| Message ID | 20220418211559.3832724-1-mka@chromium.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | LoadPin: Enable loading from trusted dm-verity devices | expand |
[oops, resending to actual CC list] On Mon, Apr 18, 2022 at 02:15:56PM -0700, Matthias Kaehlcke wrote: > This series extends LoadPin to allow loading of kernel files > from trusted dm-verity devices. It adds the concept of > trusted verity devices to LoadPin. Userspace can use the > new systl file 'loadpin/trusted_verity_root_digests' to > provide LoadPin with a list of root digests from dm-verity > devices that LoadPin should consider as trusted. When a > kernel file is read LoadPin first checks (as usual) whether > the file is located on the pinned root, if so the file can > be loaded. Otherwise, if the verity extension is enabled, > LoadPin determines whether the file is located on a verity > backed device and whether the root digest of that device > is in the list of trusted digests. The file can be loaded > if the verity device has a trusted root digest. > > The list of trusted root digests can only be written once > (typically at boot time), to limit the possiblity of > attackers setting up rogue verity devices and marking them > as trusted. Hi, Thanks for working all this out! Where does the list of trusted roothashes come from? I assume some chain of trust exists. Is the list maybe already stored on the rootfs? It'd be nice if there was some way to pass the trust chain to LoadPin more directly. -Kees
Hi Kees, On Mon, Apr 18, 2022 at 03:14:14PM -0700, Kees Cook wrote: > [oops, resending to actual CC list] > > On Mon, Apr 18, 2022 at 02:15:56PM -0700, Matthias Kaehlcke wrote: > > This series extends LoadPin to allow loading of kernel files > > from trusted dm-verity devices. It adds the concept of > > trusted verity devices to LoadPin. Userspace can use the > > new systl file 'loadpin/trusted_verity_root_digests' to > > provide LoadPin with a list of root digests from dm-verity > > devices that LoadPin should consider as trusted. When a > > kernel file is read LoadPin first checks (as usual) whether > > the file is located on the pinned root, if so the file can > > be loaded. Otherwise, if the verity extension is enabled, > > LoadPin determines whether the file is located on a verity > > backed device and whether the root digest of that device > > is in the list of trusted digests. The file can be loaded > > if the verity device has a trusted root digest. > > > > The list of trusted root digests can only be written once > > (typically at boot time), to limit the possiblity of > > attackers setting up rogue verity devices and marking them > > as trusted. > Thanks for working all this out! Where does the list of trusted > roothashes come from? I assume some chain of trust exists. Is the list > maybe already stored on the rootfs? Yes, at least the content of the list comes from the rootfs. The userspace part is still TBD (also pending on the evolution of this patchset), having the list pre-formatted in a single file on the rootfs should be fine. > It'd be nice if there was some way to pass the trust chain to LoadPin > more directly. I imagine you envision LoadPin reading the file itself, instead of just processing the content. That should be doable. One option would be to pass the path of the file with the hashes through the sysctl file and use kernel_read_file_from_path() to read it if the path is in the pinned root (or maybe even in any trusted file system ;-)
On Mon, Apr 18, 2022 at 03:43:27PM -0700, Matthias Kaehlcke wrote: > Hi Kees, > > On Mon, Apr 18, 2022 at 03:14:14PM -0700, Kees Cook wrote: > > [oops, resending to actual CC list] > > > > On Mon, Apr 18, 2022 at 02:15:56PM -0700, Matthias Kaehlcke wrote: > > > This series extends LoadPin to allow loading of kernel files > > > from trusted dm-verity devices. It adds the concept of > > > trusted verity devices to LoadPin. Userspace can use the > > > new systl file 'loadpin/trusted_verity_root_digests' to > > > provide LoadPin with a list of root digests from dm-verity > > > devices that LoadPin should consider as trusted. When a > > > kernel file is read LoadPin first checks (as usual) whether > > > the file is located on the pinned root, if so the file can > > > be loaded. Otherwise, if the verity extension is enabled, > > > LoadPin determines whether the file is located on a verity > > > backed device and whether the root digest of that device > > > is in the list of trusted digests. The file can be loaded > > > if the verity device has a trusted root digest. > > > > > > The list of trusted root digests can only be written once > > > (typically at boot time), to limit the possiblity of > > > attackers setting up rogue verity devices and marking them > > > as trusted. > > > > Thanks for working all this out! Where does the list of trusted > > roothashes come from? I assume some chain of trust exists. Is the list > > maybe already stored on the rootfs? > > Yes, at least the content of the list comes from the rootfs. The > userspace part is still TBD (also pending on the evolution of this > patchset), having the list pre-formatted in a single file on the > rootfs should be fine. Ah-ha, that's perfect. > > It'd be nice if there was some way to pass the trust chain to LoadPin > > more directly. > > I imagine you envision LoadPin reading the file itself, instead of > just processing the content. That should be doable. One option would > be to pass the path of the file with the hashes through the sysctl > file and use kernel_read_file_from_path() to read it if the path is > in the pinned root (or maybe even in any trusted file system ;-) It could be a boot param or a Kconfig too. But yeah, having LoadPin able to use itself to validate the file path would be much nicer.
As of now LoadPin restricts loading of kernel files to a single pinned filesystem, typically the rootfs. This works for many systems, however it can result in a bloated rootfs (and OTA updates) on platforms where multiple boards with different hardware configurations use the same rootfs image. Especially when 'optional' files are large it may be preferable to download/install them only when they are actually needed by a given board. Chrome OS uses Downloadable Content (DLC) [1] to deploy certain 'packages' at runtime. As an example a DLC package could contain firmware for a peripheral that is not present on all boards. DLCs use dm-verity [2] to verify the integrity of the DLC content. This series extends LoadPin to allow loading of kernel files from trusted dm-verity devices. It adds the concept of trusted verity devices to LoadPin. Userspace can use the new systl file 'loadpin/trusted_verity_root_digests' to provide LoadPin with a list of root digests from dm-verity devices that LoadPin should consider as trusted. When a kernel file is read LoadPin first checks (as usual) whether the file is located on the pinned root, if so the file can be loaded. Otherwise, if the verity extension is enabled, LoadPin determines whether the file is located on a verity backed device and whether the root digest of that device is in the list of trusted digests. The file can be loaded if the verity device has a trusted root digest. The list of trusted root digests can only be written once (typically at boot time), to limit the possiblity of attackers setting up rogue verity devices and marking them as trusted. [1] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.md [2] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html Matthias Kaehlcke (3): dm: Add verity helpers for LoadPin LoadPin: Enable loading from trusted dm-verity devices dm: verity-loadpin: Use CONFIG_SECURITY_LOADPIN_VERITY for conditional compilation drivers/md/Makefile | 1 + drivers/md/dm-verity-loadpin.c | 80 ++++++++++++++ drivers/md/dm-verity-target.c | 34 ++++++ drivers/md/dm-verity.h | 4 + include/linux/dm-verity-loadpin.h | 27 +++++ security/loadpin/Kconfig | 11 ++ security/loadpin/loadpin.c | 168 +++++++++++++++++++++++++++++- 7 files changed, 324 insertions(+), 1 deletion(-) create mode 100644 drivers/md/dm-verity-loadpin.c create mode 100644 include/linux/dm-verity-loadpin.h