[PATCH v2 16/28] namei: may_{o_}create(): handle fsid mappings

Christian Brauner
2020-02-14T18:38:16Z

Switch may_{o_}create() to lookup fsids in the fsid mappings. If no fsid
mappings are setup the behavior is unchanged, i.e. fsids are looked up in the
id mappings.

Filesystems that share a superblock in all user namespaces they are mounted in
will retain their old semantics even with the introduction of fsidmappings.

Cc: Jann Horn <jannh@google.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v2 */
- Jann Horn <jannh@google.com>:
- Ensure that the correct fsid is used when dealing with userns visible
filesystems like proc.
---
fs/namei.c | 36 ++++++++++++++++++++++++++++--------
1 file changed, 28 insertions(+), 8 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index db6565c99825..c5b014000f13 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -39,6 +39,7 @@
#include <linux/bitops.h>
#include <linux/init_task.h>
#include <linux/uaccess.h>
+#include <linux/fsuidgid.h>

#include "internal.h"
#include "mount.h"
@@ -287,6 +288,13 @@ static int check_acl(struct inode *inode, int mask)
return -EAGAIN;
}

+static inline kuid_t get_current_fsuid(const struct inode *inode)
+{
+ if (is_userns_visible(inode->i_sb->s_iflags))
+ return current_kfsuid();
+ return current_fsuid();
+}
+
/*
* This does the basic permission checking
*/
@@ -294,7 +302,7 @@ static int acl_permission_check(struct inode *inode, int mask)
{
unsigned int mode = inode->i_mode;

- if (likely(uid_eq(current_fsuid(), inode->i_uid)))
+ if (likely(uid_eq(get_current_fsuid(inode), inode->i_uid)))
mode >>= 6;
else {
if (IS_POSIXACL(inode) && (mode & S_IRWXG)) {
@@ -980,7 +988,7 @@ static inline int may_follow_link(struct nameidata *nd)

/* Allowed if owner and follower match. */
inode = nd->link_inode;
- if (uid_eq(current_cred()->fsuid, inode->i_uid))
+ if (uid_eq(get_current_fsuid(inode), inode->i_uid))
return 0;

/* Allowed if parent directory not sticky and world-writable. */
@@ -1097,7 +1105,7 @@ static int may_create_in_sticky(umode_t dir_mode, kuid_t dir_uid,
(!sysctl_protected_regular && S_ISREG(inode->i_mode)) ||
likely(!(dir_mode & S_ISVTX)) ||
uid_eq(inode->i_uid, dir_uid) ||
- uid_eq(current_fsuid(), inode->i_uid))
+ uid_eq(get_current_fsuid(inode), inode->i_uid))
return 0;

if (likely(dir_mode & 0002) ||
@@ -2832,7 +2840,7 @@ EXPORT_SYMBOL(kern_path_mountpoint);

int __check_sticky(struct inode *dir, struct inode *inode)
{
- kuid_t fsuid = current_fsuid();
+ kuid_t fsuid = get_current_fsuid(inode);

if (uid_eq(inode->i_uid, fsuid))
return 0;
@@ -2902,6 +2910,20 @@ static int may_delete(struct inode *dir, struct dentry *victim, bool isdir)
return 0;
}

+static bool fsid_has_mapping(struct user_namespace *ns, struct super_block *sb)
+{
+ if (is_userns_visible(sb->s_iflags)) {
+ if (!kuid_has_mapping(ns, current_kfsuid()) ||
+ !kgid_has_mapping(ns, current_kfsgid()))
+ return false;
+ } else if (!kfsuid_has_mapping(ns, current_fsuid()) ||
+ !kfsgid_has_mapping(ns, current_fsgid())) {
+ return false;
+ }
+
+ return true;
+}
+
/* Check whether we can create an object with dentry child in directory
* dir.
* 1. We can't do it if child already exists (open has special treatment for
@@ -2920,8 +2942,7 @@ static inline int may_create(struct inode *dir, struct dentry *child)
if (IS_DEADDIR(dir))
return -ENOENT;
s_user_ns = dir->i_sb->s_user_ns;
- if (!kuid_has_mapping(s_user_ns, current_fsuid()) ||
- !kgid_has_mapping(s_user_ns, current_fsgid()))
+ if (!fsid_has_mapping(s_user_ns, dir->i_sb))
return -EOVERFLOW;
return inode_permission(dir, MAY_WRITE | MAY_EXEC);
}
@@ -3103,8 +3124,7 @@ static int may_o_create(const struct path *dir, struct dentry *dentry, umode_t m
return error;

s_user_ns = dir->dentry->d_sb->s_user_ns;
- if (!kuid_has_mapping(s_user_ns, current_fsuid()) ||
- !kgid_has_mapping(s_user_ns, current_fsgid()))
+ if (!fsid_has_mapping(s_user_ns, dir->dentry->d_sb))
return -EOVERFLOW;

error = inode_permission(dir->dentry->d_inode, MAY_WRITE | MAY_EXEC);
--
2.25.0