[PATCH 3/8] ucounts: Fix and simplify RLIMIT_NPROC handling during setuid()+execve

[Eric W. Biederman]

Michal Koutný <mkoutny@xxxxxxxx> wrote:
> The check is currently against the current->cred but since those are
> going to change and we want to check RLIMIT_NPROC condition after the
> switch, supply the capability check with the new cred.
> But since we're checking new_user being INIT_USER any new cred's
> capability-based allowance may be redundant when the check fails and the
> alternative solution would be revert of the commit 2863643fb8b9
> ("set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds")

As of commit 2863643fb8b9 ("set_user: add capability check when
rlimit(RLIMIT_NPROC) exceeds") setting the flag to see if execve
should check RLIMIT_NPROC is buggy, as it tests the capabilites from
before the credential change and not aftwards.

As of commit 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of
ucounts") examining the rlimit is buggy as cred->ucounts has not yet
been properly set in the new credential.

Make the code correct and more robust moving the test to see if
execve() needs to test RLIMIT_NPROC into commit_creds, and defer all
of the rest of the logic into execve() itself.

As the flag only indicateds that RLIMIT_NPROC should be checked
in execve rename it from PF_NPROC_EXCEEDED to PF_NPROC_CHECK.

Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20220207121800.5079-2-mkoutny@xxxxxxxx
Link: https://lkml.kernel.org/r/20220207121800.5079-3-mkoutny@xxxxxxxx
Reported-by: Michal Koutný <mkoutny@xxxxxxxx>
Fixes: 2863643fb8b9 ("set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds")
Fixes: 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts")
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 fs/exec.c             | 15 ++++++++-------
 include/linux/sched.h |  2 +-
 kernel/cred.c         | 13 +++++++++----
 kernel/fork.c         |  2 +-
 kernel/sys.c          | 14 --------------
 5 files changed, 19 insertions(+), 27 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 79f2c9483302..1e7f757cbc2c 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1875,20 +1875,21 @@ static int do_execveat_common(int fd, struct filename *filename,
 		return PTR_ERR(filename);
 
 	/*
-	 * 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.
+	 * After calling set*uid() is RLIMT_NPROC exceeded?
+	 * This can not be checked in set*uid() because too many programs don't
+	 * check the setuid() return code.
 	 */
-	if ((current->flags & PF_NPROC_EXCEEDED) &&
-	    is_ucounts_overlimit(current_ucounts(), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
+	if ((current->flags & PF_NPROC_CHECK) &&
+	    is_ucounts_overlimit(current_ucounts(), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) &&
+	    (current_user() != INIT_USER) &&
+	    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) {
 		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;
+	current->flags &= ~PF_NPROC_CHECK;
 
 	bprm = alloc_bprm(fd, filename);
 	if (IS_ERR(bprm)) {
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 75ba8aa60248..6605a262a6be 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1678,7 +1678,7 @@ extern struct pid *cad_pid;
 #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_NPROC_CHECK		0x00001000	/* Check in execve if RLIMIT_NPROC was exceeded */
 #define PF_USED_MATH		0x00002000	/* If unset the fpu must be initialized before use */
 #define PF_NOFREEZE		0x00008000	/* This thread should not be frozen */
 #define PF_FROZEN		0x00010000	/* Frozen for system suspend */
diff --git a/kernel/cred.c b/kernel/cred.c
index 933155c96922..229cff081167 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -490,13 +490,18 @@ int commit_creds(struct cred *new)
 	if (!gid_eq(new->fsgid, old->fsgid))
 		key_fsgid_changed(new);
 
-	/* do it
-	 * RLIMIT_NPROC limits on user->processes have already been checked
-	 * in set_user().
+	/*
+	 * Remember if the NPROC limit may be exceeded.  The set*uid() functions
+	 * can not fail if the NPROC limit is exceeded as too many programs
+	 * don't check the return code.  Instead enforce the NPROC limit for
+	 * programs doing set*uid()+execve by harmlessly defering the failure
+	 * to the execve() stage.
 	 */
 	alter_cred_subscribers(new, 2);
-	if (new->user != old->user || new->user_ns != old->user_ns)
+	if (new->user != old->user || new->user_ns != old->user_ns) {
 		inc_rlimit_ucounts(new->ucounts, UCOUNT_RLIMIT_NPROC, 1);
+		task->flags |= PF_NPROC_CHECK;
+	}
 	rcu_assign_pointer(task->real_cred, new);
 	rcu_assign_pointer(task->cred, new);
 	if (new->user != old->user || new->user_ns != old->user_ns)
diff --git a/kernel/fork.c b/kernel/fork.c
index 17d8a8c85e3b..2b6a28a86325 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2031,7 +2031,7 @@ static __latent_entropy struct task_struct *copy_process(
 		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
 			goto bad_fork_cleanup_count;
 	}
-	current->flags &= ~PF_NPROC_EXCEEDED;
+	current->flags &= ~PF_NPROC_CHECK;
 
 	/*
 	 * If multiple threads are within copy_process(), then this check
diff --git a/kernel/sys.c b/kernel/sys.c
index ecc4cf019242..b1ed21d79f3b 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -472,20 +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 (is_ucounts_overlimit(new->ucounts, UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) &&
-			new_user != INIT_USER &&
-			!capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
-		current->flags |= PF_NPROC_EXCEEDED;
-	else
-		current->flags &= ~PF_NPROC_EXCEEDED;
-
 	free_uid(new->user);
 	new->user = new_user;
 	return 0;
-- 
2.29.2