Re: [PATCH v3 08/38] capability: handle idmapped mounts

Serge E. Hallyn
2020-11-28T22:35:05Z

On Sat, Nov 28, 2020 at 10:34:57PM +0100, Christian Brauner wrote:
> In order to determine whether a caller holds privilege over a given inode the
> capability framework exposes the two helpers privileged_wrt_inode_uidgid() and
> capable_wrt_inode_uidgid(). The former verifies that the inode has a mapping in
> the caller's user namespace and the latter additionally verifies that the caller
> has the requested capability in their current user namespace.
> If the inode is accessed through an idmapped mount we simply need to map it
> according to the mount's user namespace. Afterwards the checks are identical to
> non-idmapped inodes. If the initial user namespace is passed all operations are
> a nop so non-idmapped mounts will not see a change in behavior and will also not
> see any performance impact.
>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: David Howells <dhowells@redhat.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: linux-fsdevel@vger.kernel.org
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Acked-by: Serge Hallyn <serge@hallyn.com>

> ---
> /* v2 */
> - Christoph Hellwig <hch@lst.de>:
> - Don't pollute the vfs with additional helpers simply extend the existing
> helpers with an additional argument and switch all callers.
>
> /* v3 */
> unchanged
> ---
> fs/attr.c | 8 ++++----
> fs/exec.c | 2 +-
> fs/inode.c | 2 +-
> fs/namei.c | 13 ++++++++-----
> fs/overlayfs/super.c | 2 +-
> fs/posix_acl.c | 2 +-
> fs/xfs/xfs_ioctl.c | 2 +-
> include/linux/capability.h | 7 +++++--
> kernel/capability.c | 14 +++++++++-----
> security/commoncap.c | 5 +++--
> 10 files changed, 34 insertions(+), 23 deletions(-)
>
> diff --git a/fs/attr.c b/fs/attr.c
> index b4bbdbd4c8ca..d270f640a192 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -23,7 +23,7 @@ static bool chown_ok(const struct inode *inode, kuid_t uid)
> if (uid_eq(current_fsuid(), inode->i_uid) &&
> uid_eq(uid, inode->i_uid))
> return true;
> - if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> + if (capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_CHOWN))
> return true;
> if (uid_eq(inode->i_uid, INVALID_UID) &&
> ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> @@ -36,7 +36,7 @@ static bool chgrp_ok(const struct inode *inode, kgid_t gid)
> if (uid_eq(current_fsuid(), inode->i_uid) &&
> (in_group_p(gid) || gid_eq(gid, inode->i_gid)))
> return true;
> - if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> + if (capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_CHOWN))
> return true;
> if (gid_eq(inode->i_gid, INVALID_GID) &&
> ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> @@ -92,7 +92,7 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
> /* Also check the setgid bit! */
> if (!in_group_p((ia_valid & ATTR_GID) ? attr->ia_gid :
> inode->i_gid) &&
> - !capable_wrt_inode_uidgid(inode, CAP_FSETID))
> + !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
> attr->ia_mode &= ~S_ISGID;
> }
>
> @@ -193,7 +193,7 @@ void setattr_copy(struct inode *inode, const struct iattr *attr)
> umode_t mode = attr->ia_mode;
>
> if (!in_group_p(inode->i_gid) &&
> - !capable_wrt_inode_uidgid(inode, CAP_FSETID))
> + !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
> mode &= ~S_ISGID;
> inode->i_mode = mode;
> }
> diff --git a/fs/exec.c b/fs/exec.c
> index 547a2390baf5..8e75d7a33514 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1398,7 +1398,7 @@ void would_dump(struct linux_binprm *bprm, struct file *file)
