[Devel] Re: [PATCH 04/10] user namespaces: enforce user namespaces for file permission
Eric W. Biederman
ebiederm at xmission.com
Fri Aug 22 13:13:36 PDT 2008
"Serge E. Hallyn" <serue at us.ibm.com> writes:
> Add a user_ns to the sb. It is always set to the user_ns of the task which
> mounted the sb.
>
> Define 3 new super_operations. convert_uid() and convert_gid() take a uid
> or gid from an inode on the sb's fs, and attempt to convert them into ids
> meaningful in the user namespace passed in, which presumably is current's.
> is_capable() checks whether current has the requested capability with respect
> to the sb's fs, which is dependent upon the relationship between current's
> user_ns and those which the sb knows about.
>
> If the sb isn't user ns aware - which none are right now - then the new
> super_operations won't be defined. If in that case current and sb have
> different user namespaces, then the userids can't be compared.
>
> If current's and sb's userids can't be compared, then current will get
> 'user other' (we usually say 'nobody') permissions to the inode.
>
> Changelog:
> Aug 5: send the whole inode to the super_operations.
>
> Signed-off-by: Serge Hallyn <serue at us.ibm.com>
> ---
> fs/namei.c | 29 +++++++++++++++++++++++---
> fs/super.c | 3 ++
> include/linux/fs.h | 55 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 83 insertions(+), 4 deletions(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 4ea63ed..6bf5446 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -183,10 +183,27 @@ int generic_permission(struct inode *inode, int mask,
> int (*check_acl)(struct inode *inode, int mask))
> {
> umode_t mode = inode->i_mode;
> + uid_t iuid;
> + gid_t igid;
> + int ret;
> + int good_userns = 1;
> +
> + ret = s_convert_uid_gid(inode, current->user->user_ns,
> + &iuid, &igid);
> + if (!ret)
> + good_userns = 0;
>
> mask &= MAY_READ | MAY_WRITE | MAY_EXEC;
>
> - if (current->fsuid == inode->i_uid)
> + /*
> + * If we're not in the inode's user namespace, we get
> + * user nobody permissions, and we ignore acls
> + * (bc serge doesn't know how to handle acls in this case)
> + */
> + if (!good_userns)
> + goto check;
> +
> + if (current->fsuid == iuid)
> mode >>= 6;
> else {
> if (IS_POSIXACL(inode) && (mode & S_IRWXG) && check_acl) {
> @@ -197,15 +214,18 @@ int generic_permission(struct inode *inode, int mask,
> return error;
> }
>
> - if (in_group_p(inode->i_gid))
> + if (in_group_p(igid))
> mode >>= 3;
> }
>
> +check:
> /*
> * If the DACs are ok we don't need any capability check.
> */
> if ((mask & ~mode) == 0)
> return 0;
> + if (!good_userns)
> + return -EACCES;
>
> check_capabilities:
> /*
> @@ -214,14 +234,15 @@ int generic_permission(struct inode *inode, int mask,
> */
> if (!(mask & MAY_EXEC) ||
> (inode->i_mode & S_IXUGO) || S_ISDIR(inode->i_mode))
> - if (capable(CAP_DAC_OVERRIDE))
> + if (s_is_capable(inode, current->user->user_ns,
> + CAP_DAC_OVERRIDE))
> return 0;
>
> /*
> * Searching includes executable on directories, else just read.
> */
> if (mask == MAY_READ || (S_ISDIR(inode->i_mode) && !(mask & MAY_WRITE)))
> - if (capable(CAP_DAC_READ_SEARCH))
> + if (s_is_capable(inode, current->user->user_ns, CAP_DAC_READ_SEARCH))
> return 0;
>
> return -EACCES;
> diff --git a/fs/super.c b/fs/super.c
> index e931ae9..3559637 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -38,6 +38,7 @@
> #include <linux/kobject.h>
> #include <linux/mutex.h>
> #include <linux/file.h>
> +#include <linux/user_namespace.h>
> #include <asm/uaccess.h>
> #include "internal.h"
>
> @@ -93,6 +94,7 @@ static struct super_block *alloc_super(struct file_system_type
> *type)
> s->s_qcop = sb_quotactl_ops;
> s->s_op = &default_op;
> s->s_time_gran = 1000000000;
> + s->user_ns = get_user_ns(current->user->user_ns);
> }
> out:
> return s;
> @@ -109,6 +111,7 @@ static inline void destroy_super(struct super_block *s)
> security_sb_free(s);
> kfree(s->s_subtype);
> kfree(s->s_options);
> + put_user_ns(s->user_ns);
> kfree(s);
> }
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 580b513..bb58c2e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1128,6 +1128,11 @@ struct super_block {
> * generic_show_options()
> */
> char *s_options;
> +
> + /*
> + * namespace of the user which mounted the sb
> + */
> + struct user_namespace *user_ns;
We should make it clear that this is a default, and not something
a filesystem must use, just something a filesystem may use.
> };
>
> extern struct timespec current_fs_time(struct super_block *sb);
> @@ -1320,6 +1325,9 @@ struct super_operations {
> int (*remount_fs) (struct super_block *, int *, char *);
> void (*clear_inode) (struct inode *);
> void (*umount_begin) (struct super_block *);
> + int (*is_capable) (struct user_namespace *, struct inode *, int);
> + uid_t (*convert_uid_gid)(struct user_namespace *, struct inode *,
> + uid_t *, gid_t *);
>
I'm not convinced either of those methods makes sense.
> int (*show_options)(struct seq_file *, struct vfsmount *);
> int (*show_stats)(struct seq_file *, struct vfsmount *);
> @@ -1330,6 +1338,53 @@ struct super_operations {
> };
>
> /*
> + * In the next 3, i'm passing the user_ns in so that I don't need
> + * to know how to dereference struct user her (which would require
> + * #including sched.h).
> + * Note in particular that for instance in s_convert_uid, the uid is the
> + * inode->i_uid, while user_ns is current->user->user_ns.
> + */
> +
> +/*
> + * s_convert_uid: attempt to translate the inode's stored
> + * uid to a uid meaningful in user_ns passed in
> + * If possible, store the result in reuid and return 1
> + * Otherwise, return 0, meaningful the uid cannot be translated
> + */
> +static inline int s_convert_uid_gid(struct inode *ino,
> + struct user_namespace *user_ns, uid_t *retuid, gid_t *retgid)
> +{
> + struct super_block *sb = ino->i_sb;
> +
> + if (sb->user_ns == user_ns) {
> + *retuid = ino->i_uid;
> + *retgid = ino->i_gid;
> + return 1;
> + }
> + if (sb->s_op->convert_uid_gid)
> + return sb->s_op->convert_uid_gid(user_ns, ino, retuid, retgid);
We should be able to just use the existing inode->i_op->getattr.
Especially as that method will have to be updated anyway..
> + return 0;
> +}
> +
> +/*
> + * s_is_capable: check whether current is capable with respect
> + * to the filesystem represented by sb.
> + *
> + * return 0 if false, 1 if true
> + */
??? Capable should be with respect to a user namespace not with
respect to a filesystem.
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