ucounts: Base set_cred_ucounts changes on the real user

Michal Koutný <mkoutny@suse.com> 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 96a6f980d813 ("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 cred->ucount and it 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,
affecting all uses of cred->ulimit not just RLIMIT_NPROC.

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().  Remove that draft version of the check.

All that matters for setting the cred->ucounts are the user_ns and uid
fields in the cred.

Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20220207121800.5079-4-mkoutny@suse.com
Link: https://lkml.kernel.org/r/20220216155832.680775-3-ebiederm@xmission.com
Reported-by: Michal Koutný <mkoutny@suse.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Fixes: 96a6f980d813 ("Reimplement RLIMIT_NPROC on top of ucounts")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/kernel/cred.c b/kernel/cred.c
index 473d17c431f3a8e7dc10cf66c15b8ff526bf551d..933155c9692270fe700f609dff5aafc98de59524 100644 (file)
--- 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;