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SubjectRe: [PATCH 08/11] fuse: Support fuse filesystems outside of init_user_ns
Miklos Szeredi <mszeredi@redhat.com> writes:

> On Fri, Dec 22, 2017 at 3:32 PM, Dongsu Park <dongsu@kinvolk.io> wrote:
>> From: Seth Forshee <seth.forshee@canonical.com>
>>
>> In order to support mounts from namespaces other than
>> init_user_ns, fuse must translate uids and gids to/from the
>> userns of the process servicing requests on /dev/fuse. This
>> patch does that, with a couple of restrictions on the namespace:
>>
>> - The userns for the fuse connection is fixed to the namespace
>> from which /dev/fuse is opened.
>>
>> - The namespace must be the same as s_user_ns.
>>
>> These restrictions simplify the implementation by avoiding the
>> need to pass around userns references and by allowing fuse to
>> rely on the checks in inode_change_ok for ownership changes.
>> Either restriction could be relaxed in the future if needed.
>
> Can we not introduce potential userspace interface regressions?
>
> The issue with pid namespaces fixed in commit 5d6d3a301c4e ("fuse:
> allow server to run in different pid_ns") will probably bite us here
> as well.

Maybe, but unlike the pid namespace no one has been able to mount
fuse outside of init_user_ns so we are much less exposed. I agree we
should be careful.

> We basically need two modes of operation:
>
> a) old, backward compatible (not introducing any new failure mores),
> created with privileged mount
> b) new, non-backward compatible, created with unprivileged mount
>
> Technically there would still be a risk from breaking userspace, since
> we are using the same entry point for both, but let's hope that no
> practical problems come from that.

Answering from a 10,000 foot perspective:

There are two cases. Requests to read/write the filesystem from outside
of s_user_ns. These run no risk of breaking userspace as this mode has
not been implemented before.

Restrictions at mount time to ensure we are not dealing with a crazy mix
of namespaces. This has a small chance of breaking someone's crazy
setup.


Dropping requests to read/write the filesystem when the requester does
not map into s_user_ns should not be a problem to enable universally. If
s_user_ns is init_user_ns everything maps so there is no restriction.



What we can do if we want to ensure maximum backwards compatibility
is if the fuse filesystem is mounted in init_user_ns but if device for
the communication channel is opened in some other user namespace we
can just force the communication channel to operate in init_user_ns.

That will be 100% backwards compatible in all cases and as far as I can
see remove the need for having different ``modes'' of operation.



This does look like the time to give all of this a hard look and see if
we can get these patches in shape to be merged.

