[RFC PATCH 2/6] set*uid: Check RLIMIT_PROC against new credentials

[Michal Koutný]

The generic idea is that not even root or capable user can force an
unprivileged user's limit breach. (For historical and security reasons
this check is postponed from set*uid to execve.) During the switch the
resource consumption of target the user has to be checked. The commits
905ae01c4ae2 ("Add a reference to ucounts for each cred") and
21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts") made the
check in set_user() look at the old user's consumption.

This version of the fix simply moves the check to the place where the
actual switch of the accounting structure happens -- set_cred_ucounts().

The other callers are kept without the check but with the per-userns
accounting they may be newly subject to the check too.
The set_cred_ucounts() becomes inconsistent since task->flags are
passed by the caller but task_rlimit() is implicitly `current`'s, this
patch is meant to illustrate the issue, nicer implementation is
possible.

Fixes: 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts")
Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
---
 fs/exec.c               |  2 +-
 include/linux/cred.h    |  2 +-
 kernel/cred.c           | 24 +++++++++++++++++++++---
 kernel/fork.c           |  2 +-
 kernel/sys.c            | 21 +++------------------
 kernel/user_namespace.c |  2 +-
 6 files changed, 28 insertions(+), 25 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index fc598c2652b2..e759e42c61da 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1363,7 +1363,7 @@ int begin_new_exec(struct linux_binprm * bprm)
 	WRITE_ONCE(me->self_exec_id, me->self_exec_id + 1);
 	flush_signal_handlers(me, 0);
 
-	retval = set_cred_ucounts(bprm->cred);
+	retval = set_cred_ucounts(bprm->cred, NULL);
 	if (retval < 0)
 		goto out_unlock;
 
diff --git a/include/linux/cred.h b/include/linux/cred.h
index fcbc6885cc09..455525ab380d 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -170,7 +170,7 @@ extern int set_security_override_from_ctx(struct cred *, const char *);
 extern int set_create_files_as(struct cred *, struct inode *);
 extern int cred_fscmp(const struct cred *, const struct cred *);
 extern void __init cred_init(void);
-extern int set_cred_ucounts(struct cred *);
+extern int set_cred_ucounts(struct cred *, unsigned int *);
 
 /*
  * check for validity of credentials
diff --git a/kernel/cred.c b/kernel/cred.c
index 473d17c431f3..791cab70b764 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -370,7 +370,7 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
 		ret = create_user_ns(new);
 		if (ret < 0)
 			goto error_put;
-		ret = set_cred_ucounts(new);
+		ret = set_cred_ucounts(new, NULL);
 		if (ret < 0)
 			goto error_put;
 	}
@@ -492,7 +492,7 @@ int commit_creds(struct cred *new)
 
 	/* do it
 	 * RLIMIT_NPROC limits on user->processes have already been checked
-	 * in set_user().
+	 * in set_cred_ucounts().
 	 */
 	alter_cred_subscribers(new, 2);
 	if (new->user != old->user || new->user_ns != old->user_ns)
@@ -663,7 +663,7 @@ int cred_fscmp(const struct cred *a, const struct cred *b)
 }
 EXPORT_SYMBOL(cred_fscmp);
 
-int set_cred_ucounts(struct cred *new)
+int set_cred_ucounts(struct cred *new, unsigned int *nproc_flags)
 {
 	struct task_struct *task = current;
 	const struct cred *old = task->real_cred;
@@ -685,6 +685,24 @@ int set_cred_ucounts(struct cred *new)
 	new->ucounts = new_ucounts;
 	put_ucounts(old_ucounts);
 
+	if (!nproc_flags)
+		return 0;
+
+	/*
+	 * We don't fail in case of NPROC limit excess here because too many
+	 * poorly written programs don't check set*uid() return code, assuming
+	 * it never fails if called by root.  We may still enforce NPROC limit
+	 * for programs doing set*uid()+execve() by harmlessly deferring the
+	 * failure to the execve() stage.
+	 */
+	if (ucounts_limit_cmp(new->ucounts, UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) >= 0 &&
+			new->user != INIT_USER &&
+			!security_capable(new, &init_user_ns, CAP_SYS_RESOURCE, CAP_OPT_NONE) &&
+			!security_capable(new, &init_user_ns, CAP_SYS_ADMIN, CAP_OPT_NONE))
+		*nproc_flags |= PF_NPROC_EXCEEDED;
+	else
+		*nproc_flags &= ~PF_NPROC_EXCEEDED;
+
 	return 0;
 }
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 7cb21a70737d..a4005c679d29 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -3051,7 +3051,7 @@ int ksys_unshare(unsigned long unshare_flags)
 		goto bad_unshare_cleanup_cred;
 
 	if (new_cred) {
-		err = set_cred_ucounts(new_cred);
+		err = set_cred_ucounts(new_cred, NULL);
 		if (err)
 			goto bad_unshare_cleanup_cred;
 	}
diff --git a/kernel/sys.c b/kernel/sys.c
index 48c90dcceff3..4e4eea30e235 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -472,21 +472,6 @@ static int set_user(struct cred *new)
 	if (!new_user)
 		return -EAGAIN;
 
-	/*
-	 * We don't fail in case of NPROC limit excess here because too many
-	 * poorly written programs don't check set*uid() return code, assuming
-	 * it never fails if called by root.  We may still enforce NPROC limit
-	 * for programs doing set*uid()+execve() by harmlessly deferring the
-	 * failure to the execve() stage.
-	 */
-	if (ucounts_limit_cmp(new->ucounts, UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) >= 0 &&
-			new_user != INIT_USER &&
-			!security_capable(new, &init_user_ns, CAP_SYS_RESOURCE, CAP_OPT_NONE) &&
-			!security_capable(new, &init_user_ns, CAP_SYS_ADMIN, CAP_OPT_NONE))
-		current->flags |= PF_NPROC_EXCEEDED;
-	else
-		current->flags &= ~PF_NPROC_EXCEEDED;
-
 	free_uid(new->user);
 	new->user = new_user;
 	return 0;
@@ -560,7 +545,7 @@ long __sys_setreuid(uid_t ruid, uid_t euid)
 	if (retval < 0)
 		goto error;
 
-	retval = set_cred_ucounts(new);
+	retval = set_cred_ucounts(new, &current->flags);
 	if (retval < 0)
 		goto error;
 
@@ -622,7 +607,7 @@ long __sys_setuid(uid_t uid)
 	if (retval < 0)
 		goto error;
 
-	retval = set_cred_ucounts(new);
+	retval = set_cred_ucounts(new, &current->flags);
 	if (retval < 0)
 		goto error;
 
@@ -701,7 +686,7 @@ long __sys_setresuid(uid_t ruid, uid_t euid, uid_t suid)
 	if (retval < 0)
 		goto error;
 
-	retval = set_cred_ucounts(new);
+	retval = set_cred_ucounts(new, &current->flags);
 	if (retval < 0)
 		goto error;
 
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 6b2e3ca7ee99..f7eec0b0233b 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -1344,7 +1344,7 @@ static int userns_install(struct nsset *nsset, struct ns_common *ns)
 	put_user_ns(cred->user_ns);
 	set_cred_user_ns(cred, get_user_ns(user_ns));
 
-	if (set_cred_ucounts(cred) < 0)
+	if (set_cred_ucounts(cred, NULL) < 0)
 		return -EINVAL;
 
 	return 0;
-- 
2.34.1