CVE-2026-89778isofs: fix out-of-bounds page array access on empty zisofs block

Official description

In the Linux kernel, the following vulnerability has been resolved: isofs: fix out-of-bounds page array access on empty zisofs block 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

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/8b994ac5778a725982fd6a8a3afcaa068d4a93e3
• https://git.kernel.org/stable/c/ad3249cdf9d4ba34bb8b1ff3956a4020fdfb5b0a
• https://git.kernel.org/stable/c/85904076cece72ee3194646ad7ac8e6659d999aa
• https://git.kernel.org/stable/c/f03425dcbe04aec3c27b9917e97d7d23a2908dda
• https://git.kernel.org/stable/c/cd616aa0449a772a6956abf03358f0ff31720580
• https://git.kernel.org/stable/c/9c6eace8d07e90f038c89eb3b756d65a7e259d48
• https://git.kernel.org/stable/c/68d4d3e78150c7ed7d1195af63ad1e6ace30c661