From: Ard Biesheuvel <ardb@kernel.org>
To: Borislav Petkov <bp@alien8.de>
Cc: linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Michael Roth <michael.roth@amd.com>
Subject: Re: [PATCH v2 02/16] x86/compressed: efi-mixed: move 32-bit entrypoint code into .text section
Date: Thu, 6 Oct 2022 14:27:54 +0200 [thread overview]
Message-ID: <CAMj1kXEkX=dZ0uUAdXoe5JC_Zv2+ndTMYwDiixYmWT-ip8OOOg@mail.gmail.com> (raw)
In-Reply-To: <CAMj1kXHi4TgGd=vicfxVOUnzDrhSfsiR=572L6cEwi47JO17jw@mail.gmail.com>
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
next prev parent reply other threads:[~2022-10-06 12:28 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-21 14:54 [PATCH v2 00/16] x86: head_64.S spring cleaning Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 01/16] x86/compressed: efi-mixed: rename efi_thunk_64.S to efi-mixed.S Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 02/16] x86/compressed: efi-mixed: move 32-bit entrypoint code into .text section Ard Biesheuvel
2022-10-06 10:42 ` Borislav Petkov
2022-10-06 10:56 ` Ard Biesheuvel
2022-10-06 11:13 ` Borislav Petkov
2022-10-06 11:19 ` Ard Biesheuvel
2022-10-06 12:27 ` Ard Biesheuvel [this message]
2022-10-07 8:56 ` Borislav Petkov
2022-09-21 14:54 ` [PATCH v2 03/16] x86/compressed: efi-mixed: move bootargs parsing out of 32-bit startup code Ard Biesheuvel
2022-10-06 11:03 ` Borislav Petkov
2022-10-06 11:29 ` Ard Biesheuvel
2022-10-07 9:30 ` Borislav Petkov
2022-09-21 14:54 ` [PATCH v2 04/16] x86/compressed: efi-mixed: move efi32_pe_entry into .text section Ard Biesheuvel
2022-11-17 15:57 ` Borislav Petkov
2022-11-17 16:06 ` Ard Biesheuvel
2022-11-17 17:08 ` Borislav Petkov
2022-09-21 14:54 ` [PATCH v2 05/16] x86/compressed: efi-mixed: move efi32_entry out of head_64.S Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 06/16] x86/compressed: efi-mixed: move efi32_pe_entry() " Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 07/16] x86/compressed: efi: merge multiple definitions of image_offset into one Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 08/16] x86/compressed: efi-mixed: simplify IDT/GDT preserve/restore Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 09/16] x86/compressed: avoid touching ECX in startup32_set_idt_entry() Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 10/16] x86/compressed: pull global variable ref up into startup32_load_idt() Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 11/16] x86/compressed: move startup32_load_idt() into .text section Ard Biesheuvel
2022-11-18 16:44 ` Borislav Petkov
2022-09-21 14:54 ` [PATCH v2 12/16] x86/compressed: move startup32_load_idt() out of head_64.S Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 13/16] x86/compressed: move startup32_check_sev_cbit() into .text Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 14/16] x86/compressed: move startup32_check_sev_cbit() out of head_64.S Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 15/16] x86/compressed: adhere to calling convention in get_sev_encryption_bit() Ard Biesheuvel
2022-09-21 14:54 ` [PATCH v2 16/16] x86/compressed: only build mem_encrypt.S if AMD_MEM_ENCRYPT=y Ard Biesheuvel
2022-11-18 18:26 ` [PATCH v2 00/16] x86: head_64.S spring cleaning Borislav Petkov
2022-11-18 23:31 ` Ard Biesheuvel
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