Re: [PATCH] isofs: fix out-of-bounds page array access on empty zisofs block

[Xiang Mei]

On Mon, Jul 13, 2026 at 5:35 AM Jan Kara <jack@xxxxxxx> wrote:
>
> On Sun 12-07-26 16:41:50, Xiang Mei wrote:
> > zisofs_uncompress_block()'s empty-block fast path returns
> > pcount << PAGE_SHIFT, ignoring the incoming poffset, unlike the
> > decompression path which returns bytes produced relative to poffset.
> > zisofs_fill_pages() uses that return to advance its page cursor, so when
> > the zisofs block size is below PAGE_SIZE and a sub-page block leaves
> > poffset partway into a page, a following empty block over-counts and
> > advances pages[] one element past its end, after which
> > "if (poffset && *pages)" reads pages[1] out of bounds.  rock.c only
> > rejects a block-size shift > 17, so a crafted "ZF" Rock Ridge record can
> > set it below PAGE_SHIFT; the bug is reached by an ordinary read() of a
> > compressed file on such a mounted ISO9660 image.
> >
> > Return the byte count relative to poffset and zero only
> > [poffset, PAGE_SIZE) of the first page, matching the decompression path.
> > The page-aligned case (poffset == 0) is unaffected.
> >
> >   BUG: KASAN: slab-out-of-bounds in zisofs_read_folio (fs/isofs/compress.c:290)
> >   Read of size 8 at addr ffff88800f5eac48 by task exploit/142
> >    zisofs_read_folio (fs/isofs/compress.c:290)
> >    read_pages (mm/readahead.c:184)
> >    ...
> >    filemap_read (mm/filemap.c:2814)
> >    vfs_read (fs/read_write.c:574)
> >    __x64_sys_pread64 (fs/read_write.c:769)
> >    do_syscall_64 (arch/x86/entry/syscall_64.c:94)
> >    entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
> >   The buggy address is located 0 bytes to the right of the
> >   allocated 8-byte region in the kmalloc-8 cache
> >
> > Fixes: 59bc055211b8 ("zisofs: Implement reading of compressed files when PAGE_CACHE_SIZE > compress block size")
> > Reported-by: Weiming Shi <bestswngs@xxxxxxxxx>
> > Assisted-by: Claude:claude-opus-4-8
> > Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
>
> Thanks! I've added the patch to my tree.
>
>                                                                         Honza
>
Thank you!
Xiang
> > ---
> >  fs/isofs/compress.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/isofs/compress.c b/fs/isofs/compress.c
> > index 397568b9c7e7..3fda92358e22 100644
> > --- a/fs/isofs/compress.c
> > +++ b/fs/isofs/compress.c
> > @@ -65,12 +65,14 @@ static loff_t zisofs_uncompress_block(struct inode *inode, loff_t block_start,
> >       /* Empty block? */
> >       if (block_size == 0) {
> >               for ( i = 0 ; i < pcount ; i++ ) {
> > +                     unsigned int off = i ? 0 : poffset;
> > +
> >                       if (!pages[i])
> >                               continue;
> > -                     memzero_page(pages[i], 0, PAGE_SIZE);
> > +                     memzero_page(pages[i], off, PAGE_SIZE - off);
> >                       SetPageUptodate(pages[i]);
> >               }
> > -             return ((loff_t)pcount) << PAGE_SHIFT;
> > +             return (((loff_t)pcount) << PAGE_SHIFT) - poffset;
> >       }
> >
> >       /* Because zlib is not thread-safe, do all the I/O at the top. */
> > --
> > 2.43.0
> >
> --
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR