[RFC][PATCH 7/8] x86/mm/vsyscall: consider vsyscall page part of user address space

[Dave Hansen <dave.hansen@linux.intel.com>]

From: Dave Hansen <dave.hansen@linux.intel.com>

The vsyscall page is weird.  It is in what is traditionally part of the
kernel address space.  But, it has user permissions and we handle faults
on it like we would on a user page: interrupts on.

Right now, we handle vsyscall emulation in the "bad_area" code, which
is used for both user-address-space and kernel-address-space faults.  Move
the handling to the user-address-space code *only* and ensure we get there
by "excluding" the vsyscall page from the kernel address space via a check
in fault_in_kernel_space().

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86@kernel.org
Cc: Andy Lutomirski <luto@kernel.org>
---

 b/arch/x86/mm/fault.c |   36 ++++++++++++++++++++++++------------
 1 file changed, 24 insertions(+), 12 deletions(-)

diff -puN arch/x86/mm/fault.c~vsyscall-is-user-address-space arch/x86/mm/fault.c
--- a/arch/x86/mm/fault.c~vsyscall-is-user-address-space	2018-09-07 11:21:48.720751896 -0700
+++ b/arch/x86/mm/fault.c	2018-09-07 11:21:48.724751896 -0700
@@ -873,18 +873,6 @@ __bad_area_nosemaphore(struct pt_regs *r
 		if (is_errata100(regs, address))
 			return;
 
-#ifdef CONFIG_X86_64
-		/*
-		 * Instruction fetch faults in the vsyscall page might need
-		 * emulation.
-		 */
-		if (unlikely((error_code & X86_PF_INSTR) &&
-			     is_vsyscall_vaddr(address))) {
-			if (emulate_vsyscall(regs, address))
-				return;
-		}
-#endif
-
 		/*
 		 * To avoid leaking information about the kernel page table
 		 * layout, pretend that user-mode accesses to kernel addresses
@@ -1192,6 +1180,13 @@ access_error(unsigned long error_code, s
 
 static int fault_in_kernel_space(unsigned long address)
 {
+	/*
+	 * The vsyscall page is at an address above TASK_SIZE_MAX,
+	 * but is not considered part of the kernel address space.
+	 */
+	if (is_vsyscall_vaddr(address))
+		return false;
+
 	return address >= TASK_SIZE_MAX;
 }
 
@@ -1357,6 +1352,23 @@ void do_user_addr_space_fault(struct pt_
 	if (sw_error_code & X86_PF_INSTR)
 		flags |= FAULT_FLAG_INSTRUCTION;
 
+#ifdef CONFIG_X86_64
+	/*
+	 * Instruction fetch faults in the vsyscall page might need
+	 * emulation.  The vsyscall page is at a high address
+	 * (>PAGE_OFFSET), but is considered to be part of the user
+	 * address space.
+	 *
+	 * The vsyscall page does not have a "real" VMA, so do this
+	 * emulation before we go searching for VMAse
+	 */
+	if (unlikely((sw_error_code & X86_PF_INSTR) &&
+		     is_vsyscall_vaddr(address))) {
+		if (emulate_vsyscall(regs, address))
+			return;
+	}
+#endif
+
 	/*
 	 * Kernel-mode access to the user address space should only occur
 	 * inside well-defined areas of code listed in the exception
_