Move the code that stores the arguments passed to the EFI entrypoint
into the .text section, so that it can be moved into a separate
compilation unit in a subsequent patch.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/boot/compressed/head_64.S | 34 ++++++++++++--------
1 file changed, 20 insertions(+), 14 deletions(-)
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index d33f060900d2..1ba2fc2357e6 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -303,24 +303,28 @@ SYM_FUNC_START(efi32_stub_entry)
popl %ecx
popl %edx
popl %esi
+ jmp efi32_entry
+SYM_FUNC_END(efi32_stub_entry)
+ .text
+SYM_FUNC_START_LOCAL(efi32_entry)
call 1f
-1: pop %ebp
- subl $ rva(1b), %ebp
-
- movl %esi, rva(efi32_boot_args+8)(%ebp)
-SYM_INNER_LABEL(efi32_pe_stub_entry, SYM_L_LOCAL)
- movl %ecx, rva(efi32_boot_args)(%ebp)
- movl %edx, rva(efi32_boot_args+4)(%ebp)
- movb $0, rva(efi_is64)(%ebp)
+1: pop %ebx
/* Save firmware GDTR and code/data selectors */
- sgdtl rva(efi32_boot_gdt)(%ebp)
- movw %cs, rva(efi32_boot_cs)(%ebp)
- movw %ds, rva(efi32_boot_ds)(%ebp)
+ sgdtl (efi32_boot_gdt - 1b)(%ebx)
+ movw %cs, (efi32_boot_cs - 1b)(%ebx)
+ movw %ds, (efi32_boot_ds - 1b)(%ebx)
/* Store firmware IDT descriptor */
- sidtl rva(efi32_boot_idt)(%ebp)
+ sidtl (efi32_boot_idt - 1b)(%ebx)
+
+ /* Store boot arguments */
+ leal (efi32_boot_args - 1b)(%ebx), %ebx
+ movl %ecx, 0(%ebx)
+ movl %edx, 4(%ebx)
+ movl %esi, 8(%ebx)
+ movb $0x0, 12(%ebx) // efi_is64
/* Disable paging */
movl %cr0, %eax
@@ -328,7 +332,8 @@ SYM_INNER_LABEL(efi32_pe_stub_entry, SYM_L_LOCAL)
movl %eax, %cr0
jmp startup_32
-SYM_FUNC_END(efi32_stub_entry)
+SYM_FUNC_END(efi32_entry)
+ __HEAD
#endif
.code64
@@ -831,7 +836,8 @@ SYM_FUNC_START(efi32_pe_entry)
*/
subl %esi, %ebx
movl %ebx, rva(image_offset)(%ebp) // save image_offset
- jmp efi32_pe_stub_entry
+ xorl %esi, %esi
+ jmp efi32_entry
2: popl %edi // restore callee-save registers
popl %ebx
--
2.35.1On Wed, Sep 21, 2022 at 04:54:08PM +0200, Ard Biesheuvel wrote:
> Move the code that stores the arguments passed to the EFI entrypoint
> into the .text section, so that it can be moved into a separate
> compilation unit in a subsequent patch.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> arch/x86/boot/compressed/head_64.S | 34 ++++++++++++--------
> 1 file changed, 20 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index d33f060900d2..1ba2fc2357e6 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -303,24 +303,28 @@ SYM_FUNC_START(efi32_stub_entry)
> popl %ecx
> popl %edx
> popl %esi
> + jmp efi32_entry
> +SYM_FUNC_END(efi32_stub_entry)
>
> + .text
> +SYM_FUNC_START_LOCAL(efi32_entry)
> call 1f
> -1: pop %ebp
> - subl $ rva(1b), %ebp
> -
> - movl %esi, rva(efi32_boot_args+8)(%ebp)
> -SYM_INNER_LABEL(efi32_pe_stub_entry, SYM_L_LOCAL)
> - movl %ecx, rva(efi32_boot_args)(%ebp)
> - movl %edx, rva(efi32_boot_args+4)(%ebp)
> - movb $0, rva(efi_is64)(%ebp)
> +1: pop %ebx
I'm guessing according to the EFI mixed mode calling convention, %ebx is
not a live register which gets overwritten here...?
Looking at efi32_pe_entry() from where this is called, %ebx looks live.
What am I missing?
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
On Thu, 6 Oct 2022 at 12:42, Borislav Petkov <bp@alien8.de> wrote: > > On Wed, Sep 21, 2022 at 04:54:08PM +0200, Ard Biesheuvel wrote: > > Move the code that stores the arguments passed to the EFI entrypoint > > into the .text section, so that it can be moved into a separate > > compilation unit in a subsequent patch. > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > --- > > arch/x86/boot/compressed/head_64.S | 34 ++++++++++++-------- > > 1 file changed, 20 insertions(+), 14 deletions(-) > > > > diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S > > index d33f060900d2..1ba2fc2357e6 100644 > > --- a/arch/x86/boot/compressed/head_64.S > > +++ b/arch/x86/boot/compressed/head_64.S > > @@ -303,24 +303,28 @@ SYM_FUNC_START(efi32_stub_entry) > > popl %ecx > > popl %edx > > popl %esi > > + jmp efi32_entry > > +SYM_FUNC_END(efi32_stub_entry) > > > > + .text > > +SYM_FUNC_START_LOCAL(efi32_entry) > > call 1f > > -1: pop %ebp > > - subl $ rva(1b), %ebp > > - > > - movl %esi, rva(efi32_boot_args+8)(%ebp) > > -SYM_INNER_LABEL(efi32_pe_stub_entry, SYM_L_LOCAL) > > - movl %ecx, rva(efi32_boot_args)(%ebp) > > - movl %edx, rva(efi32_boot_args+4)(%ebp) > > - movb $0, rva(efi_is64)(%ebp) > > +1: pop %ebx > > I'm guessing according to the EFI mixed mode calling convention, %ebx is > not a live register which gets overwritten here...? > > Looking at efi32_pe_entry() from where this is called, %ebx looks live. > > What am I missing? > efi32_pe_entry() preserves and restores the caller's value of %ebx, because from there, we might actually return control to the firmware. The value it keeps in %ebx itself is not live when it jumps to efi32_entry - it stores its value into image_offset, which is reloaded from memory at a later point. efi32_stub_entry() is the 'EFI handover protocol' entry point, which cannot return to the firmware (and we discard the return address already) so %ebx can be clobbered.
