4.14-stable review patch. If anyone has any objections, please let me know.
------------------ 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. Fixes: 8a09317b895f ("x86/mm/pti: Prepare the x86/entry assembly code for entry/exit CR3 switching") Reported-by: Lars Wendler <[email protected]> Reported-by: Laura Abbott <[email protected]> Signed-off-by: Thomas Gleixner <[email protected]> Cc: Borislav Betkov <[email protected]> Cc: Andy Lutomirski <[email protected]>, Cc: Dave Hansen <[email protected]>, Cc: Peter Zijlstra <[email protected]>, Cc: Greg KH <[email protected]>, , Cc: Boris Ostrovsky <[email protected]>, Cc: Juergen Gross <[email protected]> Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1801031949200.1957@nanos Signed-off-by: Greg Kroah-Hartman <[email protected]> --- arch/x86/entry/entry_64_compat.S | 13 ++++++------- 1 file changed, 6 insertions(+), 7 deletions(-) --- 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_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. */
