Re: [PATCH v2] xen: Fix possible user space selector corruption

[Andrew Cooper]

On 07/10/13 10:48, 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>

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

> ---
>  arch/x86/xen/smp.c |    9 +++++++++
>  1 file changed, 9 insertions(+)
>
>
> Just changed comment on source code as suggested by Andrew Cooper.
>
>
> diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
> index d99cae8..6d89fcc 100644
> --- a/arch/x86/xen/smp.c
> +++ b/arch/x86/xen/smp.c
> @@ -245,6 +245,15 @@ 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
> +	/*
> +	 * Xen starts us with XEN_FLAT_RING1_DS, but linux code
> +	 * expects __USER_DS
> +	 */
> +	loadsegment(ds, __USER_DS);
> +	loadsegment(es, __USER_DS);
> +#endif
> +
>  	xen_filter_cpu_maps();
>  	xen_setup_vcpu_info_placement();
>  }

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