A process can access a file if the set of MCS categories for the file is equal-to *or* a subset-of, the set of MCS categories for the process. If there are two VMs: a) svirt_t:s0:c117 b) svirt_t:s0:c117,c720 Then VM (b) is able to access files labelled for VM (a). IOW, we must discard case where the categories are equal because that is a subset of many other valid category pairs. Fixes: https://gitlab.com/libvirt/libvirt/-/issues/153 CVE-2021-3631 Reviewed-by: Peter Krempa <pkrempa@redhat.com> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com> --- Pushed to git, just publishing v2 to show the CVE allocation for the historical record. src/security/security_selinux.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/src/security/security_selinux.c b/src/security/security_selinux.c index b50f4463cc..0c2cf1d1c7 100644 --- a/src/security/security_selinux.c +++ b/src/security/security_selinux.c @@ -383,7 +383,15 @@ virSecuritySELinuxMCSFind(virSecurityManager *mgr, VIR_DEBUG("Try cat %s:c%d,c%d", sens, c1 + catMin, c2 + catMin); if (c1 == c2) { - mcs = g_strdup_printf("%s:c%d", sens, catMin + c1); + /* + * A process can access a file if the set of MCS categories + * for the file is equal-to *or* a subset-of, the set of + * MCS categories for the process. + * + * IOW, we must discard case where the categories are equal + * because that is a subset of other category pairs. + */ + continue; } else { if (c1 > c2) { int t = c1; -- 2.31.1