[libvirt] [PATCH 5/5] qemu: caps: Use CAP_DAC_OVERRIDE for probing to avoid permission issues

[Daniel P. Berrangé]

On Fri, Feb 01, 2019 at 12:33:19PM +0100, Erik Skultety wrote:
> On Fri, Feb 01, 2019 at 10:31:52AM +0000, Daniel P. Berrangé wrote:
> > On Thu, Jan 31, 2019 at 04:26:18PM +0100, Erik Skultety wrote:
> > > 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
> > > filesystem permisions are cross checked in kernel and therefore a user
> > > with read permisions could issue a 'privileged' operation on SEV which
> > > is currently only limited to root.
> > >
> > > Signed-off-by: Erik Skultety <eskultet at redhat.com>
> > > ---
> > >  src/qemu/qemu_capabilities.c | 11 +++++++++++
> > >  src/util/virutil.c           | 31 +++++++++++++++++++++++++++++--
> > >  2 files changed, 40 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
> > > index 5cf4b617c6..2e84c965e8 100644
> > > --- a/src/qemu/qemu_capabilities.c
> > > +++ b/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");
> > > @@ -4515,6 +4519,13 @@ 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);
> > >
> > > diff --git a/src/util/virutil.c b/src/util/virutil.c
> > > index 5251b66454..02de92061c 100644
> > > --- a/src/util/virutil.c
> > > +++ b/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
> >
> > This is set a bit earlier than I set it in my PoC patch, but I'll assume
> > it still works given the comment you added.
> 
> I was trying to understand whether there was a particular reason why you added
> it to the ambient set later, so my first lame attempt was to merge the 2 'for'
> loops into 1, since they were identical apart from the prctl syscall which led
> to an error. So I investigated and found the restriction I mentioned in the
> comment so I moved it after the caps were first applied and it did work:
> (trial-error)+-research method™

I added it really early at first and it didn't work, so I the put it right
at the end. I didn't bother to try it in the middle as I was lazy :-)

Regards,
Daniel
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