CVE-2023-53860: dm: don't attempt to queue IO under RCU protection
In the Linux kernel, the following vulnerability has been resolved:

dm: don't attempt to queue IO under RCU protection

dm looks up the table for IO based on the request type, with an
assumption that if the request is marked REQ_NOWAIT, it's fine to
attempt to submit that IO while under RCU read lock protection. This
is not OK, as REQ_NOWAIT just means that we should not be sleeping
waiting on other IO, it does not mean that we can't potentially
schedule.

A simple test case demonstrates this quite nicely:

int main(int argc, char *argv[])
{
        struct iovec iov;
        int fd;

        fd = open("/dev/dm-0", O_RDONLY | O_DIRECT);
        posix_memalign(&iov.iov_base, 4096, 4096);
        iov.iov_len = 4096;
        preadv2(fd, &iov, 1, 0, RWF_NOWAIT);
        return 0;
}

which will instantly spew:

BUG: sleeping function called from invalid context at include/linux/sched/mm.h:306
in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 5580, name: dm-nowait
preempt_count: 0, expected: 0
RCU nest depth: 1, expected: 0
INFO: lockdep is turned off.
CPU: 7 PID: 5580 Comm: dm-nowait Not tainted 6.6.0-rc1-g39956d2dcd81 #132
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0x11d/0x1b0
 __might_resched+0x3c3/0x5e0
 ? preempt_count_sub+0x150/0x150
 mempool_alloc+0x1e2/0x390
 ? mempool_resize+0x7d0/0x7d0
 ? lock_sync+0x190/0x190
 ? lock_release+0x4b7/0x670
 ? internal_get_user_pages_fast+0x868/0x2d40
 bio_alloc_bioset+0x417/0x8c0
 ? bvec_alloc+0x200/0x200
 ? internal_get_user_pages_fast+0xb8c/0x2d40
 bio_alloc_clone+0x53/0x100
 dm_submit_bio+0x27f/0x1a20
 ? lock_release+0x4b7/0x670
 ? blk_try_enter_queue+0x1a0/0x4d0
 ? dm_dax_direct_access+0x260/0x260
 ? rcu_is_watching+0x12/0xb0
 ? blk_try_enter_queue+0x1cc/0x4d0
 __submit_bio+0x239/0x310
 ? __bio_queue_enter+0x700/0x700
 ? kvm_clock_get_cycles+0x40/0x60
 ? ktime_get+0x285/0x470
 submit_bio_noacct_nocheck+0x4d9/0xb80
 ? should_fail_request+0x80/0x80
 ? preempt_count_sub+0x150/0x150
 ? lock_release+0x4b7/0x670
 ? __bio_add_page+0x143/0x2d0
 ? iov_iter_revert+0x27/0x360
 submit_bio_noacct+0x53e/0x1b30
 submit_bio_wait+0x10a/0x230
 ? submit_bio_wait_endio+0x40/0x40
 __blkdev_direct_IO_simple+0x4f8/0x780
 ? blkdev_bio_end_io+0x4c0/0x4c0
 ? stack_trace_save+0x90/0xc0
 ? __bio_clone+0x3c0/0x3c0
 ? lock_release+0x4b7/0x670
 ? lock_sync+0x190/0x190
 ? atime_needs_update+0x3bf/0x7e0
 ? timestamp_truncate+0x21b/0x2d0
 ? inode_owner_or_capable+0x240/0x240
 blkdev_direct_IO.part.0+0x84a/0x1810
 ? rcu_is_watching+0x12/0xb0
 ? lock_release+0x4b7/0x670
 ? blkdev_read_iter+0x40d/0x530
 ? reacquire_held_locks+0x4e0/0x4e0
 ? __blkdev_direct_IO_simple+0x780/0x780
 ? rcu_is_watching+0x12/0xb0
 ? __mark_inode_dirty+0x297/0xd50
 ? preempt_count_add+0x72/0x140
 blkdev_read_iter+0x2a4/0x530
 do_iter_readv_writev+0x2f2/0x3c0
 ? generic_copy_file_range+0x1d0/0x1d0
 ? fsnotify_perm.part.0+0x25d/0x630
 ? security_file_permission+0xd8/0x100
 do_iter_read+0x31b/0x880
 ? import_iovec+0x10b/0x140
 vfs_readv+0x12d/0x1a0
 ? vfs_iter_read+0xb0/0xb0
 ? rcu_is_watching+0x12/0xb0
 ? rcu_is_watching+0x12/0xb0
 ? lock_release+0x4b7/0x670
 do_preadv+0x1b3/0x260
 ? do_readv+0x370/0x370
 __x64_sys_preadv2+0xef/0x150
 do_syscall_64+0x39/0xb0
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f5af41ad806
Code: 41 54 41 89 fc 55 44 89 c5 53 48 89 cb 48 83 ec 18 80 3d e4 dd 0d 00 00 74 7a 45 89 c1 49 89 ca 45 31 c0 b8 47 01 00 00 0f 05 <48> 3d 00 f0 ff ff 0f 87 be 00 00 00 48 85 c0 79 4a 48 8b 0d da 55
RSP: 002b:00007ffd3145c7f0 EFLAGS: 00000246 ORIG_RAX: 0000000000000147
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f5af41ad806
RDX: 0000000000000001 RSI: 00007ffd3145c850 RDI: 0000000000000003
RBP: 0000000000000008 R08: 0000000000000000 R09: 0000000000000008
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003
R13: 00007ffd3145c850 R14: 000055f5f0431dd8 R15: 0000000000000001
 </TASK>

where in fact it is
---truncated---