[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 2/2] x86/hvm: honor guest's option when updating secondary system time for guest
>>> On 25.07.14 at 10:03, <feng.wu@xxxxxxxxx> wrote: > >> -----Original Message----- >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx] >> Sent: Friday, July 25, 2014 3:40 PM >> To: Wu, Feng >> Cc: linux@xxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxx; konrad.wilk@xxxxxxxxxx; >> keir@xxxxxxx; tim@xxxxxxx >> Subject: RE: [PATCH 2/2] x86/hvm: honor guest's option when updating >> secondary system time for guest >> >> >>> On 25.07.14 at 09:33, <feng.wu@xxxxxxxxx> wrote: >> >> > >> >> -----Original Message----- >> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx] >> >> Sent: Friday, July 25, 2014 3:26 PM >> >> To: Wu, Feng >> >> Cc: linux@xxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxx; >> konrad.wilk@xxxxxxxxxx; >> >> keir@xxxxxxx; tim@xxxxxxx >> >> Subject: RE: [PATCH 2/2] x86/hvm: honor guest's option when updating >> >> secondary system time for guest >> >> >> >> >>> On 25.07.14 at 06:30, <feng.wu@xxxxxxxxx> wrote: >> >> >> >> > >> >> >> -----Original Message----- >> >> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx] >> >> >> Sent: Wednesday, July 23, 2014 8:19 PM >> >> >> To: Wu, Feng >> >> >> Cc: linux@xxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxx; >> >> konrad.wilk@xxxxxxxxxx; >> >> >> keir@xxxxxxx; tim@xxxxxxx >> >> >> Subject: Re: [PATCH 2/2] x86/hvm: honor guest's option when updating >> >> >> secondary system time for guest >> >> >> >> >> >> >>> On 08.07.14 at 01:18, <feng.wu@xxxxxxxxx> wrote: >> >> >> > --- a/xen/include/public/vcpu.h >> >> >> > +++ b/xen/include/public/vcpu.h >> >> >> > @@ -227,6 +227,16 @@ struct vcpu_register_time_memory_area { >> >> >> > typedef struct vcpu_register_time_memory_area >> >> >> > vcpu_register_time_memory_area_t; >> >> >> > DEFINE_XEN_GUEST_HANDLE(vcpu_register_time_memory_area_t); >> >> >> > >> >> >> > +/* >> >> >> > + * Flags to tell Xen whether we need to do the SMAP check when >> >> updating >> >> >> > + * the secondary copy of the vcpu time when SMAP is enabled. Since >> the >> >> >> > + * memory location for the secondary copy of the vcpu time may be >> >> mapped >> >> >> > + * into userspace by guests intendedly, we let the guest to >> >> >> > determine >> >> >> > + * whether the check is needed. The default behavior of hypevisor is >> >> >> > + * not doing the check. >> >> >> > + */ >> >> >> > +#define VCPUOP_enable_smap_check_vcpu_time_memory_area >> 14 >> >> >> >> >> >> I think the new op to be >> VCPUOP_register_vcpu_time_memory_area_smap, >> >> >> identical to VCPUOP_register_vcpu_time_memory_area apart from also >> >> >> setting the flag, would be more natural. But considering what I just >> wrote >> >> >> in the reply to Tim I guess we can expect a nun-user mapping to be >> >> >> presented here anyway, i.e. we wouldn't need to new operation at all. >> >> > >> >> > Do you mean since the user-paging is r/o, guest will pass a r/w kernel >> >> > page >> >> > to >> >> > Xen for updating the system time. So we don't need to do the SMAP check >> >> > in this case? >> >> >> >> If the user page is r/o, it's VA obviously can't be used for updating by >> >> Xen. Hence the kernel has to provide a r/w mapped VA. That should be >> >> subject to SMAP checking (consistent with the runstate area handling), >> >> to make sure it's not a user accessible mapping. >> > >> > But there are two possible problems here: >> > 1. Is it possible that guest passes a user r/w page to update the system >> > time information? >> >> The guest kernel may pass a page that was originally mapped r/w- >> kernel, but the mapping later gets (say inadvertently) changed to >> r/w-user. That's why I think the SMAP checking ought to be done >> here. > > But in another case, as what we discussed before, if the guest intended to > pass a user r/w page to get the time information. Do we need to do the > SMAP checking for this? The question is whether we should bother supporting such insecure kernels. >> > 2. Even the user page is r/o, the kernel can still use it to update the >> > system time info when WP is disabled. >> >> The kernel can - for its own accesses to the page - do whatever >> it wants. It can't, however, control Xen's updating of it, and Xen >> won't do that with CR0.WP clear (leaving aside Aravindh's PV >> mem-access work, where this option is currently being discussed). > > Just like my comments above, my point is in some case, guest may pass a > user page (r/o or r/w) to get the time information, I doubt whether the SMAP > checking should be done for this? If we want to allow kernels to expose the time data r/w to user mode, then yes, we would need to avoid the SMAP check. But as said above, I don't think helping kernels to be insecure is of any particular value. Jan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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