Re: [PATCH] lib/decompress_bunzip2: fix off-by-one in run-length bounds check

[Matt Turner]

On Sun, Sep 13, 2026 at 10:07 PM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Sat, 12 Sep 2026 14:17:35 -0400 Matt Turner <mattst88@xxxxxxxxx> wrote:
>
> > The run-length path rejects a block when dbufCount+t equals dbufSize,
> > but the loop that follows writes exactly t bytes starting at dbufCount,
> > so a block that fills the buffer exactly is legal. bzip2 allows it too:
> > its decompressor bounds a block at 100000 * blockSize100k and checks
> > that limit per byte appended. Use > instead of >=.
> >
> > bzip2's encoder stops filling a block 19 bytes early, so nothing it
> > produces ever reaches the limit and the bug stays hidden. Compressors
> > that use the full block size do reach it: an lbzip2 -9 image whose block
> > ends on a run fails to decode, and a self-extracting kernel built that
> > way does not boot.
> >
> > This code came from busybox, which fixed the same line in 2013 in commit
> > 932e233a491b ("bunzip2: fix off-by-one check").
>
> Thanks.
>
> > Fixes: bc22c17e12c1 ("bzip2/lzma: library support for gzip, bzip2 and lzma decompression")
> > Cc: stable@xxxxxxxxxxxxxxx
>
> Why is a backport proposed?  Hopefully because downstream users need
> this change, but the changelog doesn't tell anyone the reasons why.

I ran into this because my system uses lbzip2 as /bin/bzip2 -- a
common thing on Gentoo I believe. As far as I can tell, anyone using
lbzip2 as their system bzip2 would run into this and it's only because
it's very uncommon these days to compress a kernel with bzip2 that no
one has noticed. I only noticed because I was adding support for
various compression formats on alpha.

No strong preference for a backport from me. Just thought it would be desirable.