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Subject[PATCH] lib/decompress_unlz4.c: correctly handle zero-padding around initrds.
Date
lz4 compatible decompressor is simple. The format is underspecified and
relies on EOF notification to determine when to stop. Initramfs buffer
format[1] explicitly states that it can have arbitrary number of zero
padding. Thus when operating without a fill function, be extra careful to
ensure that sizes less than 4, or apperantly empty chunksizes are treated
as EOF.

To test this I have created two cpio initrds, first a normal one,
main.cpio. And second one with just a single /test-file with content
"second" second.cpio. Then i compressed both of them with gzip, and with
lz4 -l. Then I created a padding of 4 bytes (dd if=/dev/zero of=pad4 bs=1
count=4). To create four testcase initrds:

1) main.cpio.gzip + extra.cpio.gzip = pad0.gzip
2) main.cpio.lz4 + extra.cpio.lz4 = pad0.lz4
3) main.cpio.gzip + pad4 + extra.cpio.gzip = pad4.gzip
4) main.cpio.lz4 + pad4 + extra.cpio.lz4 = pad4.lz4

The pad4 test-cases replicate the initrd load by grub, as it pads and
aligns every initrd it loads.

All of the above boot, however /test-file was not accessible in the initrd
for the testcase #4, as decoding in lz4 decompressor failed. Also an error
message printed which usually is harmless.

Whith a patched kernel, all of the above testcases now pass, and /test-file
is accessible.

This fixes lz4 initrd decompress warning on every boot with grub. And more
importantly this fixes inability to load multiple lz4 compressed initrds
with grub.

I guess I should convert above decompressor streams with/without padding
into kunit tests, across all decompressor algorithms.

[1] ./Documentation/driver-api/early-userspace/buffer-format.rst

BugLink: https://bugs.launchpad.net/bugs/1835660
Signed-off-by: Dimitri John Ledkov <xnox@ubuntu.com>
---
lib/decompress_unlz4.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/lib/decompress_unlz4.c b/lib/decompress_unlz4.c
index c0cfcfd486be..e6327391b6b6 100644
--- a/lib/decompress_unlz4.c
+++ b/lib/decompress_unlz4.c
@@ -112,6 +112,9 @@ STATIC inline int INIT unlz4(u8 *input, long in_len,
error("data corrupted");
goto exit_2;
}
+ } else if (size < 4) {
+ /* empty or end-of-file */
+ goto exit_3;
}

chunksize = get_unaligned_le32(inp);
@@ -125,6 +128,10 @@ STATIC inline int INIT unlz4(u8 *input, long in_len,
continue;
}

+ if (!fill && chunksize == 0) {
+ /* empty or end-of-file */
+ goto exit_3;
+ }

if (posp)
*posp += 4;
@@ -184,6 +191,7 @@ STATIC inline int INIT unlz4(u8 *input, long in_len,
}
}

+exit_3:
ret = 0;
exit_2:
if (!input)
--
2.27.0
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