From: Dimitri John Ledkov <dimitri.ledkov@xxxxxxxxxxxxx> Subject: 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@xxxxxxxxxx/ # v0 Link: https://lkml.kernel.org/r/20210513104831.432975-1-dimitri.ledkov@xxxxxxxxxxxxx Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@xxxxxxxxxxxxx> Cc: Kyungsik Lee <kyungsik.lee@xxxxxxx> Cc: Yinghai Lu <yinghai@xxxxxxxxxx> Cc: Bongkyu Kim <bongkyu.kim@xxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Sven Schmidt <4sschmid@xxxxxxxxxxxxxxxxxxxxxxxxx> Cc: Rajat Asthana <thisisrast7@xxxxxxxxx> Cc: Nick Terrell <terrelln@xxxxxx> Cc: Gao Xiang <hsiangkao@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- lib/decompress_unlz4.c | 8 ++++++++ 1 file changed, 8 insertions(+) --- a/lib/decompress_unlz4.c~lib-decompress_unlz4c-correctly-handle-zero-padding-around-initrds +++ a/lib/decompress_unlz4.c @@ -112,6 +112,9 @@ STATIC inline int INIT unlz4(u8 *input, 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, 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, } } +exit_3: ret = 0; exit_2: if (!input) _
