[PATCH v7 01/22] x86/decompressor: Don't rely on upper 32 bits of GPRs being preserved

[Ard Biesheuvel]

The 4-to-5 level mode switch trampoline disables long mode and paging in
order to be able to flick the LA57 bit. According to section 3.4.1.1 of
the x86 architecture manual [0], we should not rely on 64-bit GPRs
retaining the upper 32 bits of their contents across such a mode switch.

Given that RBP, RBX and RSI are live at this point, let's preserve them
on the stack, along with the return address that might be above 4G as
well.

[0] Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1: Basic Architecture

  "Because the upper 32 bits of 64-bit general-purpose registers are
   undefined in 32-bit modes, the upper 32 bits of any general-purpose
   register are not preserved when switching from 64-bit mode to a 32-bit
   mode (to protected mode or compatibility mode). Software must not
   depend on these bits to maintain a value after a 64-bit to 32-bit
   mode switch."

Fixes: 194a9749c73d650c ("x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G")
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 arch/x86/boot/compressed/head_64.S | 23 +++++++++++++++-----
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 03c4328a88cbd5d0..f7c11a0018477de8 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -459,11 +459,22 @@ 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
+
 	/*
-	 * Load the address of trampoline_return() into RDI.
-	 * It will be used by the trampoline to return to the main code.
+	 * Load the address of trampoline_return() into RDI and push it onto
+	 * the stack so it will survive 32-bit truncation due to the 32-bit
+	 * protected mode switch. It will be used by the trampoline to return
+	 * to the main code.
 	 */
 	leaq	trampoline_return(%rip), %rdi
+	pushq	%rdi
+
+	/* Preserve other live 64-bit registers */
+	pushq	%rbp
+	pushq	%rbx
+	pushq	%rsi
 
 	/* Switch to compatibility mode (CS.L = 0 CS.D = 1) via far return */
 	pushq	$__KERNEL32_CS
@@ -592,9 +603,6 @@ SYM_CODE_START(trampoline_32bit_src)
 	movl	%eax, %ds
 	movl	%eax, %ss
 
-	/* Set up new stack */
-	leal	TRAMPOLINE_32BIT_STACK_END(%ecx), %esp
-
 	/* Disable paging */
 	movl	%cr0, %eax
 	btrl	$X86_CR0_PG_BIT, %eax
@@ -671,7 +679,10 @@ SYM_CODE_END(trampoline_32bit_src)
 	.code64
 SYM_FUNC_START_LOCAL_NOALIGN(.Lpaging_enabled)
 	/* Return from the trampoline */
-	jmp	*%rdi
+	popq	%rsi
+	popq	%rbx
+	popq	%rbp
+	retq
 SYM_FUNC_END(.Lpaging_enabled)
 
 	/*
-- 
2.39.2