Re: [PATCH v2 05/20] x86: decompressor: Avoid the need for a stack in the 32-bit trampoline

[Tom Lendacky <thomas.lendacky@amd.com>]

On 5/8/23 02:03, Ard Biesheuvel wrote:
> The 32-bit trampoline no longer uses the stack for anything except
> performing a long return back to long mode. Currently, this stack is
> allocated in the same page that carries the trampoline code, which means
> this page must be mapped writable and executable, and the stack is
> therefore executable as well.
> 
> So let's do a long jump instead: that way, we can pre-calculate the
> return address and poke it into the code before we call it. In a later
> patch, we will take advantage of this by removing writable permissions
> (and adding executable ones) explicitly when booting via the EFI stub.
> 
> Not playing with the stack pointer also makes it more straight-forward
> to call the trampoline code as an ordinary 64-bit function from C code.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>   arch/x86/boot/compressed/head_64.S    | 34 ++++----------------
>   arch/x86/boot/compressed/pgtable.h    |  6 ++--
>   arch/x86/boot/compressed/pgtable_64.c | 12 ++++++-
>   3 files changed, 21 insertions(+), 31 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index b1f8a867777120bb..3b5fc851737ffc39 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -460,9 +460,6 @@ SYM_CODE_START(startup_64)
>   	leaq	TRAMPOLINE_32BIT_CODE_OFFSET(%rax), %rax
>   	call	*%rax
>   
> -	/* Restore the stack, the 32-bit trampoline uses its own stack */
> -	leaq	rva(boot_stack_end)(%rbx), %rsp
> -
>   	/*
>   	 * cleanup_trampoline() would restore trampoline memory.
>   	 *
> @@ -563,24 +560,17 @@ SYM_FUNC_END(.Lrelocated)
>    * EDI contains the base address of the trampoline memory.
>    * Non-zero ESI means trampoline needs to enable 5-level paging.
>    */
> +	.section ".rodata", "a", @progbits
>   SYM_CODE_START(trampoline_32bit_src)
> -	popq	%r8
>   	/* Switch to compatibility mode (CS.L = 0 CS.D = 1) via far return */
>   	pushq	$__KERNEL32_CS
>   	leaq	0f(%rip), %rax
>   	pushq	%rax
>   	lretq
> +.Lret:	retq

Maybe just add a comment above this to explain that this is a target of 
the long jump below to get back into long mode and be able to return 
without setting up a new stack for the 32-bit code.

And then a corresponding comment on the long jump itself. I think it would 
make it easier to understand what is going on in this part of the code.

Thanks,
Tom

>   
>   	.code32
> -0:	/* Set up data and stack segments */
> -	movl	$__KERNEL_DS, %eax
> -	movl	%eax, %ds
> -	movl	%eax, %ss
> -
> -	/* Set up new stack */
> -	leal	TRAMPOLINE_32BIT_STACK_END(%edi), %esp
> -
> -	/* Disable paging */
> +0:	/* Disable paging */
>   	movl	%cr0, %eax
>   	btrl	$X86_CR0_PG_BIT, %eax
>   	movl	%eax, %cr0
> @@ -634,26 +624,16 @@ 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(%edi), %eax
> -
> -	/* Prepare the stack for far return to Long Mode */
> -	pushl	$__KERNEL_CS
> -	pushl	%eax
> -
>   	/* Enable paging again. */
>   	movl	%cr0, %eax
>   	btsl	$X86_CR0_PG_BIT, %eax
>   	movl	%eax, %cr0
>   
> -	lret
> +.Ljmp:	ljmpl	$__KERNEL_CS, $(.Lret - trampoline_32bit_src)
>   SYM_CODE_END(trampoline_32bit_src)
>   
> -	.code64
> -SYM_FUNC_START_LOCAL_NOALIGN(.Lpaging_enabled)
> -	/* Return from the trampoline */
> -	jmp	*%r8
> -SYM_FUNC_END(.Lpaging_enabled)
> +/* keep this right after trampoline_32bit_src() so we can infer its size */
> +SYM_DATA(trampoline_ljmp_imm_offset, .word  .Ljmp + 1 - trampoline_32bit_src)
>   
>   	/*
>            * The trampoline code has a size limit.
> @@ -662,7 +642,7 @@ SYM_FUNC_END(.Lpaging_enabled)
>   	 */
>   	.org	trampoline_32bit_src + TRAMPOLINE_32BIT_CODE_SIZE
>   
> -	.code32
> +	.text
>   SYM_FUNC_START_LOCAL_NOALIGN(.Lno_longmode)
>   	/* This isn't an x86-64 CPU, so hang intentionally, we cannot continue */
>   1:
> diff --git a/arch/x86/boot/compressed/pgtable.h b/arch/x86/boot/compressed/pgtable.h
> index 4e8cef135226bcbb..131488f50af55d0a 100644
> --- a/arch/x86/boot/compressed/pgtable.h
> +++ b/arch/x86/boot/compressed/pgtable.h
> @@ -6,9 +6,7 @@
>   #define TRAMPOLINE_32BIT_PGTABLE_OFFSET	0
>   
>   #define TRAMPOLINE_32BIT_CODE_OFFSET	PAGE_SIZE
> -#define TRAMPOLINE_32BIT_CODE_SIZE	0xA0
> -
> -#define TRAMPOLINE_32BIT_STACK_END	TRAMPOLINE_32BIT_SIZE
> +#define TRAMPOLINE_32BIT_CODE_SIZE	0x80
>   
>   #ifndef __ASSEMBLER__
>   
> @@ -16,5 +14,7 @@ extern unsigned long *trampoline_32bit;
>   
>   extern void trampoline_32bit_src(void *trampoline, bool enable_5lvl);
>   
> +extern const u16 trampoline_ljmp_imm_offset;
> +
>   #endif /* __ASSEMBLER__ */
>   #endif /* BOOT_COMPRESSED_PAGETABLE_H */
> diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
> index 2ac12ff4111bf8c0..09fc18180929fab3 100644
> --- a/arch/x86/boot/compressed/pgtable_64.c
> +++ b/arch/x86/boot/compressed/pgtable_64.c
> @@ -109,6 +109,7 @@ static unsigned long find_trampoline_placement(void)
>   struct paging_config paging_prepare(void *rmode)
>   {
>   	struct paging_config paging_config = {};
> +	void *tramp_code;
>   
>   	/* Initialize boot_params. Required for cmdline_find_option_bool(). */
>   	boot_params = rmode;
> @@ -143,9 +144,18 @@ struct paging_config paging_prepare(void *rmode)
>   	memset(trampoline_32bit, 0, TRAMPOLINE_32BIT_SIZE);
>   
>   	/* Copy trampoline code in place */
> -	memcpy(trampoline_32bit + TRAMPOLINE_32BIT_CODE_OFFSET / sizeof(unsigned long),
> +	tramp_code = memcpy(trampoline_32bit +
> +			TRAMPOLINE_32BIT_CODE_OFFSET / sizeof(unsigned long),
>   			&trampoline_32bit_src, TRAMPOLINE_32BIT_CODE_SIZE);
>   
> +	/*
> +	 * Avoid the need for a stack in the 32-bit trampoline code, by using
> +	 * LJMP rather than LRET to return back to long mode. LJMP takes an
> +	 * immediate absolute address, so we have to adjust that based on the
> +	 * placement of the trampoline.
> +	 */
> +	*(u32 *)(tramp_code + trampoline_ljmp_imm_offset) += (unsigned long)tramp_code;
> +
>   	/*
>   	 * The code below prepares page table in trampoline memory.
>   	 *