Messages in this thread | | | Date | Wed, 9 Apr 2014 15:41:37 +0100 | From | Andrew Cooper <> | Subject | Re: [Xen-devel] [PATCH] x86/xen: Fix 32-bit PV guests's usage of kernel_stack |
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On 09/04/14 15:29, David Vrabel wrote: > On 09/04/14 15:21, Jan Beulich wrote: >>>>> On 09.04.14 at 16:06, <boris.ostrovsky@oracle.com> wrote: >>> --- a/arch/x86/xen/xen-asm_32.S >>> +++ b/arch/x86/xen/xen-asm_32.S >>> @@ -88,7 +88,11 @@ ENTRY(xen_iret) >>> * avoid having to reload %fs >>> */ >>> #ifdef CONFIG_SMP >>> + pushw %fs >>> + movl $(__KERNEL_PERCPU), %eax >>> + movl %eax, %fs >>> GET_THREAD_INFO(%eax) >>> + popw %fs >> I don't think it's guaranteed that this can't fault. > If loading %fs faults when it is restored previously, the fixup zeros > the value. However, this later load could still fault even if the first > succeeded. > > Suggest copying the fixup section from the RESTORE_REGS macros in > arch/x86/kernel/entry_32.S > > David
If loading __KERNEL_PERCPU info fs faults, the kernel has bigger problems to worry about.
The latter load however can easy fault; The arguments for %ds in XSA-42/ CVE-2013-0228 applies to %{e,f,g}s as well.
Furthermore, I am a little concerned about the performance impact of this. I would have thought that in most cases, %fs will already be correct, at which point reloading it twice is a waste of time.
~Andrew
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