jfs: validate AG parameters in dbMount() to prevent crashes

Validate db_agheight, db_agwidth, and db_agstart in dbMount to catch
corrupted metadata early and avoid undefined behavior in dbAllocAG.
Limits are derived from L2LPERCTL, LPERCTL/MAXAG, and CTLTREESIZE:

- agheight: 0 to L2LPERCTL/2 (0 to 5) ensures shift
  (L2LPERCTL - 2*agheight) >= 0.
- agwidth: 1 to min(LPERCTL/MAXAG, 2^(L2LPERCTL - 2*agheight))
  ensures agperlev >= 1.
  - Ranges: 1-8 (agheight 0-3), 1-4 (agheight 4), 1 (agheight 5).
  - LPERCTL/MAXAG = 1024/128 = 8 limits leaves per AG;
    2^(10 - 2*agheight) prevents division to 0.
- agstart: 0 to CTLTREESIZE-1 - agwidth*(MAXAG-1) keeps ti within
  stree (size 1365).
  - Ranges: 0-1237 (agwidth 1), 0-348 (agwidth 8).

UBSAN: shift-out-of-bounds in fs/jfs/jfs_dmap.c:1400:9
shift exponent -335544310 is negative
CPU: 0 UID: 0 PID: 5822 Comm: syz-executor130 Not tainted 6.14.0-rc5-syzkaller #0
Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
 ubsan_epilogue lib/ubsan.c:231 [inline]
 __ubsan_handle_shift_out_of_bounds+0x3c8/0x420 lib/ubsan.c:468
 dbAllocAG+0x1087/0x10b0 fs/jfs/jfs_dmap.c:1400
 dbDiscardAG+0x352/0xa20 fs/jfs/jfs_dmap.c:1613
 jfs_ioc_trim+0x45a/0x6b0 fs/jfs/jfs_discard.c:105
 jfs_ioctl+0x2cd/0x3e0 fs/jfs/ioctl.c:131
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:906 [inline]
 __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Cc: stable@vger.kernel.org
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+fe8264911355151c487f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=fe8264911355151c487f
Signed-off-by: Vasiliy Kovalev <kovalev@altlinux.org>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>

Author: kovalev0

fs/jfs/jfs_dmap.c

@@ -194,7 +194,11 @@ int dbMount(struct inode *ipbmap)
 	    !bmp->db_numag || (bmp->db_numag > MAXAG) ||
 	    (bmp->db_maxag >= MAXAG) || (bmp->db_maxag < 0) ||
 	    (bmp->db_agpref >= MAXAG) || (bmp->db_agpref < 0) ||
-	    !bmp->db_agwidth ||
+	    (bmp->db_agheight < 0) || (bmp->db_agheight > (L2LPERCTL >> 1)) ||
+	    (bmp->db_agwidth < 1) || (bmp->db_agwidth > (LPERCTL / MAXAG)) ||
+	    (bmp->db_agwidth > (1 << (L2LPERCTL - (bmp->db_agheight << 1)))) ||
+	    (bmp->db_agstart < 0) ||
+	    (bmp->db_agstart > (CTLTREESIZE - 1 - bmp->db_agwidth * (MAXAG - 1))) ||
 	    (bmp->db_agl2size > L2MAXL2SIZE - L2MAXAG) ||
 	    (bmp->db_agl2size < 0) ||
 	    ((bmp->db_mapsize - 1) >> bmp->db_agl2size) > MAXAG) {