[PATCH] jfs: account for the dirent header and charset expansion in jfs_readdir()

[Yogesh Gaur <yogeshgaur.83@gmail.com>]

jfs_readdir() converts each on-disk name into a struct jfs_dirent in a
PAGE_SIZE buffer, and stops filling the buffer when the next entry
might not fit:

	if (((long) jfs_dirent + d->namlen + 1) >
	    ((long)dirent_buf + PAGE_SIZE)) {

That check undercounts twice.  The name is written at jfs_dirent->name,
after the 14-byte header, not at jfs_dirent.  And d->namlen counts
UCS-2 characters, while jfs_strfromUCS_le() lets the codepage's
uni2char() write up to NLS_MAX_CHARSET_SIZE bytes for each one; with
iocharset=utf8 most non-ASCII characters take two or three bytes.  So
an entry that passes the check can still be written past the end of
the buffer.  Since commit de9f4f0b2c0f ("jfs: replace __get_free_page()
with kmalloc()") the buffer comes from kmalloc-4k and KASAN reports it:

  BUG: KASAN: slab-out-of-bounds in utf32_to_utf8+0x24d/0x3d0 fs/nls/nls_base.c:111
  Write of size 1 at addr ffff888039ad7000 by task syz-executor304/5946
   utf32_to_utf8+0x24d/0x3d0 fs/nls/nls_base.c:111
   uni2char+0x35/0xa0 fs/nls/nls_utf8.c:21
   jfs_strfromUCS_le+0xd2/0x3d0 fs/jfs/jfs_unicode.c:31
   jfs_readdir+0x16fd/0x33f0 fs/jfs/jfs_dtree.c:2984
  The buggy address is located 0 bytes to the right of
   allocated 4096-byte region [ffff888039ad6000, ffff888039ad7000)

Before that commit the same write went into whatever page followed.

Check against the worst case instead: the name starts at
jfs_dirent->name and each character can take NLS_MAX_CHARSET_SIZE
bytes, or one byte when no codepage is set.  The largest entry is
then 255 * 6 + 15 bytes, so an empty buffer still always takes at
least one entry and readdir keeps making progress.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+a2748ba908c108e7e525@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Yogesh Gaur <yogeshgaur.83@gmail.com>
---
Built with W=1 only; not runtime-tested.

 fs/jfs/jfs_dtree.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/jfs/jfs_dtree.c b/fs/jfs/jfs_dtree.c
index 8ce6e4458cc2..d930e577a496 100644
--- a/fs/jfs/jfs_dtree.c
+++ b/fs/jfs/jfs_dtree.c
@@ -2712,6 +2712,8 @@ int jfs_readdir(struct file *file, struct dir_context *ctx)
 {
 	struct inode *ip = file_inode(file);
 	struct nls_table *codepage = JFS_SBI(ip->i_sb)->nls_tab;
+	/* most bytes one on-disk character can turn into in jfs_dirent */
+	int max_charlen = codepage ? NLS_MAX_CHARSET_SIZE : 1;
 	int rc = 0;
 	loff_t dtpos;	/* legacy OS/2 style position */
 	struct dtoffset {
@@ -2910,9 +2912,10 @@ int jfs_readdir(struct file *file, struct dir_context *ctx)
 
 			d = (struct ldtentry *) & p->slot[stbl[i]];
 
-			if (((long) jfs_dirent + d->namlen + 1) >
+			if (((long)jfs_dirent->name +
+			     d->namlen * max_charlen + 1) >
 			    ((long)dirent_buf + PAGE_SIZE)) {
-				/* DBCS codepages could overrun dirent_buf */
+				/* the converted name might not fit */
 				index = i;
 				overflow = 1;
 				break;
-- 
2.55.0.windows.5