From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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@xxxxxx>
Reported-by: Laura Abbott <labbott@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Betkov <bp@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>,
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>,
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>,
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>, ,
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>,
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
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