On Thu, Oct 06, 2022 at 12:56:09PM +0200, Ard Biesheuvel wrote:
> efi32_pe_entry() preserves and restores the caller's value of %ebx,
> because from there, we might actually return control to the firmware.
> The value it keeps in %ebx itself is not live when it jumps to
> efi32_entry - it stores its value into image_offset, which is reloaded
> from memory at a later point.
Hmm, might be prudent to have a comment there because it is using %ebx a
couple of insns before the JMP:
subl %esi, %ebx
^^^^
movl %ebx, rva(image_offset)(%ebp) // save image_offset
<--- I think you mean that after this, %ebx is not needed anymore?
xorl %esi, %esi
jmp efi32_entry
2: popl %edi // restore callee-save registers
popl %ebx
and this restores its original value ofc.
> efi32_stub_entry() is the 'EFI handover protocol' entry point, which
> cannot return to the firmware (and we discard the return address
> already) so %ebx can be clobbered.
That info would be good to have in a comment above it.
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
On Thu, 6 Oct 2022 at 13:13, Borislav Petkov <bp@alien8.de> wrote: > > On Thu, Oct 06, 2022 at 12:56:09PM +0200, Ard Biesheuvel wrote: > > efi32_pe_entry() preserves and restores the caller's value of %ebx, > > because from there, we might actually return control to the firmware. > > The value it keeps in %ebx itself is not live when it jumps to > > efi32_entry - it stores its value into image_offset, which is reloaded > > from memory at a later point. > > Hmm, might be prudent to have a comment there because it is using %ebx a > couple of insns before the JMP: > > subl %esi, %ebx > ^^^^ > movl %ebx, rva(image_offset)(%ebp) // save image_offset > > <--- I think you mean that after this, %ebx is not needed anymore? > Exactly. > xorl %esi, %esi > jmp efi32_entry > > 2: popl %edi // restore callee-save registers > popl %ebx > > and this restores its original value ofc. > > > efi32_stub_entry() is the 'EFI handover protocol' entry point, which > > cannot return to the firmware (and we discard the return address > > already) so %ebx can be clobbered. > > That info would be good to have in a comment above it. > Fair enough.
On Thu, 6 Oct 2022 at 13:19, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Thu, 6 Oct 2022 at 13:13, Borislav Petkov <bp@alien8.de> wrote:
> >
> > On Thu, Oct 06, 2022 at 12:56:09PM +0200, Ard Biesheuvel wrote:
> > > efi32_pe_entry() preserves and restores the caller's value of %ebx,
> > > because from there, we might actually return control to the firmware.
> > > The value it keeps in %ebx itself is not live when it jumps to
> > > efi32_entry - it stores its value into image_offset, which is reloaded
> > > from memory at a later point.
> >
> > Hmm, might be prudent to have a comment there because it is using %ebx a
> > couple of insns before the JMP:
> >
> > subl %esi, %ebx
> > ^^^^
> > movl %ebx, rva(image_offset)(%ebp) // save image_offset
> >
> > <--- I think you mean that after this, %ebx is not needed anymore?
> >
>
> Exactly.
>
> > xorl %esi, %esi
> > jmp efi32_entry
> >
> > 2: popl %edi // restore callee-save registers
> > popl %ebx
> >
> > and this restores its original value ofc.
> >
> > > efi32_stub_entry() is the 'EFI handover protocol' entry point, which
> > > cannot return to the firmware (and we discard the return address
> > > already) so %ebx can be clobbered.
> >
> > That info would be good to have in a comment above it.
> >
>
> Fair enough.
I'll add the below in the next revision
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -307,6 +307,19 @@ SYM_FUNC_START(efi32_stub_entry)
SYM_FUNC_END(efi32_stub_entry)
.text
+/*
+ * This is the common EFI stub entry point for mixed mode.
+ *
+ * Arguments: %ecx image handle
+ * %edx EFI system table pointer
+ * %esi struct bootparams pointer (or NULL when not using
+ * the EFI handover protocol)
+ *
+ * Since this is the point of no return for ordinary execution, no registers
+ * are considered live except for the function parameters. [Note that the EFI
+ * stub may still exit and return to the firmware using the Exit() EFI boot
+ * service.]
+ */
SYM_FUNC_START_LOCAL(efi32_entry)
call 1f
1: pop %ebx
@@ -837,7 +850,8 @@ SYM_FUNC_START(efi32_pe_entry)
subl %esi, %ebx
movl %ebx, rva(image_offset)(%ebp) // save image_offset
xorl %esi, %esi
- jmp efi32_entry
+ jmp efi32_entry // pass %ecx, %edx, %esi
+ // no other registers
remain live
2: popl %edi // restore callee-save registers
popl %ebx
On Thu, Oct 06, 2022 at 02:27:54PM +0200, Ard Biesheuvel wrote:
> I'll add the below in the next revision
LGTM.
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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