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Date: Wed, 17 May 2023 17:40:30 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Evgeniy Baskov <baskov@...ras.ru>,
Borislav Petkov <bp@...en8.de>,
Andy Lutomirski <luto@...nel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Alexey Khoroshilov <khoroshilov@...ras.ru>,
Peter Jones <pjones@...hat.com>,
Gerd Hoffmann <kraxel@...hat.com>,
Dave Young <dyoung@...hat.com>,
Mario Limonciello <mario.limonciello@....com>,
Kees Cook <keescook@...omium.org>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 05/20] x86: decompressor: Avoid the need for a stack in
the 32-bit trampoline
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@...nel.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.
> *
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