Sorry. Ignore this please.
2018-01-05 0:45 GMT+03:00 Maxim Moseychuk
<franchesko.salias.hudro.pedros@xxxxxxxxx>:
> 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
>