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

[Xiang Mei]

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>
---
 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