[PATCH v10 2/9] Add a reference to ucounts for each cred

Alexey Gladkov
2021-04-07T17:08:42Z

For RLIMIT_NPROC and some other rlimits the user_struct that holds the
global limit is kept alive for the lifetime of a process by keeping it
in struct cred. Adding a pointer to ucounts in the struct cred will
allow to track RLIMIT_NPROC not only for user in the system, but for
user in the user_namespace.

Updating ucounts may require memory allocation which may fail. So, we
cannot change cred.ucounts in the commit_creds() because this function
cannot fail and it should always return 0. For this reason, we modify
cred.ucounts before calling the commit_creds().

Changelog

v6:
* Fix null-ptr-deref in is_ucounts_overlimit() detected by trinity. This
error was caused by the fact that cred_alloc_blank() left the ucounts
pointer empty.

Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
---
fs/exec.c | 4 ++++
include/linux/cred.h | 2 ++
include/linux/user_namespace.h | 4 ++++
kernel/cred.c | 40 ++++++++++++++++++++++++++++++++++
kernel/fork.c | 6 +++++
kernel/sys.c | 12 ++++++++++
kernel/ucount.c | 40 +++++++++++++++++++++++++++++++---
kernel/user_namespace.c | 3 +++
8 files changed, 108 insertions(+), 3 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 18594f11c31f..d7c4187ca023 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1360,6 +1360,10 @@ int begin_new_exec(struct linux_binprm * bprm)
WRITE_ONCE(me->self_exec_id, me->self_exec_id + 1);
flush_signal_handlers(me, 0);

+ retval = set_cred_ucounts(bprm->cred);
+ if (retval < 0)
+ goto out_unlock;
+
/*
* install the new credentials for this executable
*/
diff --git a/include/linux/cred.h b/include/linux/cred.h
index 4c6350503697..66436e655032 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -144,6 +144,7 @@ struct cred {
#endif
struct user_struct *user; /* real user ID subscription */
struct user_namespace *user_ns; /* user_ns the caps and keyrings are relative to. */
+ struct ucounts *ucounts;
struct group_info *group_info; /* supplementary groups for euid/fsgid */
/* RCU deletion */
union {
@@ -170,6 +171,7 @@ extern int set_security_override_from_ctx(struct cred *, const char *);
extern int set_create_files_as(struct cred *, struct inode *);
extern int cred_fscmp(const struct cred *, const struct cred *);
extern void __init cred_init(void);
+extern int set_cred_ucounts(struct cred *);

/*
* check for validity of credentials
diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 0bb833fd41f4..f71b5a4a3e74 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -97,11 +97,15 @@ struct ucounts {
};

extern struct user_namespace init_user_ns;
+extern struct ucounts init_ucounts;

bool setup_userns_sysctls(struct user_namespace *ns);
void retire_userns_sysctls(struct user_namespace *ns);
struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid, enum ucount_type type);
void dec_ucount(struct ucounts *ucounts, enum ucount_type type);
+struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid);
+struct ucounts *get_ucounts(struct ucounts *ucounts);
+void put_ucounts(struct ucounts *ucounts);

#ifdef CONFIG_USER_NS

diff --git a/kernel/cred.c b/kernel/cred.c
index 421b1149c651..58a8a9e24347 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -60,6 +60,7 @@ struct cred init_cred = {
.user = INIT_USER,
.user_ns = &init_user_ns,
.group_info = &init_groups,
+ .ucounts = &init_ucounts,
};

static inline void set_cred_subscribers(struct cred *cred, int n)
@@ -119,6 +120,8 @@ static void put_cred_rcu(struct rcu_head *rcu)
if (cred->group_info)
put_group_info(cred->group_info);
free_uid(cred->user);
+ if (cred->ucounts)
+ put_ucounts(cred->ucounts);
put_user_ns(cred->user_ns);
kmem_cache_free(cred_jar, cred);
}
@@ -222,6 +225,7 @@ struct cred *cred_alloc_blank(void)
#ifdef CONFIG_DEBUG_CREDENTIALS
new->magic = CRED_MAGIC;
#endif
+ new->ucounts = get_ucounts(&init_ucounts);

if (security_cred_alloc_blank(new, GFP_KERNEL_ACCOUNT) < 0)
goto error;
@@ -284,6 +288,11 @@ struct cred *prepare_creds(void)

if (security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT) < 0)
goto error;
+
+ new->ucounts = get_ucounts(new->ucounts);
+ if (!new->ucounts)
+ goto error;
+
validate_creds(new);
return new;

@@ -363,6 +372,8 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
ret = create_user_ns(new);
if (ret < 0)
goto error_put;
+ if (set_cred_ucounts(new) < 0)
+ goto error_put;
}

