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

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

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 40f1700..98d5358 100644 (file)
--- 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 */
@@ -220,12 +225,6 @@ GLOBAL(entry_SYSCALL_compat_after_hwframe)
        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.
         */