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Subject[PATCH v2 1/1] x86/boot/compressed: Fix reloading of GDTR post-relocation
Date
Commit ef5a7b5eb13e ("efi/x86: Remove GDT setup from efi_main")
introduced GDT setup into the 32-bit kernel's startup_32, and reloads
the GDTR after relocating the kernel for paranoia's sake.

Commit 32d009137a56 ("x86/boot: Reload GDTR after copying to the end of
the buffer") introduced a similar GDTR reload in the 64-bit kernel.

The GDTR is adjusted by init_size - _end, however this may not be the
correct offset to apply if the kernel was loaded at a misaligned address
or below LOAD_PHYSICAL_ADDR, as in that case the decompression buffer
has an additional offset from the original load address.

This should never happen for a conformant bootloader, but we're being
paranoid anyway, so just store the new GDT address in there instead of
adding any offsets, which is simpler as well.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Fixes: ef5a7b5eb13e ("efi/x86: Remove GDT setup from efi_main")
Fixes: 32d009137a56 ("x86/boot: Reload GDTR after copying to the end of the buffer")
---
arch/x86/boot/compressed/head_32.S | 9 ++++-----
arch/x86/boot/compressed/head_64.S | 4 ++--
2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 356060c5332c..2f8138b71ea9 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -139,12 +139,11 @@ SYM_FUNC_START(startup_32)
/*
* The GDT may get overwritten either during the copy we just did or
* during extract_kernel below. To avoid any issues, repoint the GDTR
- * to the new copy of the GDT. EAX still contains the previously
- * calculated relocation offset of init_size - _end.
+ * to the new copy of the GDT.
*/
- leal gdt(%ebx), %edx
- addl %eax, 2(%edx)
- lgdt (%edx)
+ leal gdt(%ebx), %eax
+ movl %eax, 2(%eax)
+ lgdt (%eax)

/*
* Jump to the relocated address.
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index f7bacc4c1494..fcf8feaa57ea 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -456,8 +456,8 @@ trampoline_return:
* to the new copy of the GDT.
*/
leaq gdt64(%rbx), %rax
- subq %rbp, 2(%rax)
- addq %rbx, 2(%rax)
+ leaq gdt(%rbx), %rdx
+ movq %rdx, 2(%rax)
lgdt (%rax)

/*
--
2.24.1
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