This is a note to let you know that I've just added the patch titled
x86/decompressor: Don't rely on upper 32 bits of GPRs being preserved
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-decompressor-don-t-rely-on-upper-32-bits-of-gprs.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 6fd234885c819b460c628881f1392480c7771002
Author: Ard Biesheuvel <ardb@xxxxxxxxxx>
Date: Mon Aug 7 18:26:58 2023 +0200
x86/decompressor: Don't rely on upper 32 bits of GPRs being preserved
[ Upstream commit 264b82fdb4989cf6a44a2bcd0c6ea05e8026b2ac ]
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], 64-bit GPRs might not retain the upper
32 bits of their contents across such a mode switch.
Given that RBP, RBX and RSI are live at this point, 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>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Link: https://lore.kernel.org/r/20230807162720.545787-2-ardb@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 95ee795d97964..d8164e6abaaff 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -381,11 +381,25 @@ ENTRY(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 %rbp
+ pushq %rbx
+ pushq %rsi
+
/*
- * Load the address of trampoline_return() into RDI.
- * It will be used by the trampoline to return to the main code.
+ * 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
@@ -393,6 +407,11 @@ ENTRY(startup_64)
pushq %rax
lretq
trampoline_return:
+ /* Restore live 64-bit registers */
+ popq %rsi
+ popq %rbx
+ popq %rbp
+
/* Restore the stack, the 32-bit trampoline uses its own stack */
leaq boot_stack_end(%rbx), %rsp
@@ -573,7 +592,7 @@ SYM_FUNC_END(.Lrelocated)
/*
* This is the 32-bit trampoline that will be copied over to low memory.
*
- * RDI contains the return address (might be above 4G).
+ * Return address is at the top of the stack (might be above 4G).
* ECX contains the base address of the trampoline memory.
* Non zero RDX means trampoline needs to enable 5-level paging.
*/
@@ -583,9 +602,6 @@ ENTRY(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
@@ -644,7 +660,7 @@ ENTRY(trampoline_32bit_src)
.code64
SYM_FUNC_START_LOCAL_NOALIGN(.Lpaging_enabled)
/* Return from the trampoline */
- jmp *%rdi
+ retq
SYM_FUNC_END(.Lpaging_enabled)
/*
