CVE-2026-89778

In the Linux kernel, the following vulnerability has been resolved:

isofs: fix out-of-bounds page array access on empty zisofs block

zisofsuncompressblock()'s empty-block fast path returns

pcount << PAGE_SHIFT, ignoring the incoming poffset, unlike the

decompression path which returns bytes produced relative to poffset.

zisofsfillpages() 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 zisofsreadfolio (fs/isofs/compress.c:290)

  Read of size 8 at addr ffff88800f5eac48 by task exploit/142

   zisofsreadfolio (fs/isofs/compress.c:290)

   read_pages (mm/readahead.c:184)

   ...

   filemap_read (mm/filemap.c:2814)

   vfsread (fs/readwrite.c:574)

   _x64syspread64 (fs/readwrite.c:769)

   dosyscall64 (arch/x86/entry/syscall_64.c:94)

   entrySYSCALL64afterhwframe (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