APPLIED[F/G]: [PATCH] lib/decompress_unlz4.c: correctly handle zero-padding around initrds.

[Kleber Souza]

On 03.12.20 01:31, Dimitri John Ledkov wrote:
> lz4 compatible decompressor is simple. The format is underspecified
> and relies on EOF notification to determine when to stop. Initramfs
> buffer format[1] explicitely 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] https://www.kernel.org/doc/html/latest/driver-api/early-userspace/buffer-format.html
> 
> BugLink: https://bugs.launchpad.net/bugs/1835660
> Signed-off-by: Dimitri John Ledkov <xnox at ubuntu.com>

Applied to focal/linux and groovy/linux.

Thanks,
Kleber

> ---
> 
>   @kernel-team this is RFC requesting pre-review before submission
>   upstream.
> 
>   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)
>