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Subject[patch 08/11] unprivileged mounts: make fuse safe
From: Miklos Szeredi <mszeredi@suse.cz>

Don't require the "user_id=" and "group_id=" options for unprivileged mounts,
but if they are present, verify them for sanity.

Disallow the "allow_other" option for unprivileged mounts.

Document new way of enabling unprivileged mounts for fuse.

Document problems with unprivileged mounts.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: Serge Hallyn <serue@us.ibm.com>
---
Documentation/filesystems/fuse.txt | 88 ++++++++++++++++++++++++++++++++++---
fs/fuse/inode.c | 21 ++++++++
2 files changed, 103 insertions(+), 6 deletions(-)
Index: linux/fs/fuse/inode.c
===================================================================
--- linux.orig/fs/fuse/inode.c 2008-03-17 20:55:30.000000000 +0100
+++ linux/fs/fuse/inode.c 2008-03-17 20:55:51.000000000 +0100
@@ -373,6 +373,19 @@ static int parse_fuse_opt(char *opt, str
d->max_read = ~0;
d->blksize = FUSE_DEFAULT_BLKSIZE;

+ /*
+ * For unprivileged mounts use current uid/gid. Still allow
+ * "user_id" and "group_id" options for compatibility, but
+ * only if they match these values.
+ */
+ if (!capable(CAP_SYS_ADMIN)) {
+ d->user_id = current->uid;
+ d->user_id_present = 1;
+ d->group_id = current->gid;
+ d->group_id_present = 1;
+
+ }
+
while ((p = strsep(&opt, ",")) != NULL) {
int token;
int value;
@@ -401,6 +414,8 @@ static int parse_fuse_opt(char *opt, str
case OPT_USER_ID:
if (match_int(&args[0], &value))
return 0;
+ if (d->user_id_present && d->user_id != value)
+ return 0;
d->user_id = value;
d->user_id_present = 1;
break;
@@ -408,6 +423,8 @@ static int parse_fuse_opt(char *opt, str
case OPT_GROUP_ID:
if (match_int(&args[0], &value))
return 0;
+ if (d->group_id_present && d->group_id != value)
+ return 0;
d->group_id = value;
d->group_id_present = 1;
break;
@@ -630,6 +647,10 @@ static int fuse_fill_super(struct super_
if (!parse_fuse_opt((char *) data, &d, is_bdev))
return -EINVAL;

+ /* This is a privileged option */
+ if ((d.flags & FUSE_ALLOW_OTHER) && !capable(CAP_SYS_ADMIN))
+ return -EPERM;
+
if (is_bdev) {
#ifdef CONFIG_BLOCK
if (!sb_set_blocksize(sb, d.blksize))
Index: linux/Documentation/filesystems/fuse.txt
===================================================================
--- linux.orig/Documentation/filesystems/fuse.txt 2008-03-17 20:55:30.000000000 +0100
+++ linux/Documentation/filesystems/fuse.txt 2008-03-17 20:55:51.000000000 +0100
@@ -215,11 +215,87 @@ the filesystem. There are several ways
- Abort filesystem through the FUSE control filesystem. Most
powerful method, always works.

-How do non-privileged mounts work?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Unprivileged fuse mounts
+~~~~~~~~~~~~~~~~~~~~~~~~

-Since the mount() system call is a privileged operation, a helper
-program (fusermount) is needed, which is installed setuid root.
+Possible problems with unprivileged fuse mounts
+-----------------------------------------------
+
+FUSE was designed from the beginning to be safe for unprivileged
+users. This has also been verified in practice over many years, with
+some distributions enabling unprivileged FUSE mounts by default.
+
+However, there are cases when unprivileged mounting a fuse filesystem
+may be problematic, particularly for multi-user systems with untrusted
+users. So here are few words of warning:
+
+Due to the design of the process freezer, a hanging (due to network
+problems, etc) or malicious filesystem may prevent suspending to ram
+or hibernation to succeed. This is not actually unique to FUSE, as
+any hanging network filesystem will have the same affect.
+
+It is not always possible to use kill(2) (not even with SIGKILL) to
+terminate a process using a FUSE filesystem (see section "Interrupting
+filesystem operations" above). As a special case of the above,
+killing a self-deadlocked FUSE process is not possible, and even
+killall5 will not terminate it.
+
+If the above could pose a threat to the system, it is recommended,
+that unprivileged fuse mounts are not enabled.
+
+Ways of enabling user mounts
+----------------------------
+
+Now there are two different ways of allowing unprivileged fuse mounts:
+
+ 1) new way: unprivileged mount syscall
+
+ 2) old way: suid-root fusermount utility
+
+Unprivileged mount syscall
+--------------------------
+
+To enable this do
+
+ echo 1 > /proc/sys/fs/types/fuse/usermount_safe
+
+or add this line to /etc/sysctl.conf:
+
+ fs.types.fuse.usermount_safe = 1
+
+More information can be found in Documentation/filesystems/proc.txt
+under the /proc/sys/fs/types/ heading. Also see description of
+nr_user_mounts and max_user_mounts under /proc/sys/fs.
+
+This doesn't in itself allow users to create mounts, first root needs
+to create a mount owned by the user, under which the user can create
+submounts.
+
+For example to enable submounts under /home/xyz/mnt do:
+
+ mount --bind -ouser=xyz /home/xyz/mnt /home/xyz/mnt
+
+or add this line to /etc/fstab:
+
+ /home/xyz/mnt /home/xyz/mnt none bind,user=xyz 0 0
+
+And finally, make sure, that the user has read and write permissions
+on /dev/fuse (installing fuse should have already taken care of this):
+
+ chmod 0666 /dev/fuse
+
+or create a file under /etc/udev/rules.d/ containing:
+
+ KERNEL=="fuse", MODE="0666"
+
+After this, mounting fuse filesystems under ~xyz/mnt should work, even
+if fusermount is not installed setuid-root.
+
+Suid-root fusermount utility
+----------------------------
+
+[Some of the details described here apply to the new, unprivileged
+mount system call as well].

The implication of providing non-privileged mounts is that the mount
owner must not be able to use this capability to compromise the
@@ -235,7 +311,7 @@ system. Obvious requirements arising fr
other users' or the super user's processes

How are requirements fulfilled?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+- - - - - - - - - - - - - - - -

A) The mount owner could gain elevated privileges by either:

@@ -300,7 +376,7 @@ How are requirements fulfilled?
filesystem, since SIGSTOP can be used to get a similar effect.

I think these limitations are unacceptable?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+- - - - - - - - - - - - - - - - - - - - - -

If a sysadmin trusts the users enough, or can ensure through other
measures, that system processes will never enter non-privileged
--


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