[PATCH 2/8] ucounts: Fix set_cred_ucounts

[Eric W. Biederman]

Michal Koutný <mkoutny@xxxxxxxx> wrote:
> Tasks are associated to multiple users at once. Historically and as per
> setrlimit(2) RLIMIT_NPROC is enforce based on real user ID.
>
> The commit 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts")
> made the accounting structure "indexed" by euid and hence potentially
> account tasks differently.
>
> The effective user ID may be different e.g. for setuid programs but
> those are exec'd into already existing task (i.e. below limit), so
> different accounting is moot.
>
> Some special setresuid(2) users may notice the difference, justifying
> this fix.

I looked at the cred->ucount is only used for rlimit operations that
were previously stored in cred->user.  Making the fact cred->ucount
can refer to a different user from cred->user a bug working will all
rlimits not just RLIMIT_NPROC.

So fix set_cred_ucounts to always use the real uid not the effective uid.

Further simplify set_cred_ucounts by noticing that set_cred_ucounts
somehow retained a draft version of the check to see if alloc_ucounts
was needed that checks the new->user and new->user_ns against the
current_real_cred(), when nothing matters for setting the ucounts
field of a struct cred except the other fields in that same struct
cred.

So delete the confusing and wrong check against the
current_real_cred(), and all of it's intermediate variables.

Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20220207121800.5079-4-mkoutny@xxxxxxxx
Reported-by: Michal Koutný <mkoutny@xxxxxxxx>
Fixes: 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts")
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 kernel/cred.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/kernel/cred.c b/kernel/cred.c
index 473d17c431f3..933155c96922 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -665,21 +665,16 @@ EXPORT_SYMBOL(cred_fscmp);
 
 int set_cred_ucounts(struct cred *new)
 {
-	struct task_struct *task = current;
-	const struct cred *old = task->real_cred;
 	struct ucounts *new_ucounts, *old_ucounts = new->ucounts;
 
-	if (new->user == old->user && new->user_ns == old->user_ns)
-		return 0;
-
 	/*
 	 * This optimization is needed because alloc_ucounts() uses locks
 	 * for table lookups.
 	 */
-	if (old_ucounts->ns == new->user_ns && uid_eq(old_ucounts->uid, new->euid))
+	if (old_ucounts->ns == new->user_ns && uid_eq(old_ucounts->uid, new->uid))
 		return 0;
 
-	if (!(new_ucounts = alloc_ucounts(new->user_ns, new->euid)))
+	if (!(new_ucounts = alloc_ucounts(new->user_ns, new->uid)))
 		return -EAGAIN;
 
 	new->ucounts = new_ucounts;
-- 
2.29.2