#ifdef CONFIG_KEYS
@@ -653,6 +664,31 @@ int cred_fscmp(const struct cred *a, const struct cred *b)
}
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 *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 && old_ucounts->ns == new->user_ns && uid_eq(old_ucounts->uid, new->euid))
+ return 0;
+
+ if (!(new->ucounts = alloc_ucounts(new->user_ns, new->euid)))
+ return -EAGAIN;
+
+ if (old_ucounts)
+ put_ucounts(old_ucounts);
+
+ return 0;
+}
+
/*
* initialise the credentials stuff
*/
@@ -719,6 +755,10 @@ struct cred *prepare_kernel_cred(struct task_struct *daemon)
if (security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT) < 0)
goto error;

+ new->ucounts = get_ucounts(new->ucounts);
+ if (!new->ucounts)
+ goto error;
+
put_cred(old);
validate_creds(new);
return new;
diff --git a/kernel/fork.c b/kernel/fork.c
index 54cc905e5fe0..37498cca6a1d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2995,6 +2995,12 @@ int ksys_unshare(unsigned long unshare_flags)
if (err)
goto bad_unshare_cleanup_cred;

+ if (new_cred) {
+ err = set_cred_ucounts(new_cred);
+ if (err)
+ goto bad_unshare_cleanup_cred;
+ }
+
if (new_fs || new_fd || do_sysvsem || new_cred || new_nsproxy) {
if (do_sysvsem) {
/*
diff --git a/kernel/sys.c b/kernel/sys.c
index 2e2e3f378d97..cabfc5b86175 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -552,6 +552,10 @@ long __sys_setreuid(uid_t ruid, uid_t euid)
if (retval < 0)
goto error;

+ retval = set_cred_ucounts(new);
+ if (retval < 0)
+ goto error;
+
return commit_creds(new);

error:
@@ -610,6 +614,10 @@ long __sys_setuid(uid_t uid)
if (retval < 0)
goto error;

+ retval = set_cred_ucounts(new);
+ if (retval < 0)
+ goto error;
+
return commit_creds(new);

error:
@@ -685,6 +693,10 @@ long __sys_setresuid(uid_t ruid, uid_t euid, uid_t suid)
if (retval < 0)
goto error;

+ retval = set_cred_ucounts(new);
+ if (retval < 0)
+ goto error;
+
return commit_creds(new);

error:
diff --git a/kernel/ucount.c b/kernel/ucount.c
index 04c561751af1..50cc1dfb7d28 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -8,6 +8,12 @@
#include <linux/kmemleak.h>
#include <linux/user_namespace.h>

+struct ucounts init_ucounts = {
+ .ns = &init_user_ns,
+ .uid = GLOBAL_ROOT_UID,
+ .count = 1,
+};
+
#define UCOUNTS_HASHTABLE_BITS 10
static struct hlist_head ucounts_hashtable[(1 << UCOUNTS_HASHTABLE_BITS)];
static DEFINE_SPINLOCK(ucounts_lock);
@@ -125,7 +131,15 @@ static struct ucounts *find_ucounts(struct user_namespace *ns, kuid_t uid, struc
return NULL;
}

-static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
+static void hlist_add_ucounts(struct ucounts *ucounts)
+{
+ struct hlist_head *hashent = ucounts_hashentry(ucounts->ns, ucounts->uid);
+ spin_lock_irq(&ucounts_lock);
+ hlist_add_head(&ucounts->node, hashent);
+ spin_unlock_irq(&ucounts_lock);
+}
+
+struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
{
struct hlist_head *hashent = ucounts_hashentry(ns, uid);
struct ucounts *ucounts, *new;
@@ -160,7 +174,26 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
return ucounts;
}

-static void put_ucounts(struct ucounts *ucounts)
+struct ucounts *get_ucounts(struct ucounts *ucounts)
+{
+ unsigned long flags;
+
+ if (!ucounts)
+ return NULL;
+
+ spin_lock_irqsave(&ucounts_lock, flags);
+ if (ucounts->count == INT_MAX) {
+ WARN_ONCE(1, "ucounts: counter has reached its maximum value");
+ ucounts = NULL;
+ } else {
+ ucounts->count += 1;
+ }
+ spin_unlock_irqrestore(&ucounts_lock, flags);
+
+ return ucounts;
+}
+
+void put_ucounts(struct ucounts *ucounts)
{
unsigned long flags;

@@ -194,7 +227,7 @@ struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid,
{
struct ucounts *ucounts, *iter, *bad;
struct user_namespace *tns;
- ucounts = get_ucounts(ns, uid);
+ ucounts = alloc_ucounts(ns, uid);
for (iter = ucounts; iter; iter = tns->ucounts) {
long max;
tns = iter->ns;
@@ -237,6 +270,7 @@ static __init int user_namespace_sysctl_init(void)
BUG_ON(!user_header);
BUG_ON(!setup_userns_sysctls(&init_user_ns));
#endif
+ hlist_add_ucounts(&init_ucounts);
return 0;
}
subsys_initcall(user_namespace_sysctl_init);
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index af612945a4d0..516db53166ab 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -1281,6 +1281,9 @@ static int userns_install(struct nsset *nsset, struct ns_common *ns)
put_user_ns(cred->user_ns);
set_cred_user_ns(cred, get_user_ns(user_ns));

+ if (set_cred_ucounts(cred) < 0)
+ return -EINVAL;
+
return 0;