x86_64, traps: Fix the espfix64 #DF fixup and rewrite it in C

There's nothing special enough about the espfix64 double fault fixup to
justify writing it in assembly.  Move it to C.

This also fixes a bug: if the double fault came from an IST stack, the
old asm code would return to a partially uninitialized stack frame.

Fixes: 3891a04aafd668686239349ea58f3314ea2af86b
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 26 insertions, 32 deletions

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index df088bb03fb3..a4dc8de7c4be 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -828,6 +828,7 @@ ENTRY(native_iret)
         jnz native_irq_return_ldt
 #endif
 
+.global native_irq_return_iret
 native_irq_return_iret:
         iretq
         _ASM_EXTABLE(native_irq_return_iret, bad_iret)
@@ -922,37 +923,6 @@ ENTRY(retint_kernel)
         CFI_ENDPROC
 END(common_interrupt)
 
-        /*
-         * If IRET takes a fault on the espfix stack, then we
-         * end up promoting it to a doublefault.  In that case,
-         * modify the stack to make it look like we just entered
-         * the #GP handler from user space, similar to bad_iret.
-         */
-#ifdef CONFIG_X86_ESPFIX64
-        ALIGN
-__do_double_fault:
-        XCPT_FRAME 1 RDI+8
-        movq RSP(%rdi),%rax             /* Trap on the espfix stack? */
-        sarq $PGDIR_SHIFT,%rax
-        cmpl $ESPFIX_PGD_ENTRY,%eax
-        jne do_double_fault             /* No, just deliver the fault */
-        cmpl $__KERNEL_CS,CS(%rdi)
-        jne do_double_fault
-        movq RIP(%rdi),%rax
-        cmpq $native_irq_return_iret,%rax
-        jne do_double_fault             /* This shouldn't happen... */
-        movq PER_CPU_VAR(kernel_stack),%rax
-        subq $(6*8-KERNEL_STACK_OFFSET),%rax    /* Reset to original stack */
-        movq %rax,RSP(%rdi)
-        movq $0,(%rax)                  /* Missing (lost) #GP error code */
-        movq $general_protection,RIP(%rdi)
-        retq
-        CFI_ENDPROC
-END(__do_double_fault)
-#else
-# define __do_double_fault do_double_fault
-#endif
-
 /*
  * APIC interrupts.
  */
@@ -1124,7 +1094,7 @@ idtentry overflow do_overflow has_error_code=0
 idtentry bounds do_bounds has_error_code=0
 idtentry invalid_op do_invalid_op has_error_code=0
 idtentry device_not_available do_device_not_available has_error_code=0
-idtentry double_fault __do_double_fault has_error_code=1 paranoid=1
+idtentry double_fault do_double_fault has_error_code=1 paranoid=1
 idtentry coprocessor_segment_overrun do_coprocessor_segment_overrun has_error_code=0
 idtentry invalid_TSS do_invalid_TSS has_error_code=1
 idtentry segment_not_present do_segment_not_present has_error_code=1
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 0d0e922fafc1..819662746e23 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -259,6 +259,30 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
         static const char str[] = "double fault";
         struct task_struct *tsk = current;
 
+#ifdef CONFIG_X86_ESPFIX64
+        extern unsigned char native_irq_return_iret[];
+
+        /*
+         * If IRET takes a non-IST fault on the espfix64 stack, then we
+         * end up promoting it to a doublefault.  In that case, modify
+         * the stack to make it look like we just entered the #GP
+         * handler from user space, similar to bad_iret.
+         */
+        if (((long)regs->sp >> PGDIR_SHIFT) == ESPFIX_PGD_ENTRY &&
+                regs->cs == __KERNEL_CS &&
+                regs->ip == (unsigned long)native_irq_return_iret)
+        {
+                struct pt_regs *normal_regs = task_pt_regs(current);
+
+                /* Fake a #GP(0) from userspace. */
+                memmove(&normal_regs->ip, (void *)regs->sp, 5*8);
+                normal_regs->orig_ax = 0;  /* Missing (lost) #GP error code */
+                regs->ip = (unsigned long)general_protection;
+                regs->sp = (unsigned long)&normal_regs->orig_ax;
+                return;
+        }
+#endif
+
         exception_enter();
         /* Return not checked because double check cannot be ignored */
         notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_DF, SIGSEGV);