[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Bisected Xen-unstable: "Segment register inaccessible for d1v0" when starting HVM guest on intel
On 02/07/14 10:55, Jan Beulich wrote: >>>> On 02.07.14 at 11:44, <andrew.cooper3@xxxxxxxxxx> wrote: >> On 02/07/14 10:28, Jan Beulich wrote: >>> This being a PV extension to the base architecture, the hardware >>> specification is meaningless. What we need to do here is _extend_ what >>> the hardware has specified for those extra accesses. We have three >>> options basically: >>> 1) never do any checking on such accesses >>> 2) honor CPL and EFLAGS.AC >>> 3) always do the checking >>> The first one obviously is bad from a security POV. Since the third one is >>> more strict than the second and since I assume adding some override is >>> going to be the simpler change than altering the point in time when the >>> VMCS gets loaded during context switch (the suggestion of which no one >>> at all commented on so far), I'd prefer that one, but wouldn't mind >>> option 2 to be implemented instead. >> The problem is not the hypervisor check. We are already deep within an >> hvm_copy_to_user() which is between a stac()/clac() pair. >> >> The issue is that guest_walk_tables() is checking a Xen access using >> guest page tables as if it were a supervisor access given the current >> context of the vcpu. > And I only ever referred to the checking done there; the hypervisor > access is of no concern here. > >> What can/should Xen do if its emulated access fails with a guest SMAP >> violations? It certainly can't/shouldn't inject a pagefault, nor should >> it actually fail the write. copy_to_user() is not subject to the guest >> operating mode and whether we are writing into guest user or supervisor >> pages. > Just like copy_to_user() would produce -EFAULT for a hypercall > when used on a non-present page or a non-canonical address, it > should (and afaict will with how things are right now) similarly > produce -EFAULT for an attempted access to a guest-accessible > page when the current mode of the guest is supervisor. > > To me it is a logical extension to also fail accesses outside of > hypercalls or emulation. > > Jan Consider an HVM guest with SMAP in effect, making a hypercall. If a guest handle points to guest userspace, Xen would be unable to ever complete the hypercall without an -EFAULT. I don't think this is reasonable to fail. ~Andrew _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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