> /* Ensure mm->user_ns contains the executable */
> user_ns = old = bprm->mm->user_ns;
> while ((user_ns != &init_user_ns) &&
> - !privileged_wrt_inode_uidgid(user_ns, inode))
> + !privileged_wrt_inode_uidgid(user_ns, &init_user_ns, inode))
> user_ns = user_ns->parent;
>
> if (old != user_ns) {
> diff --git a/fs/inode.c b/fs/inode.c
> index 9d78c37b00b8..7a15372d9c2d 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2147,7 +2147,7 @@ void inode_init_owner(struct inode *inode, const struct inode *dir,
> mode |= S_ISGID;
> else if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP) &&
> !in_group_p(inode->i_gid) &&
> - !capable_wrt_inode_uidgid(dir, CAP_FSETID))
> + !capable_wrt_inode_uidgid(&init_user_ns, dir, CAP_FSETID))
> mode &= ~S_ISGID;
> } else
> inode->i_gid = current_fsgid();
> diff --git a/fs/namei.c b/fs/namei.c
> index d4a6dd772303..3f52730af6c5 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -357,10 +357,11 @@ int generic_permission(struct inode *inode, int mask)
> if (S_ISDIR(inode->i_mode)) {
> /* DACs are overridable for directories */
> if (!(mask & MAY_WRITE))
> - if (capable_wrt_inode_uidgid(inode,
> + if (capable_wrt_inode_uidgid(&init_user_ns, inode,
> CAP_DAC_READ_SEARCH))
> return 0;
> - if (capable_wrt_inode_uidgid(inode, CAP_DAC_OVERRIDE))
> + if (capable_wrt_inode_uidgid(&init_user_ns, inode,
> + CAP_DAC_OVERRIDE))
> return 0;
> return -EACCES;
> }
> @@ -370,7 +371,8 @@ int generic_permission(struct inode *inode, int mask)
> */
> mask &= MAY_READ | MAY_WRITE | MAY_EXEC;
> if (mask == MAY_READ)
> - if (capable_wrt_inode_uidgid(inode, CAP_DAC_READ_SEARCH))
> + if (capable_wrt_inode_uidgid(&init_user_ns, inode,
> + CAP_DAC_READ_SEARCH))
> return 0;
> /*
> * Read/write DACs are always overridable.
> @@ -378,7 +380,8 @@ int generic_permission(struct inode *inode, int mask)
> * at least one exec bit set.
> */
> if (!(mask & MAY_EXEC) || (inode->i_mode & S_IXUGO))
> - if (capable_wrt_inode_uidgid(inode, CAP_DAC_OVERRIDE))
> + if (capable_wrt_inode_uidgid(&init_user_ns, inode,
> + CAP_DAC_OVERRIDE))
> return 0;
>
> return -EACCES;
> @@ -2657,7 +2660,7 @@ int __check_sticky(struct inode *dir, struct inode *inode)
> return 0;
> if (uid_eq(dir->i_uid, fsuid))
> return 0;
> - return !capable_wrt_inode_uidgid(inode, CAP_FOWNER);
> + return !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FOWNER);
> }
> EXPORT_SYMBOL(__check_sticky);
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 290983bcfbb3..196fe3e3f02b 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -972,7 +972,7 @@ ovl_posix_acl_xattr_set(const struct xattr_handler *handler,
> if (unlikely(inode->i_mode & S_ISGID) &&
> handler->flags == ACL_TYPE_ACCESS &&
> !in_group_p(inode->i_gid) &&
> - !capable_wrt_inode_uidgid(inode, CAP_FSETID)) {
> + !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID)) {
> struct iattr iattr = { .ia_valid = ATTR_KILL_SGID };
>
> err = ovl_setattr(dentry, &iattr);
> diff --git a/fs/posix_acl.c b/fs/posix_acl.c
> index 95882b3f5f62..4ca6d53c6f0a 100644
> --- a/fs/posix_acl.c
> +++ b/fs/posix_acl.c
> @@ -656,7 +656,7 @@ int posix_acl_update_mode(struct inode *inode, umode_t *mode_p,
> if (error == 0)
> *acl = NULL;
> if (!in_group_p(inode->i_gid) &&
> - !capable_wrt_inode_uidgid(inode, CAP_FSETID))
> + !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
> mode &= ~S_ISGID;
> *mode_p = mode;
> return 0;
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index 3fbd98f61ea5..97bd29fc8c43 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1502,7 +1502,7 @@ xfs_ioctl_setattr(
