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

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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: 8a09317b89 ("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
This commit is contained in:
Thomas Gleixner 2018-01-03 19:52:04 +01:00
parent 3ffdeb1a02
commit d7732ba55c
1 changed files with 6 additions and 7 deletions
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13
arch/x86/entry/entry_64_compat.S
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@ -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.