Messages in this thread Patch in this message | | | From | Alexandre Chartre <> | Subject | [RFC][PATCH v2 07/21] x86/entry: Fill ESPFIX stack using C code | Date | Mon, 16 Nov 2020 15:47:43 +0100 |
| |
The ESPFIX stack is filled using assembly code. Move this code to a C function so that it is easier to read and modify.
Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com> --- arch/x86/entry/entry_64.S | 62 ++++++++++++++++++------------------- arch/x86/kernel/espfix_64.c | 41 ++++++++++++++++++++++++ 2 files changed, 72 insertions(+), 31 deletions(-)
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 73e9cd47dc83..6e0b5b010e0b 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -684,8 +684,10 @@ native_irq_return_ldt: * long (see ESPFIX_STACK_SIZE). espfix_waddr points to the bottom * of the ESPFIX stack. * - * We clobber RAX and RDI in this code. We stash RDI on the - * normal stack and RAX on the ESPFIX stack. + * We call into C code to fill the ESPFIX stack. We stash registers + * that the C function can clobber on the normal stack. The user RAX + * is stashed first so that it is adjacent to the iret frame which + * will be copied to the ESPFIX stack. * * The ESPFIX stack layout we set up looks like this: * @@ -699,39 +701,37 @@ native_irq_return_ldt: * --- bottom of ESPFIX stack --- */ - pushq %rdi /* Stash user RDI */ - SWAPGS /* to kernel GS */ - SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi /* to kernel CR3 */ - - movq PER_CPU_VAR(espfix_waddr), %rdi - movq %rax, (0*8)(%rdi) /* user RAX */ - movq (1*8)(%rsp), %rax /* user RIP */ - movq %rax, (1*8)(%rdi) - movq (2*8)(%rsp), %rax /* user CS */ - movq %rax, (2*8)(%rdi) - movq (3*8)(%rsp), %rax /* user RFLAGS */ - movq %rax, (3*8)(%rdi) - movq (5*8)(%rsp), %rax /* user SS */ - movq %rax, (5*8)(%rdi) - movq (4*8)(%rsp), %rax /* user RSP */ - movq %rax, (4*8)(%rdi) - /* Now RAX == RSP. */ - - andl $0xffff0000, %eax /* RAX = (RSP & 0xffff0000) */ + /* save registers */ + pushq %rax + pushq %rdi + pushq %rsi + pushq %rdx + pushq %rcx + pushq %r8 + pushq %r9 + pushq %r10 + pushq %r11 /* - * espfix_stack[31:16] == 0. The page tables are set up such that - * (espfix_stack | (X & 0xffff0000)) points to a read-only alias of - * espfix_waddr for any X. That is, there are 65536 RO aliases of - * the same page. Set up RSP so that RSP[31:16] contains the - * respective 16 bits of the /userspace/ RSP and RSP nonetheless - * still points to an RO alias of the ESPFIX stack. + * fill_espfix_stack will copy the iret+rax frame to the ESPFIX + * stack and return with RAX containing a pointer to the ESPFIX + * stack. */ - orq PER_CPU_VAR(espfix_stack), %rax + leaq 8*8(%rsp), %rdi /* points to the iret+rax frame */ + call fill_espfix_stack - SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi - SWAPGS /* to user GS */ - popq %rdi /* Restore user RDI */ + /* + * RAX contains a pointer to the ESPFIX, so restore registers but + * RAX. RAX will be restored from the ESPFIX stack. + */ + popq %r11 + popq %r10 + popq %r9 + popq %r8 + popq %rcx + popq %rdx + popq %rsi + popq %rdi movq %rax, %rsp UNWIND_HINT_IRET_REGS offset=8 diff --git a/arch/x86/kernel/espfix_64.c b/arch/x86/kernel/espfix_64.c index 4fe7af58cfe1..ff4b5160b39c 100644 --- a/arch/x86/kernel/espfix_64.c +++ b/arch/x86/kernel/espfix_64.c @@ -33,6 +33,7 @@ #include <asm/pgalloc.h> #include <asm/setup.h> #include <asm/espfix.h> +#include <asm/entry-common.h> /* * Note: we only need 6*8 = 48 bytes for the espfix stack, but round @@ -205,3 +206,43 @@ void init_espfix_ap(int cpu) per_cpu(espfix_waddr, cpu) = (unsigned long)stack_page + (addr & ~PAGE_MASK); } + +/* + * iret frame with an additional user_rax register. + */ +struct iret_rax_frame { + unsigned long user_rax; + unsigned long rip; + unsigned long cs; + unsigned long rflags; + unsigned long rsp; + unsigned long ss; +}; + +noinstr unsigned long fill_espfix_stack(struct iret_rax_frame *frame) +{ + struct iret_rax_frame *espfix_frame; + unsigned long rsp; + + native_swapgs(); + user_pagetable_exit(); + + espfix_frame = (struct iret_rax_frame *)this_cpu_read(espfix_waddr); + *espfix_frame = *frame; + + /* + * espfix_stack[31:16] == 0. The page tables are set up such that + * (espfix_stack | (X & 0xffff0000)) points to a read-only alias of + * espfix_waddr for any X. That is, there are 65536 RO aliases of + * the same page. Set up RSP so that RSP[31:16] contains the + * respective 16 bits of the /userspace/ RSP and RSP nonetheless + * still points to an RO alias of the ESPFIX stack. + */ + rsp = ((unsigned long)espfix_frame) & 0xffff0000; + rsp |= this_cpu_read(espfix_stack); + + user_pagetable_enter(); + native_swapgs(); + + return rsp; +} -- 2.18.4
| |