move RLIMIT_NPROC check from set_user() to do_execve_common()

The patch http://lkml.org/lkml/2003/7/13/226 introduced an RLIMIT_NPROC
check in set_user() to check for NPROC exceeding via setuid() and
similar functions.

Before the check there was a possibility to greatly exceed the allowed
number of processes by an unprivileged user if the program relied on
rlimit only.  But the check created new security threat: many poorly
written programs simply don't check setuid() return code and believe it
cannot fail if executed with root privileges.  So, the check is removed
in this patch because of too often privilege escalations related to
buggy programs.

The NPROC can still be enforced in the common code flow of daemons
spawning user processes.  Most of daemons do fork()+setuid()+execve().
The check introduced in execve() (1) enforces the same limit as in
setuid() and (2) doesn't create similar security issues.

Neil Brown suggested to track what specific process has exceeded the
limit by setting PF_NPROC_EXCEEDED process flag.  With the change only
this process would fail on execve(), and other processes' execve()
behaviour is not changed.

Solar Designer suggested to re-check whether NPROC limit is still
exceeded at the moment of execve().  If the process was sleeping for
days between set*uid() and execve(), and the NPROC counter step down
under the limit, the defered execve() failure because NPROC limit was
exceeded days ago would be unexpected.  If the limit is not exceeded
anymore, we clear the flag on successful calls to execve() and fork().

The flag is also cleared on successful calls to set_user() as the limit
was exceeded for the previous user, not the current one.

Similar check was introduced in -ow patches (without the process flag).

v3 - clear PF_NPROC_EXCEEDED on successful calls to set_user().

Reviewed-by: James Morris <[email protected]>
Signed-off-by: Vasiliy Kulikov <[email protected]>
Acked-by: NeilBrown <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/exec.c

@@ -1459,6 +1459,23 @@ static int do_execve_common(const char *filename,
 	struct files_struct *displaced;
 	bool clear_in_exec;
 	int retval;
+	const struct cred *cred = current_cred();
+
+	/*
+	 * We move the actual failure in case of RLIMIT_NPROC excess from
+	 * set*uid() to execve() because too many poorly written programs
+	 * don't check setuid() return code.  Here we additionally recheck
+	 * whether NPROC limit is still exceeded.
+	 */
+	if ((current->flags & PF_NPROC_EXCEEDED) &&
+	    atomic_read(&cred->user->processes) > rlimit(RLIMIT_NPROC)) {
+		retval = -EAGAIN;
+		goto out_ret;
+	}
+
+	/* We're below the limit (still or again), so we don't want to make
+	 * further execve() calls fail. */
+	current->flags &= ~PF_NPROC_EXCEEDED;
 
 	retval = unshare_files(&displaced);
 	if (retval)

include/linux/sched.h

@@ -1767,6 +1767,7 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
 #define PF_DUMPCORE	0x00000200	/* dumped core */
 #define PF_SIGNALED	0x00000400	/* killed by a signal */
 #define PF_MEMALLOC	0x00000800	/* Allocating memory */
+#define PF_NPROC_EXCEEDED 0x00001000	/* set_user noticed that RLIMIT_NPROC was exceeded */
 #define PF_USED_MATH	0x00002000	/* if unset the fpu must be initialized before use */
 #define PF_FREEZING	0x00004000	/* freeze in progress. do not account to load */
 #define PF_NOFREEZE	0x00008000	/* this thread should not be frozen */

kernel/cred.c

@@ -508,10 +508,8 @@ int commit_creds(struct cred *new)
 		key_fsgid_changed(task);
 
 	/* do it
-	 * - What if a process setreuid()'s and this brings the
-	 *   new uid over his NPROC rlimit?  We can check this now
-	 *   cheaply with the new uid cache, so if it matters
-	 *   we should be checking for it.  -DaveM
+	 * RLIMIT_NPROC limits on user->processes have already been checked
+	 * in set_user().
 	 */
 	alter_cred_subscribers(new, 2);
 	if (new->user != old->user)

kernel/fork.c

@@ -1111,6 +1111,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 		    p->real_cred->user != INIT_USER)
 			goto bad_fork_free;
 	}
+	current->flags &= ~PF_NPROC_EXCEEDED;
 
 	retval = copy_creds(p, clone_flags);
 	if (retval < 0)

kernel/sys.c

@@ -621,11 +621,18 @@ 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 (atomic_read(&new_user->processes) >= rlimit(RLIMIT_NPROC) &&
-			new_user != INIT_USER) {
-		free_uid(new_user);
-		return -EAGAIN;
-	}
+			new_user != INIT_USER)
+		current->flags |= PF_NPROC_EXCEEDED;
+	else
+		current->flags &= ~PF_NPROC_EXCEEDED;
 
 	free_uid(new->user);
 	new->user = new_user;