mm/pagewalk: fix stale walk->action escaping walk_pmd_range()

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

mm/pagewalk: fix stale walk->action escaping walk_pmd_range()

If ->pmd_entry() sets walk->action = ACTION_AGAIN, the pmd_none() check is
retried. The PMD entry may be cleared at the point of retry.

In this case, if walk->ops->install_pte is not specified, the code
continues to the next PMD entry in the range without resetting
walk->action to ACTION_SUBTREE.

This leaves walk->action erroneously set to ACTION_AGAIN, which is
incorrect.

This was incorrect but not problematic up until commit 3b89863c3fa4
("mm/pagewalk: fix race between concurrent split and refault") which
updated walk_pud_range() to check for walk->action == ACTION_AGAIN upon
walk_pmd_range()'s return, causing the PUD walk to be retried.

In this case this results in duplicate walk callbacks being invoked,
which is erroneous and will break any caller that is not idempotent
with respect to this (and waste time for those which are). The result
is an out-of-bounds write, triggered by a local fuzzer:

[ 2.272695] ==================================================================
[ 2.273471] BUG: KASAN: slab-out-of-bounds in __mincore_unmapped_range+0x14f/0x190
[ 2.274302] Write of size 1 at addr ffff888008d9b000 by task poc/106
[ 2.274966]
[ 2.275154] CPU: 0 UID: 1000 PID: 106 Comm: poc Not tainted 7.2.0-rc6-00429-ga7c7074b58d2 #55 PREEMPT(lazy)
[ 2.275159] Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[ 2.275164] Call Trace:
[ 2.275170] <TASK>
[ 2.275172] dump_stack_lvl+0x53/0x70
[ 2.275200] print_report+0xd0/0x630
[ 2.275210] ? __pfx__raw_spin_lock_irqsave+0x10/0x10
[ 2.275219] ? irqentry_exit+0xd2/0x670
[ 2.275224] ? irqentry_exit+0xd2/0x670
[ 2.275226] ? __virt_addr_valid+0xef/0x1a0
[ 2.275239] ? __mincore_unmapped_range+0x14f/0x190
[ 2.275242] kasan_report+0xce/0x100
[ 2.275245] ? __mincore_unmapped_range+0x14f/0x190
[ 2.275248] __mincore_unmapped_range+0x14f/0x190
[ 2.275252] mincore_unmapped_range+0x45/0x70
[ 2.275254] walk_pgd_range+0xafc/0xfc0
[ 2.275261] ? __pfx_walk_pgd_range+0x10/0x10
[ 2.275264] ? __update_load_avg_se+0x3d1/0x670
[ 2.275275] __walk_page_range+0xc0/0x310
[ 2.275278] ? __pfx_find_vma+0x10/0x10
[ 2.275281] ? finish_task_switch.isra.0+0x16d/0x4f0
[ 2.275290] walk_page_range_mm_unsafe+0x26f/0x3a0
[ 2.275293] ? __pfx_mtree_load+0x10/0x10
[ 2.275298] ? __pfx_walk_page_range_mm_unsafe+0x10/0x10
[ 2.275302] ? __free_frozen_pages+0x54d/0x7e0
[ 2.275308] __do_sys_mincore+0x132/0x380
[ 2.275311] do_syscall_64+0xf9/0x540
[ 2.275316] entry_SYSCALL_64_after_hwframe+0x77/0x7f
[ 2.275322] RIP: 0033:0x422ccd
[ 2.275326] Code: b3 66 2e 0f 1f 84 00 00 00 00 00 66 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
[ 2.275329] RSP: 002b:00007fffffffec18 EFLAGS: 00000287 ORIG_RAX: 000000000000001b
[ 2.275337] RAX: ffffffffffffffda RBX: 0000000000000066 RCX: 0000000000422ccd
[ 2.275339] RDX: 00000000004d0940 RSI: 0000000001000000 RDI: 00007ffff4000000
[ 2.275340] RBP: 00000000004d0940 R08: 0000000000000100 R09: 0000000000000100
[ 2.275342] R10: 0000000000000100 R11: 0000000000000287 R12: 20c49ba5e353f7cf
[ 2.275343] R13: 00000000004990d3 R14: 0000000000000000 R15: 0000000000000001
[ 2.275346] </TASK>
[ 2.275347]
[ 2.296904] The buggy address belongs to the object at ffff888008d9b000
[ 2.296904] which belongs to the cache sigqueue of size 80
[ 2.298151] The buggy address is located 0 bytes inside of
[ 2.298151] allocated 80-byte region [ffff888008d9b000, ffff888008d9b050)
[ 2.299408]
[ 2.299601] The buggy address belongs to the physical page:
[ 2.300191] page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x8d9b
---truncated---