ocfs2: validate bg_bits during freefrag scan

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

ocfs2: validate bg_bits during freefrag scan

[BUG]

A crafted filesystem can trigger an out-of-bounds bitmap walk when

OCFS2_IOC_INFO is issued with OCFS2_INFO_FL_NON_COHERENT.

BUG: KASAN: use-after-free in instrument_atomic_read include/linux/instrumented.h:68 [inline]

BUG: KASAN: use-after-free in _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]

BUG: KASAN: use-after-free in test_bit_le include/asm-generic/bitops/le.h:21 [inline]

BUG: KASAN: use-after-free in ocfs2_info_freefrag_scan_chain fs/ocfs2/ioctl.c:495 [inline]

BUG: KASAN: use-after-free in ocfs2_info_freefrag_scan_bitmap fs/ocfs2/ioctl.c:588 [inline]

BUG: KASAN: use-after-free in ocfs2_info_handle_freefrag fs/ocfs2/ioctl.c:662 [inline]

BUG: KASAN: use-after-free in ocfs2_info_handle_request+0x1c66/0x3370 fs/ocfs2/ioctl.c:754

Read of size 8 at addr ffff888031bce000 by task syz.0.636/1435

Call Trace:

__dump_stack lib/dump_stack.c:94 [inline]

dump_stack_lvl+0xbe/0x130 lib/dump_stack.c:120

print_address_description mm/kasan/report.c:378 [inline]

print_report+0xd1/0x650 mm/kasan/report.c:482

kasan_report+0xfb/0x140 mm/kasan/report.c:595

check_region_inline mm/kasan/generic.c:186 [inline]

kasan_check_range+0x11c/0x200 mm/kasan/generic.c:200

__kasan_check_read+0x11/0x20 mm/kasan/shadow.c:31

instrument_atomic_read include/linux/instrumented.h:68 [inline]

_test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]

test_bit_le include/asm-generic/bitops/le.h:21 [inline]

ocfs2_info_freefrag_scan_chain fs/ocfs2/ioctl.c:495 [inline]

ocfs2_info_freefrag_scan_bitmap fs/ocfs2/ioctl.c:588 [inline]

ocfs2_info_handle_freefrag fs/ocfs2/ioctl.c:662 [inline]

ocfs2_info_handle_request+0x1c66/0x3370 fs/ocfs2/ioctl.c:754

ocfs2_info_handle+0x18d/0x2a0 fs/ocfs2/ioctl.c:828

ocfs2_ioctl+0x632/0x6e0 fs/ocfs2/ioctl.c:913

vfs_ioctl fs/ioctl.c:51 [inline]

__do_sys_ioctl fs/ioctl.c:597 [inline]

__se_sys_ioctl fs/ioctl.c:583 [inline]

__x64_sys_ioctl+0x197/0x1e0 fs/ioctl.c:583

...

[CAUSE]

ocfs2_info_freefrag_scan_chain() uses on-disk bg_bits directly as the

bitmap scan limit. The coherent path reads group descriptors through

ocfs2_read_group_descriptor(), which validates the descriptor before

use. The non-coherent path uses ocfs2_read_blocks_sync() instead and

skips that validation, so an impossible bg_bits value can drive the

bitmap walk past the end of the block.

[FIX]

Compute the bitmap capacity from the filesystem format with

ocfs2_group_bitmap_size(), report descriptors whose bg_bits exceeds

that limit, and clamp the scan to the computed capacity. This keeps the

freefrag report going while avoiding reads beyond the buffer.