef CONFIG_32BIT
> >> > > >         .long  __pecoff_text_end - _start               // BaseOfData
> >> > > > @@ -121,4 +121,13 @@ section_table:
> >> > > >
> >> > > >         .balign 0x1000
> >> > > >  efi_header_end:
> >> > > > +
> >> > > > +       .global _efistub_entry
> >> > > > +_efistub_entry:
> >> > >
> >> > > This should go into .text or .init.text, not the header.
> >> > >
> >> > > > +       /* Reload the global pointer */
> >> > > > +       load_global_pointer
> >> > > > +
> >> > >
> >> > > What is supposed to happen here if CONFIG_SHADOW_CALL_STACK=y? The EFI
> >> > > stub Makefile removes the SCS CFLAGS, so the stub will be built
> >> > > without shadow call stack support, which I guess means that it might
> >> > > use GP as a global pointer as usual?
> >> > >
> >> > > > +       call __efistub_efi_pe_entry
> >> > > > +       ret
> >> > > > +
> >> > >
> >> > > You are returning to the firmware here, but after modifying the GP
> >> > > register. Shouldn't you restore it to its old value?
> >> > There is no need to restore the value of the gp register. Where gp is
> >> > needed, the gp register must first be initialized. And here is the
> >> > entry.
> >> >
> >>
> >> But how should the firmware know that GP was corrupted after calling
> >> the kernel's EFI entrypoint? The EFI stub can return to the firmware
> >> if it encounters any errors while still running in the EFI boot
> >> services.
> >>
> >
> > Actually, I wonder if GP can be modified at all before
> > ExitBootServices(). The EFI timer interrupt is still live at this
> > point, and so the firmware is being called behind your back, and might
> > rely on GP retaining its original value.
>
> [A few of us are talking on IRC as I'm writing this...]
>
> The UEFI spec says "UEFI firmware must neither trust the
> values of tp and gp nor make an assumption of owning the write access to
> these register in any circumstances".  It's kind of vague what "UEFI
> firmware" means here, but I think it's reasonable to assume that the
> kernel (and thus the EFI stub) is not included there.
>
> So under that interpretation, the kernel (including the EFI stub) would
> be allowed to overwrite GP with whatever it wants.
>

OK, so even if the UEFI spec seems to suggest that using GP in EFI
applications such as the Linux EFI stub should be safe, I'd still like
to understand why this change is necessary. The patches you are
reverting are supposed to ensure that a) the compiler does not
generate references that can be relaxed to GP based ones, and b) no
R_RISCV_RELAX relocations are present in any of the code that runs in
the context of the EFI firmware.

Are you still seeing GP based symbol references? Is there C code that
gets pulled into the EFI stub that uses GP based relocations perhaps?
(see list below). If any of those are implemented in C, they should
not be used by the EFI stub directly unless they are guaranteed to be
uninstrumented and callable at arbitrary offsets other than the one
they were linked to run at.


__efistub_memcmp         = memcmp;
__efistub_memchr         = memchr;
__efistub_memcpy         = memcpy;
__efistub_memmove        = memmove;
__efistub_memset         = memset;
__efistub_strlen         = strlen;
__efistub_strnlen        = strnlen;
__efistub_strcmp         = strcmp;
__efistub_strncmp        = strncmp;
__efistub_strrchr        = strrchr;
__efistub___memcpy       = memcpy;
__efistub___memmove      = memmove;
__efistub___memset       = memset;
__efistub__start         = _start;
__efistub__start_kernel  = _start_kernel;

(from arch/riscv/kernel/image-vars.h)