qemu: caps: Use CAP_DAC_OVERRIDE for probing to avoid permission issues

This is mainly about /dev/sev and its default permissions 0600. Of
course, rule of 'tinfoil' would be that we can't trust anything, but the
probing code in QEMU is considered safe from security's perspective + we
can't create an udev rule for this at the moment, because ioctls and
file system permissions aren't cross-checked in kernel and therefore a
user with read permissions could issue a 'privileged' operation on SEV
which is currently only limited to root.

https://bugzilla.redhat.com/show_bug.cgi?id=1665400

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

src/qemu/qemu_capabilities.c

@@ -53,6 +53,10 @@
 #include <stdarg.h>
 #include <sys/utsname.h>
 
+#if WITH_CAPNG
+# include <cap-ng.h>
+#endif
+
 #define VIR_FROM_THIS VIR_FROM_QEMU
 
 VIR_LOG_INIT("qemu.qemu_capabilities");
@@ -4516,6 +4520,13 @@ virQEMUCapsInitQMPCommandRun(virQEMUCapsInitQMPCommandPtr cmd,
                                     NULL);
     virCommandAddEnvPassCommon(cmd->cmd);
     virCommandClearCaps(cmd->cmd);
+
+#if WITH_CAPNG
+    /* QEMU might run into permission issues, e.g. /dev/sev (0600), override
+     * them just for the purpose of probing */
+    virCommandAllowCap(cmd->cmd, CAP_DAC_OVERRIDE);
+#endif
+
     virCommandSetGID(cmd->cmd, cmd->runGid);
     virCommandSetUID(cmd->cmd, cmd->runUid);
 

src/util/virutil.c

@@ -1502,8 +1502,10 @@ virSetUIDGIDWithCaps(uid_t uid, gid_t gid, gid_t *groups, int ngroups,
 {
     size_t i;
     int capng_ret, ret = -1;
-    bool need_setgid = false, need_setuid = false;
+    bool need_setgid = false;
+    bool need_setuid = false;
     bool need_setpcap = false;
+    const char *capstr = NULL;
 
     /* First drop all caps (unless the requested uid is "unchanged" or
      * root and clearExistingCaps wasn't requested), then add back
@@ -1512,14 +1514,18 @@ virSetUIDGIDWithCaps(uid_t uid, gid_t gid, gid_t *groups, int ngroups,
      */
 
     if (clearExistingCaps || (uid != (uid_t)-1 && uid != 0))
-       capng_clear(CAPNG_SELECT_BOTH);
+        capng_clear(CAPNG_SELECT_BOTH);
 
     for (i = 0; i <= CAP_LAST_CAP; i++) {
+        capstr = capng_capability_to_name(i);
+
         if (capBits & (1ULL << i)) {
             capng_update(CAPNG_ADD,
                          CAPNG_EFFECTIVE|CAPNG_INHERITABLE|
                          CAPNG_PERMITTED|CAPNG_BOUNDING_SET,
                          i);
+
+            VIR_DEBUG("Added '%s' to child capabilities' set", capstr);
         }
     }
 
@@ -1579,6 +1585,27 @@ virSetUIDGIDWithCaps(uid_t uid, gid_t gid, gid_t *groups, int ngroups,
         goto cleanup;
     }
 
+# ifdef PR_CAP_AMBIENT
+    /* we couldn't do this in the loop earlier above, because the capabilities
+     * were not applied yet, since in order to add a capability into the AMBIENT
+     * set, it has to be present in both the PERMITTED and INHERITABLE sets
+     * (capabilities(7))
+     */
+    for (i = 0; i <= CAP_LAST_CAP; i++) {
+        capstr = capng_capability_to_name(i);
+
+        if (capBits & (1ULL << i)) {
+            if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, i, 0, 0) < 0) {
+                virReportSystemError(errno,
+                                     _("prctl failed to enable '%s' in the "
+                                       "AMBIENT set"),
+                                     capstr);
+                goto cleanup;
+            }
+        }
+    }
+# endif
+
     /* Set bounding set while we have CAP_SETPCAP.  Unfortunately we cannot
      * do this if we failed to get the capability above, so ignore the
      * return value.