CVE-2024-40915: riscv: rewrite __kernel_map_pages() to fix sleeping in invalid context

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

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

kernelmappages() 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 setmemory(). setmemory() acquires initmm's semaphore, and this operation may sleep. This is problematic, because kernelmappages() 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 inatomic(): 1, irqsdisabled(): 0, nonblock: 0, pid: 2, name: kthreadd preemptcount: 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>] dumpbacktrace+0x1c/0x24 [<ffffffff8091ef6e>] showstack+0x2c/0x38 [<ffffffff8092baf8>] dumpstacklvl+0x5a/0x72 [<ffffffff8092bb24>] dumpstack+0x14/0x1c [<ffffffff8003b7ac>] mightresched+0x104/0x10e [<ffffffff8003b7f4>] mightsleep+0x3e/0x62 [<ffffffff8093276a>] downwrite+0x20/0x72 [<ffffffff8000cf00>] setmemory+0x82/0x2fa [<ffffffff8000d324>] kernelmappages+0x5a/0xd4 [<ffffffff80196cca>] allocpagesbulk+0x3b2/0x43a [<ffffffff8018ee82>] vmallocnoderange+0x196/0x6ba [<ffffffff80011904>] copyprocess+0x72c/0x17ec [<ffffffff80012ab4>] kernelclone+0x60/0x2fe [<ffffffff80012f62>] kernelthread+0x82/0xa0 [<ffffffff8003552c>] kthreadd+0x14a/0x1be [<ffffffff809357de>] retfromfork+0xe/0x1c

Rewrite this function with applytoexistingpagerange(). It is fine to not have any locking, because kernelmappages() works with pages being allocated/deallocated and those pages are not changed by anyone else in the meantime.