The zip crate gained support for streaming writes in its master branch,
so we can finally upgrade from our ancient fork of it. The new
implementation is done a bit differently, with seekable writers having
the ZipWriter<W> type and streaming writers having the
ZipWriter<StreamWriter<W>> type. This forces us to add a new wrapper
type since we have to switch between them at runtime.
We still need to maintain a (hopefully temporary) fork of the crate due
to a few issues:
1. There's no way to get the original underlying writer instance back
after finalizing a streaming zip. A fix for this has been submitted
upstream:
https://github.com/zip-rs/zip2/pull/367
2. The streaming writes implementation does not include the magic
signature for data descriptors. While the zip spec says the magic
value is optional and parsers should not require it, older versions
of Android's libziparchive do. A fix for this has been submitted
upstream:
https://github.com/zip-rs/zip2/pull/368
3. There is currently no way to get the data offset of zip entries.
avbroot requires this to fill in the OTA metadata's "property files"
entries, which Android uses to read file data without parsing the zip
file structures.
This new zip update produces files that are slightly different to
before. The "version made by" and "version needed to extract" fields are
now set to their minimum possible values. Previously, the zip crate was
hardcoded to use versions 4.6 and 2.0, respectively.
Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>
This also fixes a number of disabled-by-default clippy warnings and
updates the Rust edition to 2024.
Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>
Previously, overriding otacerts.zip in the system partition required the
user to flash a Magisk/KernelSU module that would bind mount over the
file during boot. While this worked well enough, it's insufficient for
unrooted setups, which has become more important since unrooting is the
only safe way to use the new OEM repair mode feature. With the stock
otacerts.zip, the OEM's default OTA updater app could run and install an
OS upgrade that's not signed by the user's key.
With this commit, the raw otacerts.zip bytes in the system partition are
directly replaced with a new zip that contains the user's certificate.
This method was inspired by @pascallj's comment in #216 suggesting
intentionally corrupting the otacerts.zip data in the filesystem.
Because avbroot does not have filesystem parsers for ext4/f2fs/erofs, we
rely on a heuristic-based search on the raw filesystem image. The file
is always smaller than one block (which is at least 4096 bytes on all
known devices), so the file data is stored contiguously on disk and in
the case of erofs, won't be compressed. None of the three filesystems
are copy-on-write and thus, have no filesystem-level data checksums. For
the dm-verity layer one level up, avbroot already knows how to recompute
the hash tree and FEC data.
To ensure that there are no false positives, any match that the search
finds must correctly parse as a valid zip and every entry within the zip
must have a filename that ends in .x509.pem. This matches what
update_engine expects from a proper otacerts.zip file.
Since the new approach is doing a raw search and replace, the old and
new files must have the same size. When the new zip is smaller, null
bytes are added to the zip archive comment field to pad to the correct
size. When the new zip is larger, avbroot will attempt the following to
try and make the file size smaller:
1. Enable zip deflate compression
2. Strip the X.509 signature from the certificate
3. Clear out the issuer RDN sequence from the certificate
4. Clear out the subject RDN sequence from the certificate
The latter three changes work because Android never performs any PKI
operations with the certificate. There is no CA certificate chain. The
X.509 certificate file is nothing more than a way to transport an RSA
public key.
avbroot requires the user's key to be RSA 4096. If the original zip had
the same key size, then none of these shrinking methods are needed. If
it contained an RSA 2048 key, then the first two modifications are
usually sufficient. The latter two modifications should only be needed
if the user picked a really long subject value when generating the
certificate.
With these new changes, the OTA patching time will approximately double
on a system with an SSD and modern CPU. This is dominated by the time it
takes to XZ-compress the system partition image. The compression is
already parallelized and scales linearly with the number of cores.
There's likely not much more that can be done to further speed this up.
Finally, these new changes are currently excluded from the e2e tests
because including the system partition in the stripped OTAs would
increase the file size by an order of magnitude. This could potentially
be solved in the future by generating our own small OTAs to use for
testing instead of running against real device OTAs.
Fixes: #225
Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>
The longer PR links have weird line wrapping behavior on the Github web
UI, making it more difficult to read.
Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>
Andrew Gunnerson