[PATCH 03/10] LSM: SafeSetID: refactor policy hash table

[Kees Cook]

On Wed, Apr 10, 2019 at 9:55 AM Micah Morton <mortonm at chromium.org> wrote:
>
> From: Jann Horn <jannh at google.com>
>
> parent_kuid and child_kuid are kuids, there is no reason to make them
> uint64_t. (And anyway, in the kernel, the normal name for that would be
> u64, not uint64_t.)
>
> check_setuid_policy_hashtable_key() and
> check_setuid_policy_hashtable_key_value() are basically the same thing,
> merge them.
>
> Also fix the comment that claimed that (1<<8)==128.
>
> Signed-off-by: Jann Horn <jannh at google.com>
> Signed-off-by: Micah Morton <mortonm at chromium.org>

Reviewed-by: Kees Cook <keescook at chromium.org>

-Kees

> ---
>  security/safesetid/lsm.c | 62 ++++++++++++----------------------------
>  security/safesetid/lsm.h | 19 ++++++++++++
>  2 files changed, 37 insertions(+), 44 deletions(-)
>
> diff --git a/security/safesetid/lsm.c b/security/safesetid/lsm.c
> index 5310fcf3052a..15cd13b5a211 100644
> --- a/security/safesetid/lsm.c
> +++ b/security/safesetid/lsm.c
> @@ -14,67 +14,40 @@
>
>  #define pr_fmt(fmt) "SafeSetID: " fmt
>
> -#include <linux/hashtable.h>
>  #include <linux/lsm_hooks.h>
>  #include <linux/module.h>
>  #include <linux/ptrace.h>
>  #include <linux/sched/task_stack.h>
>  #include <linux/security.h>
> +#include "lsm.h"
>
>  /* Flag indicating whether initialization completed */
>  int safesetid_initialized;
>
> -#define NUM_BITS 8 /* 128 buckets in hash table */
> +#define NUM_BITS 8 /* 256 buckets in hash table */
>
>  static DEFINE_HASHTABLE(safesetid_whitelist_hashtable, NUM_BITS);
>
> -/*
> - * Hash table entry to store safesetid policy signifying that 'parent' user
> - * can setid to 'child' user.
> - */
> -struct entry {
> -       struct hlist_node next;
> -       struct hlist_node dlist; /* for deletion cleanup */
> -       uint64_t parent_kuid;
> -       uint64_t child_kuid;
> -};
> -
>  static DEFINE_SPINLOCK(safesetid_whitelist_hashtable_spinlock);
>
> -static bool check_setuid_policy_hashtable_key(kuid_t parent)
> +static enum sid_policy_type setuid_policy_lookup(kuid_t src, kuid_t dst)
>  {
>         struct entry *entry;
> +       enum sid_policy_type result = SIDPOL_DEFAULT;
>
>         rcu_read_lock();
>         hash_for_each_possible_rcu(safesetid_whitelist_hashtable,
> -                                  entry, next, __kuid_val(parent)) {
> -               if (entry->parent_kuid == __kuid_val(parent)) {
> +                                  entry, next, __kuid_val(src)) {
> +               if (!uid_eq(entry->src_uid, src))
> +                       continue;
> +               if (uid_eq(entry->dst_uid, dst)) {
>                         rcu_read_unlock();
> -                       return true;
> +                       return SIDPOL_ALLOWED;
>                 }
> +               result = SIDPOL_CONSTRAINED;
>         }
>         rcu_read_unlock();
> -
> -       return false;
> -}
> -
> -static bool check_setuid_policy_hashtable_key_value(kuid_t parent,
> -                                                   kuid_t child)
> -{
> -       struct entry *entry;
> -
> -       rcu_read_lock();
> -       hash_for_each_possible_rcu(safesetid_whitelist_hashtable,
> -                                  entry, next, __kuid_val(parent)) {
> -               if (entry->parent_kuid == __kuid_val(parent) &&
> -                   entry->child_kuid == __kuid_val(child)) {
> -                       rcu_read_unlock();
> -                       return true;
> -               }
> -       }
> -       rcu_read_unlock();
> -
