fs: Limit sys_mount to only request filesystem modules.

Modify the request_module to prefix the file system type with "fs-"
and add aliases to all of the filesystems that can be built as modules
to match.

A common practice is to build all of the kernel code and leave code
that is not commonly needed as modules, with the result that many
users are exposed to any bug anywhere in the kernel.

Looking for filesystems with a fs- prefix limits the pool of possible
modules that can be loaded by mount to just filesystems trivially
making things safer with no real cost.

Using aliases means user space can control the policy of which
filesystem modules are auto-loaded by editing /etc/modprobe.d/*.conf
with blacklist and alias directives.  Allowing simple, safe,
well understood work-arounds to known problematic software.

This also addresses a rare but unfortunate problem where the filesystem
name is not the same as it's module name and module auto-loading
would not work.  While writing this patch I saw a handful of such
cases.  The most significant being autofs that lives in the module
autofs4.

This is relevant to user namespaces because we can reach the request
module in get_fs_type() without having any special permissions, and
people get uncomfortable when a user specified string (in this case
the filesystem type) goes all of the way to request_module.

After having looked at this issue I don't think there is any
particular reason to perform any filtering or permission checks beyond
making it clear in the module request that we want a filesystem
module.  The common pattern in the kernel is to call request_module()
without regards to the users permissions.  In general all a filesystem
module does once loaded is call register_filesystem() and go to sleep.
Which means there is not much attack surface exposed by loading a
filesytem module unless the filesystem is mounted.  In a user
namespace filesystems are not mounted unless .fs_flags = FS_USERNS_MOUNT,
which most filesystems do not set today.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Acked-by: Kees Cook <keescook@chromium.org>
Reported-by: Kees Cook <keescook@google.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

10 files changed, 10 insertions, 0 deletions

diff --git a/drivers/firmware/efivars.c b/drivers/firmware/efivars.c
index 7320bf891706..3edade07b249 100644
--- a/drivers/firmware/efivars.c
+++ b/drivers/firmware/efivars.c
@@ -1234,6 +1234,7 @@ static struct file_system_type efivarfs_type = {
 	.mount   = efivarfs_mount,
 	.kill_sb = efivarfs_kill_sb,
 };
+MODULE_ALIAS_FS("efivarfs");
 
 /*
  * Handle negative dentry.
diff --git a/drivers/infiniband/hw/ipath/ipath_fs.c b/drivers/infiniband/hw/ipath/ipath_fs.c
index a479375a8fd8..e0c404bdc4a8 100644
--- a/drivers/infiniband/hw/ipath/ipath_fs.c
+++ b/drivers/infiniband/hw/ipath/ipath_fs.c
@@ -410,6 +410,7 @@ static struct file_system_type ipathfs_fs_type = {
 	.mount =	ipathfs_mount,
 	.kill_sb =	ipathfs_kill_super,
 };
+MODULE_ALIAS_FS("ipathfs");
 
 int __init ipath_init_ipathfs(void)
 {
diff --git a/drivers/infiniband/hw/qib/qib_fs.c b/drivers/infiniband/hw/qib/qib_fs.c
index 644bd6f6467c..f247fc6e6182 100644
--- a/drivers/infiniband/hw/qib/qib_fs.c
+++ b/drivers/infiniband/hw/qib/qib_fs.c
@@ -604,6 +604,7 @@ static struct file_system_type qibfs_fs_type = {
 	.mount =        qibfs_mount,
 	.kill_sb =      qibfs_kill_super,
 };
+MODULE_ALIAS_FS("ipathfs");
 
 int __init qib_init_qibfs(void)
 {
diff --git a/drivers/misc/ibmasm/ibmasmfs.c b/drivers/misc/ibmasm/ibmasmfs.c
index 6673e578b3e9..ce5b75616b45 100644
--- a/drivers/misc/ibmasm/ibmasmfs.c
+++ b/drivers/misc/ibmasm/ibmasmfs.c
@@ -110,6 +110,7 @@ static struct file_system_type ibmasmfs_type = {
 	.mount          = ibmasmfs_mount,
 	.kill_sb        = kill_litter_super,
 };
+MODULE_ALIAS_FS("ibmasmfs");
 
