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

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(a)redhat.com&gt; > > --- > > 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™