CVE-2025-40349 hfs: validate record offset in hfsplus_bmap_alloc
In the Linux kernel, the following vulnerability has been resolved:

hfs: validate record offset in hfsplus_bmap_alloc

hfsplus_bmap_alloc can trigger a crash if a
record offset or length is larger than node_size

[   15.264282] BUG: KASAN: slab-out-of-bounds in hfsplus_bmap_alloc+0x887/0x8b0
[   15.265192] Read of size 8 at addr ffff8881085ca188 by task test/183
[   15.265949]
[   15.266163] CPU: 0 UID: 0 PID: 183 Comm: test Not tainted 6.17.0-rc2-gc17b750b3ad9 #14 PREEMPT(voluntary)
[   15.266165] Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[   15.266167] Call Trace:
[   15.266168]  <TASK>
[   15.266169]  dump_stack_lvl+0x53/0x70
[   15.266173]  print_report+0xd0/0x660
[   15.266181]  kasan_report+0xce/0x100
[   15.266185]  hfsplus_bmap_alloc+0x887/0x8b0
[   15.266208]  hfs_btree_inc_height.isra.0+0xd5/0x7c0
[   15.266217]  hfsplus_brec_insert+0x870/0xb00
[   15.266222]  __hfsplus_ext_write_extent+0x428/0x570
[   15.266225]  __hfsplus_ext_cache_extent+0x5e/0x910
[   15.266227]  hfsplus_ext_read_extent+0x1b2/0x200
[   15.266233]  hfsplus_file_extend+0x5a7/0x1000
[   15.266237]  hfsplus_get_block+0x12b/0x8c0
[   15.266238]  __block_write_begin_int+0x36b/0x12c0
[   15.266251]  block_write_begin+0x77/0x110
[   15.266252]  cont_write_begin+0x428/0x720
[   15.266259]  hfsplus_write_begin+0x51/0x100
[   15.266262]  cont_write_begin+0x272/0x720
[   15.266270]  hfsplus_write_begin+0x51/0x100
[   15.266274]  generic_perform_write+0x321/0x750
[   15.266285]  generic_file_write_iter+0xc3/0x310
[   15.266289]  __kernel_write_iter+0x2fd/0x800
[   15.266296]  dump_user_range+0x2ea/0x910
[   15.266301]  elf_core_dump+0x2a94/0x2ed0
[   15.266320]  vfs_coredump+0x1d85/0x45e0
[   15.266349]  get_signal+0x12e3/0x1990
[   15.266357]  arch_do_signal_or_restart+0x89/0x580
[   15.266362]  irqentry_exit_to_user_mode+0xab/0x110
[   15.266364]  asm_exc_page_fault+0x26/0x30
[   15.266366] RIP: 0033:0x41bd35
[   15.266367] Code: bc d1 f3 0f 7f 27 f3 0f 7f 6f 10 f3 0f 7f 77 20 f3 0f 7f 7f 30 49 83 c0 0f 49 29 d0 48 8d 7c 17 31 e9 9f 0b 00 00 66 0f ef c0 <f3> 0f 6f 0e f3 0f 6f 56 10 66 0f 74 c1 66 0f d7 d0 49 83 f8f
[   15.266369] RSP: 002b:00007ffc9e62d078 EFLAGS: 00010283
[   15.266371] RAX: 00007ffc9e62d100 RBX: 0000000000000000 RCX: 0000000000000000
[   15.266372] RDX: 00000000000000e0 RSI: 0000000000000000 RDI: 00007ffc9e62d100
[   15.266373] RBP: 0000400000000040 R08: 00000000000000e0 R09: 0000000000000000
[   15.266374] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
[   15.266375] R13: 0000000000000000 R14: 0000000000000000 R15: 0000400000000000
[   15.266376]  </TASK>

When calling hfsplus_bmap_alloc to allocate a free node, this function
first retrieves the bitmap from header node and map node using node->page
together with the offset and length from hfs_brec_lenoff

```
len = hfs_brec_lenoff(node, 2, &off16);
off = off16;

off += node->page_offset;
pagep = node->page + (off >> PAGE_SHIFT);
data = kmap_local_page(*pagep);
```

However, if the retrieved offset or length is invalid(i.e. exceeds
node_size), the code may end up accessing pages outside the allocated
range for this node.

This patch adds proper validation of both offset and length before use,
preventing out-of-bounds page access. Move is_bnode_offset_valid and
check_and_correct_requested_length to hfsplus_fs.h, as they may be
required by other functions.
CVSS v3.1 score: 7.8