Eric



>> For cuse the namespace used for the connection is also simply
>> current_user_ns() at the time /dev/cuse is opened.
>>
>> Patch v4 is available: https://patchwork.kernel.org/patch/8944661/
>>
>> Cc: linux-fsdevel@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: Miklos Szeredi <mszeredi@redhat.com>
>> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
>> Signed-off-by: Dongsu Park <dongsu@kinvolk.io>
>> ---
>> fs/fuse/cuse.c | 3 ++-
>> fs/fuse/dev.c | 11 ++++++++---
>> fs/fuse/dir.c | 14 +++++++-------
>> fs/fuse/fuse_i.h | 6 +++++-
>> fs/fuse/inode.c | 31 +++++++++++++++++++------------
>> 5 files changed, 41 insertions(+), 24 deletions(-)
>>
>> diff --git a/fs/fuse/cuse.c b/fs/fuse/cuse.c
>> index e9e97803..b1b83259 100644
>> --- a/fs/fuse/cuse.c
>> +++ b/fs/fuse/cuse.c
>> @@ -48,6 +48,7 @@
>> #include <linux/stat.h>
>> #include <linux/module.h>
>> #include <linux/uio.h>
>> +#include <linux/user_namespace.h>
>>
>> #include "fuse_i.h"
>>
>> @@ -498,7 +499,7 @@ static int cuse_channel_open(struct inode *inode, struct file *file)
>> if (!cc)
>> return -ENOMEM;
>>
>> - fuse_conn_init(&cc->fc);
>> + fuse_conn_init(&cc->fc, current_user_ns());
>>
>> fud = fuse_dev_alloc(&cc->fc);
>> if (!fud) {
>> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
>> index 17f0d05b..0f780e16 100644
>> --- a/fs/fuse/dev.c
>> +++ b/fs/fuse/dev.c
>> @@ -114,8 +114,8 @@ static void __fuse_put_request(struct fuse_req *req)
>>
>> static void fuse_req_init_context(struct fuse_conn *fc, struct fuse_req *req)
>> {
>> - req->in.h.uid = from_kuid_munged(&init_user_ns, current_fsuid());
>> - req->in.h.gid = from_kgid_munged(&init_user_ns, current_fsgid());
>> + req->in.h.uid = from_kuid(fc->user_ns, current_fsuid());
>> + req->in.h.gid = from_kgid(fc->user_ns, current_fsgid());
>> req->in.h.pid = pid_nr_ns(task_pid(current), fc->pid_ns);
>> }
>>
>> @@ -167,6 +167,10 @@ static struct fuse_req *__fuse_get_req(struct fuse_conn *fc, unsigned npages,
>> __set_bit(FR_WAITING, &req->flags);
>> if (for_background)
>> __set_bit(FR_BACKGROUND, &req->flags);
>> + if (req->in.h.uid == (uid_t)-1 || req->in.h.gid == (gid_t)-1) {
>> + fuse_put_request(fc, req);
>> + return ERR_PTR(-EOVERFLOW);
>> + }
>>
>> return req;
>>
>> @@ -1260,7 +1264,8 @@ static ssize_t fuse_dev_do_read(struct fuse_dev *fud, struct file *file,
>> in = &req->in;
>> reqsize = in->h.len;
>>
>> - if (task_active_pid_ns(current) != fc->pid_ns) {
>> + if (task_active_pid_ns(current) != fc->pid_ns ||
>> + current_user_ns() != fc->user_ns) {
>
> I don't get it. Why recalculate the pid if the user_ns does not match?
>
>> rcu_read_lock();
>> in->h.pid = pid_vnr(find_pid_ns(in->h.pid, fc->pid_ns));
>> rcu_read_unlock();
>> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
>> index 24967382..ad1cfac1 100644
>> --- a/fs/fuse/dir.c
>> +++ b/fs/fuse/dir.c
>> @@ -858,8 +858,8 @@ static void fuse_fillattr(struct inode *inode, struct fuse_attr *attr,
>> stat->ino = attr->ino;
>> stat->mode = (inode->i_mode & S_IFMT) | (attr->mode & 07777);
>> stat->nlink = attr->nlink;
>> - stat->uid = make_kuid(&init_user_ns, attr->uid);
>> - stat->gid = make_kgid(&init_user_ns, attr->gid);
>> + stat->uid = make_kuid(fc->user_ns, attr->uid);
>> + stat->gid = make_kgid(fc->user_ns, attr->gid);
>> stat->rdev = inode->i_rdev;
>> stat->atime.tv_sec = attr->atime;
