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Subject[PATCH 5.7 125/265] efi/x86: Setup stack correctly for efi_pe_entry
From: Arvind Sankar <>

[ Upstream commit 41d90b0c1108d1e46c48cf79964636c553844f4c ]


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.

Fixes: 17054f492dfd ("efi/x86: Implement mixed mode boot without the handover protocol")
Signed-off-by: Arvind Sankar <>
Signed-off-by: Ard Biesheuvel <>
Signed-off-by: Sasha Levin <>
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 76d1d64d51e38..41f7922086220 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
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
+ pushl $__KERNEL_CS
pushl %eax

/* Enter paged protected Mode, activating Long Mode */
@@ -776,6 +784,7 @@ SYM_DATA_LOCAL(boot_heap, .fill BOOT_HEAP_SIZE, 1, 0)

.fill BOOT_STACK_SIZE, 1, 0
+ .balign 16
SYM_DATA_END_LABEL(boot_stack, SYM_L_LOCAL, boot_stack_end)

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