exfat: check if cluster num is valid

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: exfat: check if cluster num is valid Syzbot reported slab-out-of-bounds read in exfat_clear_bitmap. This was triggered by reproducer calling truncute with size 0, which causes the following trace: BUG: KASAN: slab-out-of-bounds in exfat_clear_bitmap+0x147/0x490 fs/exfat/balloc.c:174 Read of size 8 at addr ffff888115aa9508 by task syz-executor251/365 Call Trace: __dump_stack lib/dump_stack.c:77 [inline] dump_stack_lvl+0x1e2/0x24b lib/dump_stack.c:118 print_address_description+0x81/0x3c0 mm/kasan/report.c:233 __kasan_report mm/kasan/report.c:419 [inline] kasan_report+0x1a4/0x1f0 mm/kasan/report.c:436 __asan_report_load8_noabort+0x14/0x20 mm/kasan/report_generic.c:309 exfat_clear_bitmap+0x147/0x490 fs/exfat/balloc.c:174 exfat_free_cluster+0x25a/0x4a0 fs/exfat/fatent.c:181 __exfat_truncate+0x99e/0xe00 fs/exfat/file.c:217 exfat_truncate+0x11b/0x4f0 fs/exfat/file.c:243 exfat_setattr+0xa03/0xd40 fs/exfat/file.c:339 notify_change+0xb76/0xe10 fs/attr.c:336 do_truncate+0x1ea/0x2d0 fs/open.c:65 Move the is_valid_cluster() helper from fatent.c to a common header to make it reusable in other *.c files. And add is_valid_cluster() to validate if cluster number is within valid range in exfat_clear_bitmap() and exfat_set_bitmap().

References

• https://git.kernel.org/stable/c/82f723b8a5adf497f9e34c702a30ca7298615654
• https://git.kernel.org/stable/c/c504167adc3248095a905fa0700a9693897cb5ed
• https://git.kernel.org/stable/c/7c58b14b6f9cde9f69e7fa053ab73f6e013a7131
• https://git.kernel.org/stable/c/2193286402df2d9c53294f7a858d5e6fd7346e08
• https://git.kernel.org/stable/c/64ba4b15e5c045f8b746c6da5fc9be9a6b00b61d