>> stat->atime.tv_nsec = attr->atimensec;
>> @@ -1475,17 +1475,17 @@ static bool update_mtime(unsigned ivalid, bool trust_local_mtime)
>> return true;
>> }
>>
>> -static void iattr_to_fattr(struct iattr *iattr, struct fuse_setattr_in *arg,
>> - bool trust_local_cmtime)
>> +static void iattr_to_fattr(struct fuse_conn *fc, struct iattr *iattr,
>> + struct fuse_setattr_in *arg, bool trust_local_cmtime)
>> {
>> unsigned ivalid = iattr->ia_valid;
>>
>> if (ivalid & ATTR_MODE)
>> arg->valid |= FATTR_MODE, arg->mode = iattr->ia_mode;
>> if (ivalid & ATTR_UID)
>> - arg->valid |= FATTR_UID, arg->uid = from_kuid(&init_user_ns, iattr->ia_uid);
>> + arg->valid |= FATTR_UID, arg->uid = from_kuid(fc->user_ns, iattr->ia_uid);
>> if (ivalid & ATTR_GID)
>> - arg->valid |= FATTR_GID, arg->gid = from_kgid(&init_user_ns, iattr->ia_gid);
>> + arg->valid |= FATTR_GID, arg->gid = from_kgid(fc->user_ns, iattr->ia_gid);
>> if (ivalid & ATTR_SIZE)
>> arg->valid |= FATTR_SIZE, arg->size = iattr->ia_size;
>> if (ivalid & ATTR_ATIME) {
>> @@ -1646,7 +1646,7 @@ int fuse_do_setattr(struct dentry *dentry, struct iattr *attr,
>>
>> memset(&inarg, 0, sizeof(inarg));
>> memset(&outarg, 0, sizeof(outarg));
>> - iattr_to_fattr(attr, &inarg, trust_local_cmtime);
>> + iattr_to_fattr(fc, attr, &inarg, trust_local_cmtime);
>> if (file) {
>> struct fuse_file *ff = file->private_data;
>> inarg.valid |= FATTR_FH;
>> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
>> index d5773ca6..364e65c8 100644
>> --- a/fs/fuse/fuse_i.h
>> +++ b/fs/fuse/fuse_i.h
>> @@ -26,6 +26,7 @@
>> #include <linux/xattr.h>
>> #include <linux/pid_namespace.h>
>> #include <linux/refcount.h>
>> +#include <linux/user_namespace.h>
>>
>> /** Max number of pages that can be used in a single read request */
>> #define FUSE_MAX_PAGES_PER_REQ 32
>> @@ -466,6 +467,9 @@ struct fuse_conn {
>> /** The pid namespace for this mount */
>> struct pid_namespace *pid_ns;
>>
>> + /** The user namespace for this mount */
>> + struct user_namespace *user_ns;
>> +
>> /** Maximum read size */
>> unsigned max_read;
>>
>> @@ -870,7 +874,7 @@ struct fuse_conn *fuse_conn_get(struct fuse_conn *fc);
>> /**
>> * Initialize fuse_conn
>> */
>> -void fuse_conn_init(struct fuse_conn *fc);
>> +void fuse_conn_init(struct fuse_conn *fc, struct user_namespace *user_ns);
>>
>> /**
>> * Release reference to fuse_conn
>> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
>> index 2f504d61..7f6b2e55 100644
>> --- a/fs/fuse/inode.c
>> +++ b/fs/fuse/inode.c
>> @@ -171,8 +171,8 @@ void fuse_change_attributes_common(struct inode *inode, struct fuse_attr *attr,
>> inode->i_ino = fuse_squash_ino(attr->ino);
>> inode->i_mode = (inode->i_mode & S_IFMT) | (attr->mode & 07777);
>> set_nlink(inode, attr->nlink);
>> - inode->i_uid = make_kuid(&init_user_ns, attr->uid);
>> - inode->i_gid = make_kgid(&init_user_ns, attr->gid);
>> + inode->i_uid = make_kuid(fc->user_ns, attr->uid);
>> + inode->i_gid = make_kgid(fc->user_ns, attr->gid);
>> inode->i_blocks = attr->blocks;
>> inode->i_atime.tv_sec = attr->atime;
>> inode->i_atime.tv_nsec = attr->atimensec;
>> @@ -477,7 +477,8 @@ static int fuse_match_uint(substring_t *s, unsigned int *res)
>> return err;
>> }
>>
>> -static int parse_fuse_opt(char *opt, struct fuse_mount_data *d, int is_bdev)
