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Subject[PATCH 08/11] fuse: Support fuse filesystems outside of init_user_ns
Date
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.

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) {
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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