x86/espfix/64: Stop assuming that pt_regs is on the entry stack

When we start using an entry trampoline, a #GP from userspace will
be delivered on the entry stack, not on the task stack.  Fix the
espfix64 #DF fixup to set up #GP according to TSS.SP0, rather than
assuming that pt_regs + 1 == SP0.  This won't change anything
without an entry stack, but it will make the code continue to work
when an entry stack is added.

While we're at it, improve the comments to explain what's actually
going on.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Laight <David.Laight@aculab.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eduardo Valentin <eduval@amazon.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: aliguori@amazon.com
Cc: daniel.gruss@iaik.tugraz.at
Cc: hughd@google.com
Cc: keescook@google.com
Link: https://lkml.kernel.org/r/20171204150606.130778051@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/traps.c

@@ -348,23 +348,42 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
 
 	/*
 	 * 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.
+	 * end up promoting it to a doublefault.  In that case, take
+	 * advantage of the fact that we're not using the normal (TSS.sp0)
+	 * stack right now.  We can write a fake #GP(0) frame at TSS.sp0
+	 * and then modify our own IRET frame so that, when we return,
+	 * we land directly at the #GP(0) vector with the stack already
+	 * set up according to its expectations.
+	 *
+	 * The net result is that our #GP handler will think that we
+	 * entered from usermode with the bad user context.
 	 *
 	 * No need for ist_enter here because we don't use RCU.
 	 */
 	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);
+		struct pt_regs *gpregs = (struct pt_regs *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1;
+
+		/*
+		 * regs->sp points to the failing IRET frame on the
+		 * ESPFIX64 stack.  Copy it to the entry stack.  This fills
+		 * in gpregs->ss through gpregs->ip.
+		 *
+		 */
+		memmove(&gpregs->ip, (void *)regs->sp, 5*8);
+		gpregs->orig_ax = 0;  /* Missing (lost) #GP error code */
 
-		/* 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 */
+		/*
+		 * Adjust our frame so that we return straight to the #GP
+		 * vector with the expected RSP value.  This is safe because
+		 * we won't enable interupts or schedule before we invoke
+		 * general_protection, so nothing will clobber the stack
+		 * frame we just set up.
+		 */
 		regs->ip = (unsigned long)general_protection;
-		regs->sp = (unsigned long)&normal_regs->orig_ax;
+		regs->sp = (unsigned long)&gpregs->orig_ax;
 
 		return;
 	}
@@ -389,7 +408,7 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
 	 *
 	 *   Processors update CR2 whenever a page fault is detected. If a
 	 *   second page fault occurs while an earlier page fault is being
-	 *   deliv- ered, the faulting linear address of the second fault will
+	 *   delivered, the faulting linear address of the second fault will
 	 *   overwrite the contents of CR2 (replacing the previous
 	 *   address). These updates to CR2 occur even if the page fault
 	 *   results in a double fault or occurs during the delivery of a