[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] 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@xxxxxxxxxx> Looks okay to me but I don't really understand it... David _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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