linux-kernel - [RESEND PATCH v1] lib/decompress_unlz4.c: correctly handle zero-padding around initrds.

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Date:   Thu, 13 May 2021 11:48:31 +0100
From:   Dimitri John Ledkov <dimitri.ledkov@...onical.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dimitri John Ledkov <dimitri.ledkov@...onical.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kyungsik Lee <kyungsik.lee@....com>,
        Yinghai Lu <yinghai@...nel.org>,
        Bongkyu Kim <bongkyu.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Sven Schmidt <4sschmid@...ormatik.uni-hamburg.de>
Subject: [RESEND PATCH v1] lib/decompress_unlz4.c: correctly handle zero-padding around initrds.

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. This patch has been shipping in Ubuntu kernels
since January 2021.

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

BugLink: https://bugs.launchpad.net/bugs/1835660
Link: https://lore.kernel.org/lkml/20210114200256.196589-1-xnox@ubuntu.com/ # v0
Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@...onical.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Kyungsik Lee <kyungsik.lee@....com>
Cc: Yinghai Lu <yinghai@...nel.org>
Cc: Bongkyu Kim <bongkyu.kim@....com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Sven Schmidt <4sschmid@...ormatik.uni-hamburg.de>
---
 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