Messages in this thread | | | Date | Wed, 17 May 2023 17:40:30 -0500 | Subject | Re: [PATCH v2 05/20] x86: decompressor: Avoid the need for a stack in the 32-bit trampoline | From | Tom Lendacky <> |
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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. > *
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