High [CVE-2026-63952] deny writeable mappings when implying SEAL_WRITE

In the Linux kernel, the following vulnerability has been resolved: memfd: deny writeable mappings when implying SEAL_WRITE When SEAL_EXEC is added, SEAL_WRITE is implied to make W^X. But the implied seal is set after the check that makes sure the memfd can not have any writable mappings.

This means one can use SEAL_EXEC to apply SEAL_WRITE while having writeable mappings. This breaks the contract that SEAL_WRITE provides and can be used by an attacker to pass a memfd that appears to be write sealed but can still be modified arbitrarily.

Fix this by adding the implied seals before the call for mapping_deny_writable() is done. When the `SEAL_EXEC` flag is used to create a memory file descriptor, the `SEAL_WRITE` flag is implicitly applied to prevent writing to executable memory.

However, this implicit write protection is applied after checks that should deny writable mappings, allowing an attacker to maintain writable access to a `memfd` that appears to be write-sealed. This vulnerability could allow a local attacker to arbitrarily modify memory, potentially leading to privilege escalation or other impacts.

Red Hat severity: Important — CVSS 7 (CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H). Weakness: CWE-266.

Affected Red Hat products: Red Hat Enterprise Linux 10; Red Hat Enterprise Linux 9. Red Hat fixing advisory: RHSA-2026:57251, RHSA-2026:57252.