> */
>
> if ((VFS_I(ip)->i_mode & (S_ISUID|S_ISGID)) &&
> - !capable_wrt_inode_uidgid(VFS_I(ip), CAP_FSETID))
> + !capable_wrt_inode_uidgid(&init_user_ns, VFS_I(ip), CAP_FSETID))
> VFS_I(ip)->i_mode &= ~(S_ISUID|S_ISGID);
>
> /* Change the ownerships and register project quota modifications */
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index 1e7fe311cabe..041e336f3369 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -247,8 +247,11 @@ static inline bool ns_capable_setid(struct user_namespace *ns, int cap)
> return true;
> }
> #endif /* CONFIG_MULTIUSER */
> -extern bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct inode *inode);
> -extern bool capable_wrt_inode_uidgid(const struct inode *inode, int cap);
> +extern bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
> + struct user_namespace *mnt_user_ns,
> + const struct inode *inode);
> +extern bool capable_wrt_inode_uidgid(struct user_namespace *mnt_user_ns,
> + const struct inode *inode, int cap);
> extern bool file_ns_capable(const struct file *file, struct user_namespace *ns, int cap);
> extern bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns);
> static inline bool perfmon_capable(void)
> diff --git a/kernel/capability.c b/kernel/capability.c
> index de7eac903a2a..28e3a599ff7a 100644
> --- a/kernel/capability.c
> +++ b/kernel/capability.c
> @@ -484,10 +484,12 @@ EXPORT_SYMBOL(file_ns_capable);
> *
> * Return true if the inode uid and gid are within the namespace.
> */
> -bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct inode *inode)
> +bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
> + struct user_namespace *mnt_user_ns,
> + const struct inode *inode)
> {
> - return kuid_has_mapping(ns, inode->i_uid) &&
> - kgid_has_mapping(ns, inode->i_gid);
> + return kuid_has_mapping(ns, i_uid_into_mnt(mnt_user_ns, inode)) &&
> + kgid_has_mapping(ns, i_gid_into_mnt(mnt_user_ns, inode));
> }
>
> /**
> @@ -499,11 +501,13 @@ bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct inode *
> * its own user namespace and that the given inode's uid and gid are
> * mapped into the current user namespace.
> */
> -bool capable_wrt_inode_uidgid(const struct inode *inode, int cap)
> +bool capable_wrt_inode_uidgid(struct user_namespace *mnt_user_ns,
> + const struct inode *inode, int cap)
> {
> struct user_namespace *ns = current_user_ns();
>
> - return ns_capable(ns, cap) && privileged_wrt_inode_uidgid(ns, inode);
> + return ns_capable(ns, cap) &&
> + privileged_wrt_inode_uidgid(ns, mnt_user_ns, inode);
> }
> EXPORT_SYMBOL(capable_wrt_inode_uidgid);
>
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 59bf3c1674c8..4cd2bdfd0a8b 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -489,7 +489,7 @@ int cap_convert_nscap(struct dentry *dentry, void **ivalue, size_t size)
> return -EINVAL;
> if (!validheader(size, cap))
> return -EINVAL;
> - if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
> + if (!capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_SETFCAP))
> return -EPERM;
> if (size == XATTR_CAPS_SZ_2)
> if (ns_capable(inode->i_sb->s_user_ns, CAP_SETFCAP))
> @@ -957,7 +957,8 @@ int cap_inode_removexattr(struct dentry *dentry, const char *name)
> struct inode *inode = d_backing_inode(dentry);
> if (!inode)
> return -EINVAL;
> - if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
> + if (!capable_wrt_inode_uidgid(&init_user_ns, inode,
> + CAP_SETFCAP))
> return -EPERM;
> return 0;
> }
> --
> 2.29.2