On Thu, 6 Oct 2022 at 13:19, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Thu, 6 Oct 2022 at 13:13, Borislav Petkov <bp@xxxxxxxxx> 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
