Patch "x86/pti: Switch to kernel CR3 at early in entry_SYSCALL_compat()" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

x86/pti: Switch to kernel CR3 at early in entry_SYSCALL_compat()

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-pti-switch-to-kernel-cr3-at-early-in-entry_syscall_compat.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Thomas Gleixner <[email protected]>
Date: Wed, 3 Jan 2018 19:52:04 +0100
Subject: x86/pti: Switch to kernel CR3 at early in entry_SYSCALL_compat()

From: Thomas Gleixner <[email protected]>

commit d7732ba55c4b6a2da339bb12589c515830cfac2c upstream.

The preparation for PTI which added CR3 switching to the entry code misplaced the CR3 switch in entry_SYSCALL_compat().

With PTI enabled the entry code tries to access a per cpu variable after switching to kernel GS. This fails because that variable is not mapped to user space. This results in a double fault and in the worst case a kernel crash.

Move the switch ahead of the access and clobber RSP which has been saved already.

---
 arch/x86/entry/entry_64_compat.S |   13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

@@ -190,8 +190,13 @@ ENTRY(entry_SYSCALL_compat)
 	/* Interrupts are off on entry. */
 	swapgs
 
-	/* Stash user ESP and switch to the kernel stack. */
+	/* Stash user ESP */
 	movl	%esp, %r8d
+
+	/* Use %rsp as scratch reg. User ESP is stashed in r8 */
+	SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
+
+	/* Switch to the kernel stack */
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 
 	/* Construct struct pt_regs on stack */

@@ -220,12 +225,6 @@ GLOBAL(entry_SYSCALL_compat_after_hwfram
 	pushq   $0			/* pt_regs->r15 = 0 */
 
 	/*
-	 * We just saved %rdi so it is safe to clobber.  It is not
-	 * preserved during the C calls inside TRACE_IRQS_OFF anyway.
-	 */
-	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
-
-	/*
 	 * User mode is traced as though IRQs are on, and SYSENTER
 	 * turned them off.
 	 */

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/x86-pti-switch-to-kernel-cr3-at-early-in-entry_syscall_compat.patch queue-4.14/x86-dumpstack-print-registers-for-first-stack-frame.patch queue-4.14/x86-process-define-cpu_tss_rw-in-same-section-as-declaration.patch queue-4.14/x86-pti-make-sure-the-user-kernel-ptes-match.patch queue-4.14/x86-cpu-x86-pti-do-not-enable-pti-on-amd-processors.patch queue-4.14/x86-dumpstack-fix-partial-register-dumps.patch