[PATCH RFC 34/41] loop: refuse files on pseudo filesystems as backing files

[Christian Brauner <brauner@kernel.org>]

Currently loop accepts any regular file that implements ->read_iter() as
a backing file and any that also implements ->write_iter() as a writable
one. That includes files on procfs, sysfs and other pseudo filesystems.
For example, /proc/kcore attaches read-only as a loop device the size of
the kernel's virtual address space, 64 PiB with 5-level paging. sysfs
attributes attach read-write as 4096 byte loop devices. Writing a block
to the loop device of /sys/kernel/mm/transparent_hugepage/enabled
changes the THP setting.

That's not just nonsensical, it can be abused. Loop calls ->read_iter()
and ->write_iter() of the backing file directly from its worker which
runs with kernel credentials. But pseudo files enforce part of their
access rules at read and write time against current. Dozens of sysfs
->show() and ->store() methods call capable(). Behind a loop device all
of these checks are done against the loop worker instead of the task
that opened the file. And setting up a loop device requires no
capabilities, only access to the device nodes. So on v7.4-rc1:

- uid 0 without any capabilities can't write to /sys/kernel/reboot/mode
  as mode_store() requires CAP_SYS_BOOT. Writing "warm" through a loop
  device works.

- uid 0 without any capabilities can't read the files in
  /sys/firmware/dmi/entries/*/ as dmi_sysfs_attr_show() requires
  CAP_SYS_ADMIN. Reading them through a loop device works.

- An unprivileged container that was handed a loop device can't change
  gro_flush_timeout of its own network devices as
  gro_flush_timeout_store() requires CAP_NET_ADMIN in the initial user
  namespace. Writing it through a loop device works.

The same goes for hard_offline_page, the md and sd attributes, rfkill
and others.

Refuse files on pseudo filesystems as loop backing files. Note that
file_is_pseudo() looks through overlayfs so stacking overlayfs on top of
sysfs doesn't get around this.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 drivers/block/loop.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index c96e35c3ef42..1482580a1979 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -537,6 +537,9 @@ static int loop_check_backing_file(struct file *file)
 	if ((file->f_mode & FMODE_WRITE) && !file->f_op->write_iter)
 		return -EINVAL;
 
+	if (file_is_pseudo(file))
+		return -EINVAL;
+
 	return 0;
 }
 

-- 
2.53.0