> -       return false;
> +       return result;
>  }
>
>  static int safesetid_security_capable(const struct cred *cred,
> @@ -83,7 +56,7 @@ static int safesetid_security_capable(const struct cred *cred,
>                                       unsigned int opts)
>  {
>         if (cap == CAP_SETUID &&
> -           check_setuid_policy_hashtable_key(cred->uid)) {
> +           setuid_policy_lookup(cred->uid, INVALID_UID) != SIDPOL_DEFAULT) {
>                 if (!(opts & CAP_OPT_INSETID)) {
>                         /*
>                          * Deny if we're not in a set*uid() syscall to avoid
> @@ -116,7 +89,8 @@ static bool uid_permitted_for_cred(const struct cred *old, kuid_t new_uid)
>          * Transitions to new UIDs require a check against the policy of the old
>          * RUID.
>          */
> -       permitted = check_setuid_policy_hashtable_key_value(old->uid, new_uid);
> +       permitted =
> +           setuid_policy_lookup(old->uid, new_uid) != SIDPOL_CONSTRAINED;
>         if (!permitted) {
>                 pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n",
>                         __kuid_val(old->uid), __kuid_val(old->euid),
> @@ -136,7 +110,7 @@ static int safesetid_task_fix_setuid(struct cred *new,
>  {
>
>         /* Do nothing if there are no setuid restrictions for our old RUID. */
> -       if (!check_setuid_policy_hashtable_key(old->uid))
> +       if (setuid_policy_lookup(old->uid, INVALID_UID) == SIDPOL_DEFAULT)
>                 return 0;
>
>         if (uid_permitted_for_cred(old, new->uid) &&
> @@ -159,14 +133,14 @@ int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child)
>         struct entry *new;
>
>         /* Return if entry already exists */
> -       if (check_setuid_policy_hashtable_key_value(parent, child))
> +       if (setuid_policy_lookup(parent, child) == SIDPOL_ALLOWED)
>                 return 0;
>
>         new = kzalloc(sizeof(struct entry), GFP_KERNEL);
>         if (!new)
>                 return -ENOMEM;
> -       new->parent_kuid = __kuid_val(parent);
> -       new->child_kuid = __kuid_val(child);
> +       new->src_uid = parent;
> +       new->dst_uid = child;
>         spin_lock(&safesetid_whitelist_hashtable_spinlock);
>         hash_add_rcu(safesetid_whitelist_hashtable,
>                      &new->next,
> diff --git a/security/safesetid/lsm.h b/security/safesetid/lsm.h
> index c1ea3c265fcf..6806f902794c 100644
> --- a/security/safesetid/lsm.h
> +++ b/security/safesetid/lsm.h
> @@ -15,6 +15,8 @@
>  #define _SAFESETID_H
>
>  #include <linux/types.h>
> +#include <linux/uidgid.h>
> +#include <linux/hashtable.h>
>
>  /* Flag indicating whether initialization completed */
>  extern int safesetid_initialized;
> @@ -25,6 +27,23 @@ enum safesetid_whitelist_file_write_type {
>         SAFESETID_WHITELIST_FLUSH, /* Flush whitelist policies. */
>  };
>
> +enum sid_policy_type {
> +       SIDPOL_DEFAULT, /* source ID is unaffected by policy */
> +       SIDPOL_CONSTRAINED, /* source ID is affected by policy */
> +       SIDPOL_ALLOWED /* target ID explicitly allowed */
> +};
> +
> +/*
> + * Hash table entry to store safesetid policy signifying that 'src_uid'
> + * can setid to 'dst_uid'.
> + */
> +struct entry {
> +       struct hlist_node next;
> +       struct hlist_node dlist; /* for deletion cleanup */
> +       kuid_t src_uid;
> +       kuid_t dst_uid;
> +};
> +
>  /* Add entry to safesetid whitelist to allow 'parent' to setid to 'child'. */
>  int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child);
>
> --
> 2.21.0.392.gf8f6787159e-goog
>


-- 
Kees Cook