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From: Arvind Sankar <nivedita@alum.mit.edu>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: linux-efi <linux-efi@vger.kernel.org>, X86 ML <x86@kernel.org>
Subject: [PATCH v2] efi/x86: Setup stack correctly for efi_pe_entry
Date: Wed, 17 Jun 2020 09:19:57 -0400
Message-ID: <20200617131957.2507632-1-nivedita@alum.mit.edu> (raw)
In-Reply-To: <CAMj1kXGovNeVTm3sSwpk6Lqk=JkBq_gV0t3WKd1=kJ11+C2e5g@mail.gmail.com>

Commit
  17054f492dfd ("efi/x86: Implement mixed mode boot without the handover protocol")
introduced a new entry point for the EFI stub to be booted in mixed mode
on 32-bit firmware.

When entered via efi32_pe_entry, control is first transferred to
startup_32 to setup for the switch to long mode, and then the EFI stub
proper is entered via efi_pe_entry. efi_pe_entry is an MS ABI function,
and the ABI requires 32 bytes of shadow stack space to be allocated by
the caller, as well as the stack being aligned to 8 mod 16 on entry.

Allocate 40 bytes on the stack before switching to 64-bit mode when
calling efi_pe_entry to account for this.

For robustness, explicitly align boot_stack_end to 16 bytes. It is
currently implicitly aligned since .bss is cacheline-size aligned,
head_64.o is the first object file with a .bss section, and the heap and
boot sizes are aligned.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
---
 arch/x86/boot/compressed/head_64.S | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index e821a7d7d5c4..97d37f0a34f5 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -213,7 +213,6 @@ SYM_FUNC_START(startup_32)
 	 * We place all of the values on our mini stack so lret can
 	 * used to perform that far jump.
 	 */
-	pushl	$__KERNEL_CS
 	leal	startup_64(%ebp), %eax
 #ifdef CONFIG_EFI_MIXED
 	movl	efi32_boot_args(%ebp), %edi
@@ -224,11 +223,20 @@ SYM_FUNC_START(startup_32)
 	movl	efi32_boot_args+8(%ebp), %edx	// saved bootparams pointer
 	cmpl	$0, %edx
 	jnz	1f
+	/*
+	 * efi_pe_entry uses MS calling convention, which requires 32 bytes of
+	 * shadow space on the stack even if all arguments are passed in
+	 * registers. We also need an additional 8 bytes for the space that
+	 * would be occupied by the return address, and this also results in
+	 * the correct stack alignment for entry.
+	 */
+	subl	$40, %esp
 	leal	efi_pe_entry(%ebp), %eax
 	movl	%edi, %ecx			// MS calling convention
 	movl	%esi, %edx
 1:
 #endif
+	pushl	$__KERNEL_CS
 	pushl	%eax
 
 	/* Enter paged protected Mode, activating Long Mode */
@@ -784,6 +792,7 @@ SYM_DATA_LOCAL(boot_heap,	.fill BOOT_HEAP_SIZE, 1, 0)
 
 SYM_DATA_START_LOCAL(boot_stack)
 	.fill BOOT_STACK_SIZE, 1, 0
+	.balign 16
 SYM_DATA_END_LABEL(boot_stack, SYM_L_LOCAL, boot_stack_end)
 
 /*
-- 
2.26.2


  reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-26 17:02 [PATCH 0/1] efi/x86: Use firmware stack for mixed-mode EFI stub Arvind Sankar
2020-05-26 17:02 ` [PATCH 1/1] " Arvind Sankar
2020-06-15  9:58   ` Ard Biesheuvel
2020-06-15 15:56     ` Arvind Sankar
2020-06-16 18:48       ` Arvind Sankar
2020-06-16 18:50         ` Ard Biesheuvel
2020-06-16 19:48           ` [PATCH] efi/x86: Setup stack correctly for efi_pe_entry Arvind Sankar
2020-06-16 22:06             ` Ard Biesheuvel
2020-06-17 10:33               ` Ard Biesheuvel
2020-06-17 13:19                 ` Arvind Sankar [this message]
2020-06-17 13:26                   ` [PATCH v2] " Ard Biesheuvel

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