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LSM: SafeSetID: fix check for setresuid(new1, new2, new3) 

With the old code, when a process with the (real,effective,saved) UID set
(1,1,1) calls setresuid(2,3,4), safesetid_task_fix_setuid() only checks
whether the transition 1->2 is permitted; the transitions 1->3 and 1->4 are
not checked. Fix this.

This is also a good opportunity to refactor safesetid_task_fix_setuid() to
be less verbose - having one branch per set*uid() syscall is unnecessary.

Note that this slightly changes semantics: The UID transition check for
UIDs that were not in the old cred struct is now always performed against
the policy of the RUID. I think that's more consistent anyway, since the
RUID is also the one that decides whether any policy is enforced at all.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Micah Morton <mortonm@chromium.org>
This commit is contained in:
Jann Horn 2019-04-10 09:55:19 -07:00 committed by Micah Morton
parent c783d525f9
commit 7ef6b3062f
1 changed files with 35 additions and 90 deletions
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125
security/safesetid/lsm.c
View File

@ -99,20 +99,30 @@ static int safesetid_security_capable(const struct cred *cred,
return 0;
}
static int check_uid_transition(kuid_t parent, kuid_t child)
/*
* Check whether a caller with old credentials @old is allowed to switch to
* credentials that contain @new_uid.
*/
static bool uid_permitted_for_cred(const struct cred *old, kuid_t new_uid)
{
if (check_setuid_policy_hashtable_key_value(parent, child))
return 0;
pr_warn("UID transition (%d -> %d) blocked\n",
__kuid_val(parent),
__kuid_val(child));
bool permitted;
/* If our old creds already had this UID in it, it's fine. */
if (uid_eq(new_uid, old->uid) || uid_eq(new_uid, old->euid) ||
uid_eq(new_uid, old->suid))
return true;
/*
* Kill this process to avoid potential security vulnerabilities
* that could arise from a missing whitelist entry preventing a
* privileged process from dropping to a lesser-privileged one.
* Transitions to new UIDs require a check against the policy of the old
* RUID.
*/
force_sig(SIGKILL);
return -EACCES;
permitted = check_setuid_policy_hashtable_key_value(old->uid, new_uid);
if (!permitted) {
pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n",
__kuid_val(old->uid), __kuid_val(old->euid),
__kuid_val(old->suid), __kuid_val(new_uid));
}
return permitted;
}
/*
@ -125,88 +135,23 @@ static int safesetid_task_fix_setuid(struct cred *new,
int flags)
{
/* Do nothing if there are no setuid restrictions for this UID. */
/* Do nothing if there are no setuid restrictions for our old RUID. */
if (!check_setuid_policy_hashtable_key(old->uid))
return 0;
switch (flags) {
case LSM_SETID_RE:
/*
* Users for which setuid restrictions exist can only set the
* real UID to the real UID or the effective UID, unless an
* explicit whitelist policy allows the transition.
*/
if (!uid_eq(old->uid, new->uid) &&
!uid_eq(old->euid, new->uid)) {
return check_uid_transition(old->uid, new->uid);
}
/*
* Users for which setuid restrictions exist can only set the
* effective UID to the real UID, the effective UID, or the
* saved set-UID, unless an explicit whitelist policy allows
* the transition.
*/
if (!uid_eq(old->uid, new->euid) &&
!uid_eq(old->euid, new->euid) &&
!uid_eq(old->suid, new->euid)) {
return check_uid_transition(old->euid, new->euid);
}
break;
case LSM_SETID_ID:
/*
* Users for which setuid restrictions exist cannot change the
* real UID or saved set-UID unless an explicit whitelist
* policy allows the transition.
*/
if (!uid_eq(old->uid, new->uid))
return check_uid_transition(old->uid, new->uid);
if (!uid_eq(old->suid, new->suid))
return check_uid_transition(old->suid, new->suid);
break;
case LSM_SETID_RES:
/*
* Users for which setuid restrictions exist cannot change the
* real UID, effective UID, or saved set-UID to anything but
* one of: the current real UID, the current effective UID or
* the current saved set-user-ID unless an explicit whitelist
* policy allows the transition.
*/
if (!uid_eq(new->uid, old->uid) &&
!uid_eq(new->uid, old->euid) &&
!uid_eq(new->uid, old->suid)) {
return check_uid_transition(old->uid, new->uid);
}
if (!uid_eq(new->euid, old->uid) &&
!uid_eq(new->euid, old->euid) &&
!uid_eq(new->euid, old->suid)) {
return check_uid_transition(old->euid, new->euid);
}
if (!uid_eq(new->suid, old->uid) &&
!uid_eq(new->suid, old->euid) &&
!uid_eq(new->suid, old->suid)) {
return check_uid_transition(old->suid, new->suid);
}
break;
case LSM_SETID_FS:
/*
* Users for which setuid restrictions exist cannot change the
* filesystem UID to anything but one of: the current real UID,
* the current effective UID or the current saved set-UID
* unless an explicit whitelist policy allows the transition.
*/
if (!uid_eq(new->fsuid, old->uid) &&
!uid_eq(new->fsuid, old->euid) &&
!uid_eq(new->fsuid, old->suid) &&
!uid_eq(new->fsuid, old->fsuid)) {
return check_uid_transition(old->fsuid, new->fsuid);
}
break;
default:
pr_warn("Unknown setid state %d\n", flags);
force_sig(SIGKILL);
return -EINVAL;
}
return 0;
if (uid_permitted_for_cred(old, new->uid) &&
uid_permitted_for_cred(old, new->euid) &&
uid_permitted_for_cred(old, new->suid) &&
uid_permitted_for_cred(old, new->fsuid))
return 0;
/*
* Kill this process to avoid potential security vulnerabilities
* that could arise from a missing whitelist entry preventing a
* privileged process from dropping to a lesser-privileged one.
*/
force_sig(SIGKILL);
return -EACCES;
}
int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child)