On Fri, Dec 22, 2017 at 03:32:27PM +0100, Dongsu Park wrote: > From: Eric W. Biederman <ebied...@xmission.com> > > Allow users with CAP_SYS_CHOWN over the superblock of a filesystem to
Note it is CAP_CHOWN > chown files. Ordinarily the capable_wrt_inode_uidgid check is > sufficient to allow access to files but when the underlying filesystem > has uids or gids that don't map to the current user namespace it is > not enough, so the chown permission checks need to be extended to > allow this case. > > Calling chown on filesystem nodes whose uid or gid don't map is > necessary if those nodes are going to be modified as writing back > inodes which contain uids or gids that don't map is likely to cause > filesystem corruption of the uid or gid fields. > > Once chown has been called the existing capable_wrt_inode_uidgid > checks are sufficient, to allow the owner of a superblock to do anything > the global root user can do with an appropriate set of capabilities. > > For the proc filesystem this relaxation of permissions is not safe, as > some files are owned by users (particularly GLOBAL_ROOT_UID) outside > of the control of the mounter of the proc and that would be unsafe to > grant chown access to. So update setattr on proc to disallow changing > files whose uids or gids are outside of proc's s_user_ns. > > The original version of this patch was written by: Seth Forshee. I > have rewritten and rethought this patch enough so it's really not the > same thing (certainly it needs a different description), but he > deserves credit for getting out there and getting the conversation > started, and finding the potential gotcha's and putting up with my > semi-paranoid feedback. > > Patch v4 is available: https://patchwork.kernel.org/patch/8944611/ > > Cc: linux-fsde...@vger.kernel.org > Cc: linux-kernel@vger.kernel.org > Cc: Alexander Viro <v...@zeniv.linux.org.uk> > Cc: "Luis R. Rodriguez" <mcg...@kernel.org> > Cc: Kees Cook <keesc...@chromium.org> > Inspired-by: Seth Forshee <seth.fors...@canonical.com> > Signed-off-by: Eric W. Biederman <ebied...@xmission.com> > [saf: Resolve conflicts caused by s/inode_change_ok/setattr_prepare/] > Signed-off-by: Dongsu Park <don...@kinvolk.io> Reviewed-by: Serge Hallyn <se...@hallyn.com> > --- > fs/attr.c | 34 ++++++++++++++++++++++++++-------- > fs/proc/base.c | 7 +++++++ > fs/proc/generic.c | 7 +++++++ > fs/proc/proc_sysctl.c | 7 +++++++ > 4 files changed, 47 insertions(+), 8 deletions(-) > > 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; > > /* 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; > diff --git a/fs/proc/generic.c b/fs/proc/generic.c > index 793a6757..527d46c8 100644 > --- a/fs/proc/generic.c > +++ b/fs/proc/generic.c > @@ -106,8 +106,15 @@ static int proc_notify_change(struct dentry *dentry, > struct iattr *iattr) > { > struct inode *inode = d_inode(dentry); > struct proc_dir_entry *de = PDE(inode); > + struct user_namespace *s_user_ns; > int error; > > + /* 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, iattr); > if (error) > return error; > diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c > index c5cbbdff..0f9562d1 100644 > --- a/fs/proc/proc_sysctl.c > +++ b/fs/proc/proc_sysctl.c > @@ -802,11 +802,18 @@ static int proc_sys_permission(struct inode *inode, int > mask) > static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr) > { > struct inode *inode = d_inode(dentry); > + struct user_namespace *s_user_ns; > int error; > > if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)) > 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; > -- > 2.13.6
