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SubjectRe: [PATCH 03/11] fs: Allow superblock owner to change ownership of inodes
On Fri, Dec 22, 2017 at 03:32:27PM +0100, Dongsu Park wrote:
> diff --git a/fs/attr.c b/fs/attr.c
> index 12ffdb6f..bf8e94f3 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -18,6 +18,30 @@
> #include <linux/evm.h>
> #include <linux/ima.h>
>
> +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))
> + return true;
> + if (ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> + return true;
> + return false;
> +}
> +
> +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))
> + return true;
> + if (ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> + return true;
> + return false;
> +}
> +
> /**
> * setattr_prepare - check if attribute changes to a dentry are allowed
> * @dentry: dentry to check
> @@ -52,17 +76,11 @@ int setattr_prepare(struct dentry *dentry, struct iattr *attr)
> goto kill_priv;
>
> /* Make sure a caller can chown. */
> - if ((ia_valid & ATTR_UID) &&
> - (!uid_eq(current_fsuid(), inode->i_uid) ||
> - !uid_eq(attr->ia_uid, inode->i_uid)) &&
> - !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> + if ((ia_valid & ATTR_UID) && !chown_ok(inode, attr->ia_uid))
> return -EPERM;

I think this patch would read much better and easier to review if it was
split up by first adding the helpers, and then extending them afterwards.

>
> /* Make sure caller can chgrp. */
> - if ((ia_valid & ATTR_GID) &&
> - (!uid_eq(current_fsuid(), inode->i_uid) ||
> - (!in_group_p(attr->ia_gid) && !gid_eq(attr->ia_gid, inode->i_gid))) &&
> - !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> + if ((ia_valid & ATTR_GID) && !chgrp_ok(inode, attr->ia_gid))
> return -EPERM;
>
> /* Make sure a caller can chmod. */
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 31934cb9..9d50ec92 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -665,10 +665,17 @@ int proc_setattr(struct dentry *dentry, struct iattr *attr)
> {
> int error;
> struct inode *inode = d_inode(dentry);
> + struct user_namespace *s_user_ns;
>
> if (attr->ia_valid & ATTR_MODE)
> return -EPERM;
>
> + /* Don't let anyone mess with weird proc files */
> + s_user_ns = inode->i_sb->s_user_ns;
> + if (!kuid_has_mapping(s_user_ns, inode->i_uid) ||
> + !kgid_has_mapping(s_user_ns, inode->i_gid))
> + return -EPERM;
> +
> error = setattr_prepare(dentry, attr);
> if (error)
> return error;

Are we sure proc is the only special one? How was it observed first that this was
require for proc? Has anyone tried fuzzing by trying this op with a slew of other
filesystems on all files?

Luis

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