kasan: avoid sleepable page allocation from atomic context

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

kasan: avoid sleepable page allocation from atomic context

apply_to_pte_range() enters the lazy MMU mode and then invokes
kasan_populate_vmalloc_pte() callback on each page table walk iteration.
However, the callback can go into sleep when trying to allocate a single
page, e.g. if an architecutre disables preemption on lazy MMU mode enter.

On s390 if make arch_enter_lazy_mmu_mode() -> preempt_enable() and
arch_leave_lazy_mmu_mode() -> preempt_disable(), such crash occurs:

[ 0.663336] BUG: sleeping function called from invalid context at ./include/linux/sched/mm.h:321
[ 0.663348] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2, name: kthreadd
[ 0.663358] preempt_count: 1, expected: 0
[ 0.663366] RCU nest depth: 0, expected: 0
[ 0.663375] no locks held by kthreadd/2.
[ 0.663383] Preemption disabled at:
[ 0.663386] [<0002f3284cbb4eda>] apply_to_pte_range+0xfa/0x4a0
[ 0.663405] CPU: 0 UID: 0 PID: 2 Comm: kthreadd Not tainted 6.15.0-rc5-gcc-kasan-00043-gd76bb1ebb558-dirty #162 PREEMPT
[ 0.663408] Hardware name: IBM 3931 A01 701 (KVM/Linux)
[ 0.663409] Call Trace:
[ 0.663410] [<0002f3284c385f58>] dump_stack_lvl+0xe8/0x140
[ 0.663413] [<0002f3284c507b9e>] __might_resched+0x66e/0x700
[ 0.663415] [<0002f3284cc4f6c0>] __alloc_frozen_pages_noprof+0x370/0x4b0
[ 0.663419] [<0002f3284ccc73c0>] alloc_pages_mpol+0x1a0/0x4a0
[ 0.663421] [<0002f3284ccc8518>] alloc_frozen_pages_noprof+0x88/0xc0
[ 0.663424] [<0002f3284ccc8572>] alloc_pages_noprof+0x22/0x120
[ 0.663427] [<0002f3284cc341ac>] get_free_pages_noprof+0x2c/0xc0
[ 0.663429] [<0002f3284cceba70>] kasan_populate_vmalloc_pte+0x50/0x120
[ 0.663433] [<0002f3284cbb4ef8>] apply_to_pte_range+0x118/0x4a0
[ 0.663435] [<0002f3284cbc7c14>] apply_to_pmd_range+0x194/0x3e0
[ 0.663437] [<0002f3284cbc99be>] __apply_to_page_range+0x2fe/0x7a0
[ 0.663440] [<0002f3284cbc9e88>] apply_to_page_range+0x28/0x40
[ 0.663442] [<0002f3284ccebf12>] kasan_populate_vmalloc+0x82/0xa0
[ 0.663445] [<0002f3284cc1578c>] alloc_vmap_area+0x34c/0xc10
[ 0.663448] [<0002f3284cc1c2a6>] __get_vm_area_node+0x186/0x2a0
[ 0.663451] [<0002f3284cc1e696>] __vmalloc_node_range_noprof+0x116/0x310
[ 0.663454] [<0002f3284cc1d950>] __vmalloc_node_noprof+0xd0/0x110
[ 0.663457] [<0002f3284c454b88>] alloc_thread_stack_node+0xf8/0x330
[ 0.663460] [<0002f3284c458d56>] dup_task_struct+0x66/0x4d0
[ 0.663463] [<0002f3284c45be90>] copy_process+0x280/0x4b90
[ 0.663465] [<0002f3284c460940>] kernel_clone+0xd0/0x4b0
[ 0.663467] [<0002f3284c46115e>] kernel_thread+0xbe/0xe0
[ 0.663469] [<0002f3284c4e440e>] kthreadd+0x50e/0x7f0
[ 0.663472] [<0002f3284c38c04a>] __ret_from_fork+0x8a/0xf0
[ 0.663475] [<0002f3284ed57ff2>] ret_from_fork+0xa/0x38

Instead of allocating single pages per-PTE, bulk-allocate the shadow
memory prior to applying kasan_populate_vmalloc_pte() callback on a page
range.