Commit e8972a76 authored by Ard Biesheuvel's avatar Ard Biesheuvel Committed by Borislav Petkov (AMD)
Browse files

x86/decompressor: Call trampoline as a normal function 



Move the long return to switch to 32-bit mode into the trampoline code
so it can be called as an ordinary function. This will allow it to be
called directly from C code in a subsequent patch.

While at it, reorganize the code somewhat to keep the prologue and
epilogue of the function together, making the code a bit easier to
follow. Also, given that the trampoline is now entered in 64-bit mode, a
simple RIP-relative reference can be used to take the address of the
exit point.

Signed-off-by: default avatarArd Biesheuvel <ardb@kernel.org>
Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Acked-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lore.kernel.org/r/20230807162720.545787-10-ardb@kernel.org
parent 00c6b097

arch/x86/boot/compressed/head_64.S

+35 −44
Original line number Diff line number Diff line
@@ -447,39 +447,8 @@ SYM_CODE_START(startup_64) 
	/* Save the trampoline address in RCX */

	movq	%rax, %rcx



	/* Set up 32-bit addressable stack */

	leaq	TRAMPOLINE_32BIT_STACK_END(%rcx), %rsp



	/*

	 * Preserve live 64-bit registers on the stack: this is necessary

	 * because the architecture does not guarantee that GPRs will retain

	 * their full 64-bit values across a 32-bit mode switch.

	 */

	pushq	%r15

	pushq	%rbp

	pushq	%rbx



	/*

	 * Push the 64-bit address of trampoline_return() onto the new stack.

	 * It will be used by the trampoline to return to the main code. Due to

	 * the 32-bit mode switch, it cannot be kept it in a register either.

	 */

	leaq	trampoline_return(%rip), %rdi

	pushq	%rdi



	/* Switch to compatibility mode (CS.L = 0 CS.D = 1) via far return */

	pushq	$__KERNEL32_CS

	leaq	TRAMPOLINE_32BIT_CODE_OFFSET(%rax), %rax

	pushq	%rax

	lretq

trampoline_return:

	/* Restore live 64-bit registers */

	popq	%rbx

	popq	%rbp

	popq	%r15



	/* Restore the stack, the 32-bit trampoline uses its own stack */

	leaq	rva(boot_stack_end)(%rbx), %rsp

	call	*%rax



	/*

	 * cleanup_trampoline() would restore trampoline memory.
@@ -561,7 +530,6 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated) 
	jmp	*%rax

SYM_FUNC_END(.Lrelocated)



	.code32

/*

 * This is the 32-bit trampoline that will be copied over to low memory.

 *
@@ -570,6 +538,39 @@ SYM_FUNC_END(.Lrelocated) 
 * Non zero RDX means trampoline needs to enable 5-level paging.

 */

SYM_CODE_START(trampoline_32bit_src)

	/*

	 * Preserve live 64-bit registers on the stack: this is necessary

	 * because the architecture does not guarantee that GPRs will retain

	 * their full 64-bit values across a 32-bit mode switch.

	 */

	pushq	%r15

	pushq	%rbp

	pushq	%rbx



	/* Set up 32-bit addressable stack and push the old RSP value */

	leaq	(TRAMPOLINE_32BIT_STACK_END - 8)(%rcx), %rbx

	movq	%rsp, (%rbx)

	movq	%rbx, %rsp



	/* Take the address of the trampoline exit code */

	leaq	.Lret(%rip), %rbx



	/* Switch to compatibility mode (CS.L = 0 CS.D = 1) via far return */

	pushq	$__KERNEL32_CS

	leaq	0f(%rip), %rax

	pushq	%rax

	lretq



.Lret:

	/* Restore the preserved 64-bit registers */

	movq	(%rsp), %rsp

	popq	%rbx

	popq	%rbp

	popq	%r15

	retq



	.code32

0:

	/* Set up data and stack segments */

	movl	$__KERNEL_DS, %eax

	movl	%eax, %ds
@@ -633,12 +634,9 @@ SYM_CODE_START(trampoline_32bit_src) 
1:

	movl	%eax, %cr4



	/* Calculate address of paging_enabled() once we are executing in the trampoline */

	leal	.Lpaging_enabled - trampoline_32bit_src + TRAMPOLINE_32BIT_CODE_OFFSET(%ecx), %eax



	/* Prepare the stack for far return to Long Mode */

	pushl	$__KERNEL_CS

	pushl	%eax

	pushl	%ebx



	/* Enable paging again. */

	movl	%cr0, %eax
@@ -648,12 +646,6 @@ SYM_CODE_START(trampoline_32bit_src) 
	lret

SYM_CODE_END(trampoline_32bit_src)



	.code64

SYM_FUNC_START_LOCAL_NOALIGN(.Lpaging_enabled)

	/* Return from the trampoline */

	retq

SYM_FUNC_END(.Lpaging_enabled)



	/*

         * The trampoline code has a size limit.

         * Make sure we fail to compile if the trampoline code grows
@@ -661,7 +653,6 @@ SYM_FUNC_END(.Lpaging_enabled) 
	 */

	.org	trampoline_32bit_src + TRAMPOLINE_32BIT_CODE_SIZE



	.code32

SYM_FUNC_START_LOCAL_NOALIGN(.Lno_longmode)

	/* This isn't an x86-64 CPU, so hang intentionally, we cannot continue */

1:

arch/x86/boot/compressed/pgtable.h

+1 −1
Original line number Diff line number Diff line
@@ -6,7 +6,7 @@ 
#define TRAMPOLINE_32BIT_PGTABLE_OFFSET	0



#define TRAMPOLINE_32BIT_CODE_OFFSET	PAGE_SIZE

#define TRAMPOLINE_32BIT_CODE_SIZE	0x80

#define TRAMPOLINE_32BIT_CODE_SIZE	0xA0



#define TRAMPOLINE_32BIT_STACK_END	TRAMPOLINE_32BIT_SIZE