CVE-2024-57884

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

mm: vmscan: account for free pages to prevent infinite Loop in throttle_direct_reclaim()

The task sometimes continues looping in throttle_direct_reclaim() because

allow_direct_reclaim(pgdat) keeps returning false.

#0 [ffff80002cb6f8d0] __switch_to at ffff8000080095ac

#1 [ffff80002cb6f900] __schedule at ffff800008abbd1c

#2 [ffff80002cb6f990] schedule at ffff800008abc50c

#3 [ffff80002cb6f9b0] throttle_direct_reclaim at ffff800008273550

#4 [ffff80002cb6fa20] try_to_free_pages at ffff800008277b68

#5 [ffff80002cb6fae0] __alloc_pages_nodemask at ffff8000082c4660

#6 [ffff80002cb6fc50] alloc_pages_vma at ffff8000082e4a98

#7 [ffff80002cb6fca0] do_anonymous_page at ffff80000829f5a8

#8 [ffff80002cb6fce0] __handle_mm_fault at ffff8000082a5974

#9 [ffff80002cb6fd90] handle_mm_fault at ffff8000082a5bd4

At this point, the pgdat contains the following two zones:

NODE: 4 ZONE: 0 ADDR: ffff00817fffe540 NAME: "DMA32"

SIZE: 20480 MIN/LOW/HIGH: 11/28/45

VM_STAT:

NR_FREE_PAGES: 359

NR_ZONE_INACTIVE_ANON: 18813

NR_ZONE_ACTIVE_ANON: 0

NR_ZONE_INACTIVE_FILE: 50

NR_ZONE_ACTIVE_FILE: 0

NR_ZONE_UNEVICTABLE: 0

NR_ZONE_WRITE_PENDING: 0

NR_MLOCK: 0

NR_BOUNCE: 0

NR_ZSPAGES: 0

NR_FREE_CMA_PAGES: 0

NODE: 4 ZONE: 1 ADDR: ffff00817fffec00 NAME: "Normal"

SIZE: 8454144 PRESENT: 98304 MIN/LOW/HIGH: 68/166/264

VM_STAT:

NR_FREE_PAGES: 146

NR_ZONE_INACTIVE_ANON: 94668

NR_ZONE_ACTIVE_ANON: 3

NR_ZONE_INACTIVE_FILE: 735

NR_ZONE_ACTIVE_FILE: 78

NR_ZONE_UNEVICTABLE: 0

NR_ZONE_WRITE_PENDING: 0

NR_MLOCK: 0

NR_BOUNCE: 0

NR_ZSPAGES: 0

NR_FREE_CMA_PAGES: 0

In allow_direct_reclaim(), while processing ZONE_DMA32, the sum of

inactive/active file-backed pages calculated in zone_reclaimable_pages()

based on the result of zone_page_state_snapshot() is zero.

Additionally, since this system lacks swap, the calculation of inactive/

active anonymous pages is skipped.

crash> p nr_swap_pages

nr_swap_pages = $1937 = {

counter = 0

}

As a result, ZONE_DMA32 is deemed unreclaimable and skipped, moving on to

the processing of the next zone, ZONE_NORMAL, despite ZONE_DMA32 having

free pages significantly exceeding the high watermark.

The problem is that the pgdat->kswapd_failures hasn't been incremented.

crash> px ((struct pglist_data *) 0xffff00817fffe540)->kswapd_failures

$1935 = 0x0

This is because the node deemed balanced. The node balancing logic in

balance_pgdat() evaluates all zones collectively. If one or more zones

(e.g., ZONE_DMA32) have enough free pages to meet their watermarks, the

entire node is deemed balanced. This causes balance_pgdat() to exit early

before incrementing the kswapd_failures, as it considers the overall

memory state acceptable, even though some zones (like ZONE_NORMAL) remain

under significant pressure.

The patch ensures that zone_reclaimable_pages() includes free pages

(NR_FREE_PAGES) in its calculation when no other reclaimable pages are

available (e.g., file-backed or anonymous pages). This change prevents

zones like ZONE_DMA32, which have sufficient free pages, from being

mistakenly deemed unreclaimable. By doing so, the patch ensures proper

node balancing, avoids masking pressure on other zones like ZONE_NORMAL,

and prevents infinite loops in throttle_direct_reclaim() caused by

allow_direct_reclaim(pgdat) repeatedly returning false.

The kernel hangs due to a task stuck in throttle_direct_reclaim(), caused

by a node being incorrectly deemed balanced despite pressure in certain

zones, such as ZONE_NORMAL. This issue arises from

zone_reclaimable_pages

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