CVE-2024-41000

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

block/ioctl: prefer different overflow check

Running syzkaller with the newly reintroduced signed integer overflow
sanitizer shows this report:

[ 62.982337] ------------[ cut here ]------------
[ 62.985692] cgroup: Invalid name
[ 62.986211] UBSAN: signed-integer-overflow in ../block/ioctl.c:36:46
[ 62.989370] 9pnet_fd: p9_fd_create_tcp (7343): problem connecting socket to 127.0.0.1
[ 62.992992] 9223372036854775807 + 4095 cannot be represented in type 'long long'
[ 62.997827] 9pnet_fd: p9_fd_create_tcp (7345): problem connecting socket to 127.0.0.1
[ 62.999369] random: crng reseeded on system resumption
[ 63.000634] GUP no longer grows the stack in syz-executor.2 (7353): 20002000-20003000 (20001000)
[ 63.000668] CPU: 0 PID: 7353 Comm: syz-executor.2 Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1
[ 63.000677] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[ 63.000682] Call Trace:
[ 63.000686] <TASK>
[ 63.000731] dump_stack_lvl+0x93/0xd0
[ 63.000919] __get_user_pages+0x903/0xd30
[ 63.001030] __gup_longterm_locked+0x153e/0x1ba0
[ 63.001041] ? _raw_read_unlock_irqrestore+0x17/0x50
[ 63.001072] ? try_get_folio+0x29c/0x2d0
[ 63.001083] internal_get_user_pages_fast+0x1119/0x1530
[ 63.001109] iov_iter_extract_pages+0x23b/0x580
[ 63.001206] bio_iov_iter_get_pages+0x4de/0x1220
[ 63.001235] iomap_dio_bio_iter+0x9b6/0x1410
[ 63.001297] __iomap_dio_rw+0xab4/0x1810
[ 63.001316] iomap_dio_rw+0x45/0xa0
[ 63.001328] ext4_file_write_iter+0xdde/0x1390
[ 63.001372] vfs_write+0x599/0xbd0
[ 63.001394] ksys_write+0xc8/0x190
[ 63.001403] do_syscall_64+0xd4/0x1b0
[ 63.001421] ? arch_exit_to_user_mode_prepare+0x3a/0x60
[ 63.001479] entry_SYSCALL_64_after_hwframe+0x6f/0x77
[ 63.001535] RIP: 0033:0x7f7fd3ebf539
[ 63.001551] Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 f1 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 b8 ff ff ff f7 d8 64 89 01 48
[ 63.001562] RSP: 002b:00007f7fd32570c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[ 63.001584] RAX: ffffffffffffffda RBX: 00007f7fd3ff3f80 RCX: 00007f7fd3ebf539
[ 63.001590] RDX: 4db6d1e4f7e43360 RSI: 0000000020000000 RDI: 0000000000000004
[ 63.001595] RBP: 00007f7fd3f1e496 R08: 0000000000000000 R09: 0000000000000000
[ 63.001599] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
[ 63.001604] R13: 0000000000000006 R14: 00007f7fd3ff3f80 R15: 00007ffd415ad2b8
...
[ 63.018142] ---[ end trace ]---

Historically, the signed integer overflow sanitizer did not work in the
kernel due to its interaction with `-fwrapv` but this has since been
changed [1] in the newest version of Clang; It was re-enabled in the
kernel with Commit 557f8c582a9ba8ab ("ubsan: Reintroduce signed overflow
sanitizer").

Let's rework this overflow checking logic to not actually perform an
overflow during the check itself, thus avoiding the UBSAN splat.

[1]: https://github.com/llvm/llvm-project/pull/82432