Release Date: | 2025-07-09 | |
Impact: | Moderate | What is this? |
In the Linux kernel, the following vulnerability has been resolved:
mm: userfaultfd: fix race of userfaultfd_move and swap cache
This commit fixes two kinds of races, they may have different results:
Barry reported a BUG_ON in commit c50f8e6053b0, we may see the same
BUG_ON if the filemap lookup returned NULL and folio is added to swap
cache after that.
If another kind of race is triggered (folio changed after lookup) we
may see RSS counter is corrupted:
[ 406.893936] BUG: Bad rss-counter state mm:ffff0000c5a9ddc0
type:MM_ANONPAGES val:-1
[ 406.894071] BUG: Bad rss-counter state mm:ffff0000c5a9ddc0
type:MM_SHMEMPAGES val:1
Because the folio is being accounted to the wrong VMA.
I'm not sure if there will be any data corruption though, seems no.
The issues above are critical already.
On seeing a swap entry PTE, userfaultfd_move does a lockless swap cache
lookup, and tries to move the found folio to the faulting vma. Currently,
it relies on checking the PTE value to ensure that the moved folio still
belongs to the src swap entry and that no new folio has been added to the
swap cache, which turns out to be unreliable.
While working and reviewing the swap table series with Barry, following
existing races are observed and reproduced [1]:
In the example below, move_pages_pte is moving src_pte to dst_pte, where
src_pte is a swap entry PTE holding swap entry S1, and S1 is not in the
swap cache:
CPU1 CPU2
userfaultfd_move
move_pages_pte()
entry = pte_to_swp_entry(orig_src_pte);
// Here it got entry = S1
... < interrupted> ...
See more information about CVE-2025-38242 from MITRE CVE dictionary and NIST NVD
NOTE: The following CVSS metrics and score provided are preliminary and subject to review.
Base Score: | 5.5 |
Vector String: | CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H |
Version: | 3.1 |
Attack Vector: | Local |
Attack Complexity: | Low |
Privileges Required: | Low |
User Interaction: | None |
Scope: | Unchanged |
Confidentiality Impact: | None |
Integrity Impact: | None |
Availability Impact: | High |
Platform | Errata | Release Date |
Oracle Linux version 10 (kernel-uek) | ELSA-2025-20551 | 2025-09-08 |
Oracle Linux version 9 (kernel-uek) | ELSA-2025-20551 | 2025-09-08 |
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