fs: jfs: Fix UBSAN: array-index-out-of-bounds in dbAllocDmapLev

In the Linux kernel, the following vulnerability has been resolved:

fs: jfs: Fix UBSAN: array-index-out-of-bounds in dbAllocDmapLev

Syzkaller reported the following issue:

UBSAN: array-index-out-of-bounds in fs/jfs/jfs_dmap.c:1965:6
index -84 is out of range for type 's8[341]' (aka 'signed char[341]')
CPU: 1 PID: 4995 Comm: syz-executor146 Not tainted 6.4.0-rc6-syzkaller-00037-gb6dad5178cea #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/27/2023
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x1e7/0x2d0 lib/dump_stack.c:106
ubsan_epilogue lib/ubsan.c:217 [inline]
__ubsan_handle_out_of_bounds+0x11c/0x150 lib/ubsan.c:348
dbAllocDmapLev+0x3e5/0x430 fs/jfs/jfs_dmap.c:1965
dbAllocCtl+0x113/0x920 fs/jfs/jfs_dmap.c:1809
dbAllocAG+0x28f/0x10b0 fs/jfs/jfs_dmap.c:1350
dbAlloc+0x658/0xca0 fs/jfs/jfs_dmap.c:874
dtSplitUp fs/jfs/jfs_dtree.c:974 [inline]
dtInsert+0xda7/0x6b00 fs/jfs/jfs_dtree.c:863
jfs_create+0x7b6/0xbb0 fs/jfs/namei.c:137
lookup_open fs/namei.c:3492 [inline]
open_last_lookups fs/namei.c:3560 [inline]
path_openat+0x13df/0x3170 fs/namei.c:3788
do_filp_open+0x234/0x490 fs/namei.c:3818
do_sys_openat2+0x13f/0x500 fs/open.c:1356
do_sys_open fs/open.c:1372 [inline]
__do_sys_openat fs/open.c:1388 [inline]
__se_sys_openat fs/open.c:1383 [inline]
__x64_sys_openat+0x247/0x290 fs/open.c:1383
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f1f4e33f7e9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 51 14 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc21129578 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f1f4e33f7e9
RDX: 000000000000275a RSI: 0000000020000040 RDI: 00000000ffffff9c
RBP: 00007f1f4e2ff080 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f1f4e2ff110
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
</TASK>

The bug occurs when the dbAllocDmapLev()function attempts to access
dp->tree.stree[leafidx + LEAFIND] while the leafidx value is negative.

To rectify this, the patch introduces a safeguard within the
dbAllocDmapLev() function. A check has been added to verify if leafidx is
negative. If it is, the function immediately returns an I/O error, preventing
any further execution that could potentially cause harm.

Tested via syzbot.