6.4-stable review patch. If anyone has any objections, please let me know.
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From: Ard Biesheuvel ardb@kernel.org
[ 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@kernel.org Signed-off-by: Borislav Petkov (AMD) bp@alien8.de Link: https://lore.kernel.org/r/20230807162720.545787-2-ardb@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/boot/compressed/head_64.S | 30 +++++++++++++++++++++++------- 1 file changed, 23 insertions(+), 7 deletions(-)
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S index 03c4328a88cbd..f732426d3b483 100644 --- a/arch/x86/boot/compressed/head_64.S +++ b/arch/x86/boot/compressed/head_64.S @@ -459,11 +459,25 @@ 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 %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 @@ -471,6 +485,11 @@ SYM_CODE_START(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 rva(boot_stack_end)(%rbx), %rsp
@@ -582,7 +601,7 @@ 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. */ @@ -592,9 +611,6 @@ 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 +687,7 @@ SYM_CODE_END(trampoline_32bit_src) .code64 SYM_FUNC_START_LOCAL_NOALIGN(.Lpaging_enabled) /* Return from the trampoline */ - jmp *%rdi + retq SYM_FUNC_END(.Lpaging_enabled)
/*