CVE-2024-40915

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 __set_memory(). __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>] dump_stack+0x14/0x1c [<ffffffff8003b7ac>] __might_resched+0x104/0x10e [<ffffffff8003b7f4>] __mightsleep+0x3e/0x62 [<ffffffff8093276a>] downwrite+0x20/0x72 [<ffffffff8000cf00>] __set_memory+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.