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Commit d7732ba5 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

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: 8a09317b ("x86/mm/pti: Prepare the x86/entry assembly code for entry/exit CR3 switching")
Reported-by: default avatarLars Wendler <wendler.lars@web.de>
Reported-by: default avatarLaura Abbott <labbott@redhat.com>
Signed-off-by: default avatarThomas 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
parent 3ffdeb1a

arch/x86/entry/entry_64_compat.S

+6 −7
Original line number Diff line number Diff line
@@ -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.