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@xxxxxxxxxx>
> > ---
> > 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™
>
> Reviewed-by: Daniel P. Berrangé <berrange@xxxxxxxxxx>
Thanks,
Erik
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