linux-kernel - Re: [Xen-devel] [PATCH] xen: Fix possible user space selector corruption

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Date:	Thu, 3 Oct 2013 10:47:44 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Frediano Ziglio <frediano.ziglio@...rix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen: Fix possible user space selector corruption

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@...rix.com>

Looks okay to me but I don't really understand it...

David
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