From f5bca8110b16a5467a07d17b7596fc9bc5c03905 Mon Sep 17 00:00:00 2001 From: Konrad Rzeszutek Wilk Date: Wed, 19 Nov 2014 12:57:11 -0500 Subject: [PATCH] Limit XEN_DOMCTL_memory_mapping hypercall to only process up to 64 GFNs (or less) Said hypercall for large BARs can take quite a while. As such we can require that the hypercall MUST break up the request in smaller values. Another approach is to add preemption to it - whether we do the preemption using hypercall_create_continuation or returning EAGAIN to userspace (and have it re-invocate the call) - either way the issue we cannot easily solve is that in 'map_mmio_regions' if we encounter an error we MUST call 'unmap_mmio_regions' for the whole BAR region. Since the preemption would re-use input fields such as nr_mfns, first_gfn, first_mfn - we would lose the original values - and only undo what was done in the current round (i.e. ignoring anything that was done prior to earlier preemptions). Unless we re-used the return value as 'EAGAIN|nr_mfns_done<<10' but that puts a limit (since the return value is a long) on the amount of nr_mfns that can provided. This patch sidesteps this problem by: - Setting an hard limit of nr_mfns having to be 64 or less. - Toolstack adjusts correspondingly to the nr_mfn limit. - If the there is an error when adding the toolstack will call the remove operation to remove the whole region. The need to break this hypercall down is for large BARs can take more than the guest (initial domain usually) time-slice. This has the negative result in that the guest is locked out for a long duration and is unable to act on any pending events. We also augment the code to return zero if nr_mfns instead of trying to the hypercall. This is XSA-125 / CVE-2015-2752. Suggested-by: Jan Beulich Acked-by: Jan Beulich Signed-off-by: Konrad Rzeszutek Wilk Acked-by: Ian Campbell --- tools/libxc/xc_domain.c | 46 +++++++++++++++++++++++++++++++++---- xen/common/domctl.c | 5 ++++ xen/include/public/domctl.h | 1 + 3 files changed, 48 insertions(+), 4 deletions(-) diff --git a/tools/libxc/xc_domain.c b/tools/libxc/xc_domain.c index b8648720f7..eb88eeedf6 100644 --- a/tools/libxc/xc_domain.c +++ b/tools/libxc/xc_domain.c @@ -1992,6 +1992,8 @@ int xc_domain_memory_mapping( { DECLARE_DOMCTL; xc_dominfo_t info; + int ret = 0, err; + unsigned long done = 0, nr, max_batch_sz; if ( xc_domain_getinfo(xch, domid, 1, &info) != 1 || info.domid != domid ) @@ -2002,14 +2004,50 @@ int xc_domain_memory_mapping( if ( !xc_core_arch_auto_translated_physmap(&info) ) return 0; + if ( !nr_mfns ) + return 0; + domctl.cmd = XEN_DOMCTL_memory_mapping; domctl.domain = domid; - domctl.u.memory_mapping.first_gfn = first_gfn; - domctl.u.memory_mapping.first_mfn = first_mfn; - domctl.u.memory_mapping.nr_mfns = nr_mfns; domctl.u.memory_mapping.add_mapping = add_mapping; + max_batch_sz = nr_mfns; + do + { + nr = min(nr_mfns - done, max_batch_sz); + domctl.u.memory_mapping.nr_mfns = nr; + domctl.u.memory_mapping.first_gfn = first_gfn + done; + domctl.u.memory_mapping.first_mfn = first_mfn + done; + err = do_domctl(xch, &domctl); + if ( err && errno == E2BIG ) + { + if ( max_batch_sz <= 1 ) + break; + max_batch_sz >>= 1; + continue; + } + /* Save the first error... */ + if ( !ret ) + ret = err; + /* .. and ignore the rest of them when removing. */ + if ( err && add_mapping != DPCI_REMOVE_MAPPING ) + break; - return do_domctl(xch, &domctl); + done += nr; + } while ( done < nr_mfns ); + + /* + * Undo what we have done unless unmapping, by unmapping the entire region. + * Errors here are ignored. + */ + if ( ret && add_mapping != DPCI_REMOVE_MAPPING ) + xc_domain_memory_mapping(xch, domid, first_gfn, first_mfn, nr_mfns, + DPCI_REMOVE_MAPPING); + + /* We might get E2BIG so many times that we never advance. */ + if ( !done && !ret ) + ret = -1; + + return ret; } int xc_domain_ioport_mapping( diff --git a/xen/common/domctl.c b/xen/common/domctl.c index 8bca6da738..c55b289d80 100644 --- a/xen/common/domctl.c +++ b/xen/common/domctl.c @@ -1036,6 +1036,11 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl) (gfn + nr_mfns - 1) < gfn ) /* wrap? */ break; + ret = -E2BIG; + /* Must break hypercall up as this could take a while. */ + if ( nr_mfns > 64 ) + break; + ret = -EPERM; if ( !iomem_access_permitted(current->domain, mfn, mfn_end) || !iomem_access_permitted(d, mfn, mfn_end) ) diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h index 57e2ed7460..9e1bc636ff 100644 --- a/xen/include/public/domctl.h +++ b/xen/include/public/domctl.h @@ -543,6 +543,7 @@ DEFINE_XEN_GUEST_HANDLE(xen_domctl_bind_pt_irq_t); /* Bind machine I/O address range -> HVM address range. */ +/* If this returns -E2BIG lower nr_mfns value. */ /* XEN_DOMCTL_memory_mapping */ #define DPCI_ADD_MAPPING 1 #define DPCI_REMOVE_MAPPING 0 -- 2.39.5