 static int ibmasmfs_fill_super (struct super_block *sb, void *data, int silent)
 {
diff --git a/drivers/mtd/mtdchar.c b/drivers/mtd/mtdchar.c
index 82c06165d3d2..92ab30ab00dc 100644
--- a/drivers/mtd/mtdchar.c
+++ b/drivers/mtd/mtdchar.c
@@ -1238,6 +1238,7 @@ static struct file_system_type mtd_inodefs_type = {
        .mount = mtd_inodefs_mount,
        .kill_sb = kill_anon_super,
 };
+MODULE_ALIAS_FS("mtd_inodefs");
 
 static int __init init_mtdchar(void)
 {
diff --git a/drivers/oprofile/oprofilefs.c b/drivers/oprofile/oprofilefs.c
index 445ffda715ad..7c12d9c2b230 100644
--- a/drivers/oprofile/oprofilefs.c
+++ b/drivers/oprofile/oprofilefs.c
@@ -276,6 +276,7 @@ static struct file_system_type oprofilefs_type = {
 	.mount		= oprofilefs_mount,
 	.kill_sb	= kill_litter_super,
 };
+MODULE_ALIAS_FS("oprofilefs");
 
 
 int __init oprofilefs_register(void)
diff --git a/drivers/staging/ccg/f_fs.c b/drivers/staging/ccg/f_fs.c
index 8adc79d1b402..f6373dade7fb 100644
--- a/drivers/staging/ccg/f_fs.c
+++ b/drivers/staging/ccg/f_fs.c
@@ -1223,6 +1223,7 @@ static struct file_system_type ffs_fs_type = {
 	.mount		= ffs_fs_mount,
 	.kill_sb	= ffs_fs_kill_sb,
 };
+MODULE_ALIAS_FS("functionfs");
 
 
 /* Driver's main init/cleanup functions *************************************/
diff --git a/drivers/usb/gadget/f_fs.c b/drivers/usb/gadget/f_fs.c
index 38388d7844fc..c377ff84bf2c 100644
--- a/drivers/usb/gadget/f_fs.c
+++ b/drivers/usb/gadget/f_fs.c
@@ -1235,6 +1235,7 @@ static struct file_system_type ffs_fs_type = {
 	.mount		= ffs_fs_mount,
 	.kill_sb	= ffs_fs_kill_sb,
 };
+MODULE_ALIAS_FS("functionfs");
 
 
 /* Driver's main init/cleanup functions *************************************/
diff --git a/drivers/usb/gadget/inode.c b/drivers/usb/gadget/inode.c
index 8ac840f25ba9..e2b2e9cf254a 100644
--- a/drivers/usb/gadget/inode.c
+++ b/drivers/usb/gadget/inode.c
@@ -2105,6 +2105,7 @@ static struct file_system_type gadgetfs_type = {
 	.mount		= gadgetfs_mount,
 	.kill_sb	= gadgetfs_kill_sb,
 };
+MODULE_ALIAS_FS("gadgetfs");
 
 /*----------------------------------------------------------------------*/
 
diff --git a/drivers/xen/xenfs/super.c b/drivers/xen/xenfs/super.c
index ec0abb6df3c3..71679875f056 100644
--- a/drivers/xen/xenfs/super.c
+++ b/drivers/xen/xenfs/super.c
@@ -75,6 +75,7 @@ static struct file_system_type xenfs_type = {
 	.mount =	xenfs_mount,
 	.kill_sb =	kill_litter_super,
 };
+MODULE_ALIAS_FS("xenfs");
 
 static int __init xenfs_init(void)
 {