[RFC][PATCH v2 07/21] x86/entry: Fill ESPFIX stack using C code

Alexandre Chartre
2020-11-16T14:47:20Z

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