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

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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>
This commit is contained in:
Erik Skultety 2019-01-24 10:33:01 +01:00
parent 17f6a257f1
commit a2d3dea9d4
2 changed files with 40 additions and 2 deletions
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11
src/qemu/qemu_capabilities.c
View File

@ -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);

31
src/util/virutil.c
View File

@ -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.