[PATCH] jfs: reserve the header and use freelist from second

[Edward Adam Davis <eadavis@qq.com>]

[syzbot reported]
general protection fault, probably for non-canonical address 0xdffffc0000000001: 0000 [#1] PREEMPT SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
CPU: 0 PID: 5061 Comm: syz-executor404 Not tainted 6.8.0-syzkaller-08951-gfe46a7dd189e #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
RIP: 0010:dtInsertEntry+0xd0c/0x1780 fs/jfs/jfs_dtree.c:3713
...
[Analyze]
When the pointer h has the same value as p, after writing name in UniStrncpy_to_le(),
p->header.flag will be cleared.
This will cause the previously true judgment "p->header.flag & BT-LEAF" to change
to no after writing the name operation, this leads to entering an incorrect branch
and accessing the uninitialized object ih when judging this condition for the
second time.
[Fix]
When allocating slots from the freelist, we start from the second one to preserve
the header of p from being incorrectly modified.

Reported-by: syzbot+bba84aef3a26fb93deb9@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 fs/jfs/jfs_dtree.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/jfs/jfs_dtree.c b/fs/jfs/jfs_dtree.c
index 031d8f570f58..deb2a5cc78d8 100644
--- a/fs/jfs/jfs_dtree.c
+++ b/fs/jfs/jfs_dtree.c
@@ -3618,7 +3618,8 @@ static void dtInsertEntry(dtpage_t * p, int index, struct component_name * key,
 	kname = key->name;
 
 	/* allocate a free slot */
-	hsi = fsi = p->header.freelist;
+	hsi = fsi = p->header.freelist = p->header.freelist == 0 ? 
+		1 : p->header.freelist;
 	h = &p->slot[fsi];
 	p->header.freelist = h->next;
 	--p->header.freecnt;
-- 
2.43.0