[PATCH 5.16 173/227] ucounts: Base set_cred_ucounts changes on the real user

Greg Kroah-Hartman
2022-02-21T10:03:05Z

From: Eric W. Biederman <ebiederm@xmission.com>

commit a55d07294f1e9b576093bdfa95422f8119941e83 upstream.

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 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 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: 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/cred.c | 9 ++-------
1 file changed, 2 insertions(+), 7 deletions(-)

--- 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;