Re: [Xen-devel] [PATCH] xen: Fix possible user space selectorcorruption

[Frediano Ziglio]

On Thu, 2013-10-03 at 11:04 +0100, Andrew Cooper wrote:
> On 03/10/13 09:24, Frediano Ziglio wrote:
> > Due to the way kernel is initialized under Xen is possible that the ring1
> > selector used by the kernel for the boot cpu end up to be copied to
> > userspace leading to segmentation fault in the userspace.
> >
> >
> > Xen code in the kernel initialize no-boot cpus with correct selectors (ds
> > and es set to __USER_DS) but the boot one keep the ring1 (passed by Xen).
> > On task context switch (switch_to) we assume that ds, es and cs already
> > point to __USER_DS and __KERNEL_CSso these selector are not changed.
> >
> > If processor is an Intel that support sysenter instruction sysenter/sysexit
> > is used so ds and es are not restored switching back from kernel to
> > userspace. In the case the selectors point to a ring1 instead of __USER_DS
> > the userspace code will crash on first memory access attempt (to be
> > precise Xen on the emulated iret used to do sysexit will detect and set ds
> > and es to zero which lead to GPF anyway).
> >
> > Now if an userspace process call kernel using sysenter and get rescheduled
> > (for me it happen on a specific init calling wait4) could happen that the
> > ring1 selector is set to ds and es.
> >
> > This is quite hard to detect cause after a while these selectors are fixed
> > (__USER_DS seems sticky).
> >
> > Bisecting the code commit 7076aada1040de4ed79a5977dbabdb5e5ea5e249 appears
> > to be the first one that have this issue.
> >
> > Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
> 
> In terms of the correctness of the fix,
> 
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> 
> However, I am not sure the comment is necessary.  The prevailing style
> is for no justification of loads of segment selectors on boot, and the
> comment itself refers simply to an interaction issue of 32bit on Xen
> when making use of sysenter.
> 

Suggestion for the comment ??

Frediano

> > ---
> >  arch/x86/xen/smp.c |   12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
> > index d1e4777..2a47241 100644
> > --- a/arch/x86/xen/smp.c
> > +++ b/arch/x86/xen/smp.c
> > @@ -278,6 +278,18 @@ static void __init xen_smp_prepare_boot_cpu(void)
> >  		   old memory can be recycled */
> >  		make_lowmem_page_readwrite(xen_initial_gdt);
> >  
> > +#ifdef CONFIG_X86_32
> > +		/*
> > +		 * Assure we use segments with user level access.
> > +		 * During switching of task these segments got not reloaded
> > +		 * so it could happen that userspace tasks get Xen ring1
> > +		 * selector causing exit with sysenter failures on next
> > +		 * userspace memory operation.
> > +		 */
> > +		loadsegment(ds, __USER_DS);
> > +		loadsegment(es, __USER_DS);
> > +#endif
> > +
> >  		xen_filter_cpu_maps();
> >  		xen_setup_vcpu_info_placement();
> >  	}
> 


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