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Date: Sun, 2 Feb 2020 12:13:48 -0500
From: Arvind Sankar <nivedita@...m.mit.edu>
To: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>
Cc: linux-efi@...r.kernel.org, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/7] efi/x86: Don't depend on firmware GDT layout
At handover entry in efi32_stub_entry, the firmware's GDT is still
installed. We save the GDTR for later use in __efi64_thunk but we are
assuming that descriptor 2 (__KERNEL_CS) is a valid 32-bit code segment
descriptor and that descriptor 3 (__KERNEL_DS/__BOOT_DS) is a valid data
segment descriptor.
This happens to be true for OVMF (it actually uses descriptor 1 for data
segments, but descriptor 3 is also setup as data), but we shouldn't
depend on this being the case.
Fix this by saving the code and data selectors in addition to the GDTR
in efi32_stub_entry, and restoring them in __efi64_thunk before calling
the firmware. The UEFI specification guarantees that selectors will be
flat, so using the DS selector for all the segment registers should be
enough.
We also need to install our own GDT before initializing segment
registers in startup_32, so move the GDT load up to the beginning of the
function.
Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
---
arch/x86/boot/compressed/efi_thunk_64.S | 29 +++++++++++++++++++-----
arch/x86/boot/compressed/head_64.S | 30 +++++++++++++++----------
2 files changed, 42 insertions(+), 17 deletions(-)
diff --git a/arch/x86/boot/compressed/efi_thunk_64.S b/arch/x86/boot/compressed/efi_thunk_64.S
index 8fb7f6799c52..2b2049259619 100644
--- a/arch/x86/boot/compressed/efi_thunk_64.S
+++ b/arch/x86/boot/compressed/efi_thunk_64.S
@@ -54,11 +54,16 @@ SYM_FUNC_START(__efi64_thunk)
* Switch to gdt with 32-bit segments. This is the firmware GDT
* that was installed when the kernel started executing. This
* pointer was saved at the EFI stub entry point in head_64.S.
+ *
+ * Pass the saved DS selector to the 32-bit code, and use far return to
+ * restore the saved CS selector.
*/
leaq efi32_boot_gdt(%rip), %rax
lgdt (%rax)
- pushq $__KERNEL_CS
+ movzwl efi32_boot_ds(%rip), %edx
+ movzwq efi32_boot_cs(%rip), %rax
+ pushq %rax
leaq efi_enter32(%rip), %rax
pushq %rax
lretq
@@ -73,6 +78,10 @@ SYM_FUNC_START(__efi64_thunk)
movl %ebx, %es
pop %rbx
movl %ebx, %ds
+ /* Clear out 32-bit selector from FS and GS */
+ xorl %ebx, %ebx
+ movl %ebx, %fs
+ movl %ebx, %gs
/*
* Convert 32-bit status code into 64-bit.
@@ -92,10 +101,12 @@ SYM_FUNC_END(__efi64_thunk)
* The stack should represent the 32-bit calling convention.
*/
SYM_FUNC_START_LOCAL(efi_enter32)
- movl $__KERNEL_DS, %eax
- movl %eax, %ds
- movl %eax, %es
- movl %eax, %ss
+ /* Load firmware selector into data and stack segment registers */
+ movl %edx, %ds
+ movl %edx, %es
+ movl %edx, %fs
+ movl %edx, %gs
+ movl %edx, %ss
/* Reload pgtables */
movl %cr3, %eax
@@ -157,6 +168,14 @@ SYM_DATA_START(efi32_boot_gdt)
.quad 0
SYM_DATA_END(efi32_boot_gdt)
+SYM_DATA_START(efi32_boot_cs)
+ .word 0
+SYM_DATA_END(efi32_boot_cs)
+
+SYM_DATA_START(efi32_boot_ds)
+ .word 0
+SYM_DATA_END(efi32_boot_ds)
+
SYM_DATA_START(efi_gdt64)
.word efi_gdt64_end - efi_gdt64
.long 0 /* Filled out by user */
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index bd44d89540d3..c56b30bd9c7b 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -54,10 +54,6 @@ SYM_FUNC_START(startup_32)
*/
cld
cli
- movl $(__BOOT_DS), %eax
- movl %eax, %ds
- movl %eax, %es
- movl %eax, %ss
/*
* Calculate the delta between where we were compiled to run
@@ -72,10 +68,20 @@ SYM_FUNC_START(startup_32)
1: popl %ebp
subl $1b, %ebp
+ /* Load new GDT with the 64bit segments using 32bit descriptor */
+ addl %ebp, gdt+2(%ebp)
+ lgdt gdt(%ebp)
+
+ /* Load segment registers with our descriptors */
+ movl $__BOOT_DS, %eax
+ movl %eax, %ds
+ movl %eax, %es
+ movl %eax, %fs
+ movl %eax, %gs
+ movl %eax, %ss
+
/* setup a stack and make sure cpu supports long mode. */
- movl $boot_stack_end, %eax
- addl %ebp, %eax
- movl %eax, %esp
+ leal boot_stack_end(%ebp), %esp
call verify_cpu
testl %eax, %eax
@@ -112,10 +118,6 @@ SYM_FUNC_START(startup_32)
* Prepare for entering 64 bit mode
*/
- /* Load new GDT with the 64bit segments using 32bit descriptor */
- addl %ebp, gdt+2(%ebp)
- lgdt gdt(%ebp)
-
/* Enable PAE mode */
movl %cr4, %eax
orl $X86_CR4_PAE, %eax
@@ -232,9 +234,13 @@ SYM_FUNC_START(efi32_stub_entry)
movl %ecx, efi32_boot_args(%ebp)
movl %edx, efi32_boot_args+4(%ebp)
- sgdtl efi32_boot_gdt(%ebp)
movb $0, efi_is64(%ebp)
+ /* Save firmware GDTR and code/data selectors */
+ sgdtl efi32_boot_gdt(%ebp)
+ movw %cs, efi32_boot_cs(%ebp)
+ movw %ds, efi32_boot_ds(%ebp)
+
/* Disable paging */
movl %cr0, %eax
btrl $X86_CR0_PG_BIT, %eax
--
2.24.1
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