riscv: rewrite __kernel_map_pages() to fix sleeping in invalid context

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

riscv: rewrite __kernel_map_pages() to fix sleeping in invalid context

__kernel_map_pages() is a debug function which clears the valid bit in page

table entry for deallocated pages to detect illegal memory accesses to

freed pages.

This function set/clear the valid bit using __set_memory(). __set_memory()

acquires init_mm's semaphore, and this operation may sleep. This is

problematic, because __kernel_map_pages() can be called in atomic context,

and thus is illegal to sleep. An example warning that this causes:

BUG: sleeping function called from invalid context at kernel/locking/rwsem.c:1578

in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2, name: kthreadd

preempt_count: 2, expected: 0

CPU: 0 PID: 2 Comm: kthreadd Not tainted 6.9.0-g1d4c6d784ef6 #37

Hardware name: riscv-virtio,qemu (DT)

Call Trace:

[<ffffffff800060dc>] dump_backtrace+0x1c/0x24

[<ffffffff8091ef6e>] show_stack+0x2c/0x38

[<ffffffff8092baf8>] dump_stack_lvl+0x5a/0x72

[<ffffffff8092bb24>] dump_stack+0x14/0x1c

[<ffffffff8003b7ac>] __might_resched+0x104/0x10e

[<ffffffff8003b7f4>] __might_sleep+0x3e/0x62

[<ffffffff8093276a>] down_write+0x20/0x72

[<ffffffff8000cf00>] __set_memory+0x82/0x2fa

[<ffffffff8000d324>] __kernel_map_pages+0x5a/0xd4

[<ffffffff80196cca>] __alloc_pages_bulk+0x3b2/0x43a

[<ffffffff8018ee82>] __vmalloc_node_range+0x196/0x6ba

[<ffffffff80011904>] copy_process+0x72c/0x17ec

[<ffffffff80012ab4>] kernel_clone+0x60/0x2fe

[<ffffffff80012f62>] kernel_thread+0x82/0xa0

[<ffffffff8003552c>] kthreadd+0x14a/0x1be

[<ffffffff809357de>] ret_from_fork+0xe/0x1c

Rewrite this function with apply_to_existing_page_range(). It is fine to

not have any locking, because __kernel_map_pages() works with pages being

allocated/deallocated and those pages are not changed by anyone else in the

meantime.