>> +static int parse_fuse_opt(char *opt, struct fuse_mount_data *d, int is_bdev,
>> + struct user_namespace *user_ns)
>> {
>> char *p;
>> memset(d, 0, sizeof(struct fuse_mount_data));
>> @@ -513,7 +514,7 @@ static int parse_fuse_opt(char *opt, struct fuse_mount_data *d, int is_bdev)
>> case OPT_USER_ID:
>> if (fuse_match_uint(&args[0], &uv))
>> return 0;
>> - d->user_id = make_kuid(current_user_ns(), uv);
>> + d->user_id = make_kuid(user_ns, uv);
>> if (!uid_valid(d->user_id))
>> return 0;
>> d->user_id_present = 1;
>> @@ -522,7 +523,7 @@ static int parse_fuse_opt(char *opt, struct fuse_mount_data *d, int is_bdev)
>> case OPT_GROUP_ID:
>> if (fuse_match_uint(&args[0], &uv))
>> return 0;
>> - d->group_id = make_kgid(current_user_ns(), uv);
>> + d->group_id = make_kgid(user_ns, uv);
>> if (!gid_valid(d->group_id))
>> return 0;
>> d->group_id_present = 1;
>> @@ -565,8 +566,8 @@ static int fuse_show_options(struct seq_file *m, struct dentry *root)
>> struct super_block *sb = root->d_sb;
>> struct fuse_conn *fc = get_fuse_conn_super(sb);
>>
>> - seq_printf(m, ",user_id=%u", from_kuid_munged(&init_user_ns, fc->user_id));
>> - seq_printf(m, ",group_id=%u", from_kgid_munged(&init_user_ns, fc->group_id));
>> + seq_printf(m, ",user_id=%u", from_kuid_munged(fc->user_ns, fc->user_id));
>> + seq_printf(m, ",group_id=%u", from_kgid_munged(fc->user_ns, fc->group_id));
>> if (fc->default_permissions)
>> seq_puts(m, ",default_permissions");
>> if (fc->allow_other)
>> @@ -597,7 +598,7 @@ static void fuse_pqueue_init(struct fuse_pqueue *fpq)
>> fpq->connected = 1;
>> }
>>
>> -void fuse_conn_init(struct fuse_conn *fc)
>> +void fuse_conn_init(struct fuse_conn *fc, struct user_namespace *user_ns)
>> {
>> memset(fc, 0, sizeof(*fc));
>> spin_lock_init(&fc->lock);
>> @@ -621,6 +622,7 @@ void fuse_conn_init(struct fuse_conn *fc)
>> fc->attr_version = 1;
>> get_random_bytes(&fc->scramble_key, sizeof(fc->scramble_key));
>> fc->pid_ns = get_pid_ns(task_active_pid_ns(current));
>> + fc->user_ns = get_user_ns(user_ns);
>> }
>> EXPORT_SYMBOL_GPL(fuse_conn_init);
>>
>> @@ -630,6 +632,7 @@ void fuse_conn_put(struct fuse_conn *fc)
>> if (fc->destroy_req)
>> fuse_request_free(fc->destroy_req);
>> put_pid_ns(fc->pid_ns);
>> + put_user_ns(fc->user_ns);
>> fc->release(fc);
>> }
>> }
>> @@ -1061,7 +1064,7 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
>>
>> sb->s_flags &= ~(MS_NOSEC | SB_I_VERSION);
>>
>> - if (!parse_fuse_opt(data, &d, is_bdev))
>> + if (!parse_fuse_opt(data, &d, is_bdev, sb->s_user_ns))
>> goto err;
>>
>> if (is_bdev) {
>> @@ -1086,8 +1089,12 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
>> if (!file)
>> goto err;
>>
>> - if ((file->f_op != &fuse_dev_operations) ||
>> - (file->f_cred->user_ns != &init_user_ns))
>> + /*
>> + * Require mount to happen from the same user namespace which
>> + * opened /dev/fuse to prevent potential attacks.
>> + */
>> + if (file->f_op != &fuse_dev_operations ||
>> + file->f_cred->user_ns != sb->s_user_ns)
>> goto err_fput;
>>
>> fc = kmalloc(sizeof(*fc), GFP_KERNEL);
>> @@ -1095,7 +1102,7 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
>> if (!fc)
>> goto err_fput;
>>
>> - fuse_conn_init(fc);
>> + fuse_conn_init(fc, sb->s_user_ns);
>> fc->release = fuse_free_conn;
>>
>> fud = fuse_dev_alloc(fc);
>> --
>> 2.13.6
>>

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