New subject: [Linux-stable-mirror] [PATCH v4 1/3] x86/pti: Switch to kernel CR3 at early in entry_SYSCALL_compat()

4 Jan 2018

From: Thomas Gleixner tglx@linutronix.de
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.
Fixes: 8a09317b895f ("x86/mm/pti: Prepare the x86/entry assembly code for entry/exit CR3 switching")
Reported-by: Lars Wendler wendler.lars@web.de
Reported-by: Laura Abbott labbott@redhat.com
Signed-off-by: Thomas Gleixner tglx@linutronix.de
Cc: Borislav Betkov bp@alien8.de
Cc: Andy Lutomirski luto@kernel.org,
Cc: Dave Hansen dave.hansen@linux.intel.com,
Cc: Peter Zijlstra peterz@infradead.org,
Cc: Greg KH gregkh@linuxfoundation.org, ,
Cc: Boris Ostrovsky boris.ostrovsky@oracle.com,
Cc: Juergen Gross jgross@suse.com
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1801031949200.1957@nanos
---
 arch/x86/entry/entry_64_compat.S | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 40f17009ec20..98d5358e4041 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -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 */
@@ -219,12 +224,6 @@ GLOBAL(entry_SYSCALL_compat_after_hwframe)
    pushq   $0			/* pt_regs->r14 = 0 */
    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.
-- 
2.15.1

    