CVE-2025-21872 efi: Don't map the entire mokvar table to determine its size
A flaw in the Linux kernel EFI MOKvar table initialization causes a kernel warning and potential failure when mapping large tables, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-21872?
CVE-2025-21872 is a medium-severity denial of service vulnerability in Linux kernel, published on 27 Mar 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
efi: Don't map the entire mokvar table to determine its size
Currently, when validating the mokvar table, we (re)map the entire table on each iteration of the loop, adding space as we discover new entries. If the table grows over a certain size, this fails due to limitations of early_memmap(), and we get a failure and traceback:
------------[ cut here ]------------ WARNING: CPU: 0 PID: 0 at mm/early_ioremap.c:139 __early_ioremap+0xef/0x220 ... Call Trace: <TASK> ? __early_ioremap+0xef/0x220 ? __warn.cold+0x93/0xfa ? __early_ioremap+0xef/0x220 ? report_bug+0xff/0x140 ? early_fixup_exception+0x5d/0xb0 ? early_idt_handler_common+0x2f/0x3a ? __early_ioremap+0xef/0x220 ? efi_mokvar_table_init+0xce/0x1d0 ? setup_arch+0x864/0xc10 ? start_kernel+0x6b/0xa10 ? x86_64_start_reservations+0x24/0x30 ? x86_64_start_kernel+0xed/0xf0 ? common_startup_64+0x13e/0x141 </TASK> ---[ end trace 0000000000000000 ]--- mokvar: Failed to map EFI MOKvar config table pa=0x7c4c3000, size=265187.
Mapping the entire structure isn't actually necessary, as we don't ever need more than one entry header mapped at once.
Changes efi_mokvar_table_init() to only map each entry header, not the entire table, when determining the table size. Since we're not mapping any data past the variable name, it also changes the code to enforce that each variable name is NUL terminated, rather than attempting to verify it in place.
Which versions are affected by CVE-2025-21872?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-21872?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-21872 being exploited?
There is no known exploitation of CVE-2025-21872 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-69491, BELL-CVE-2025-21872, DEBIAN-CVE-2025-21872, GHSA-9pcq-fqff-43qc, ROOT-OS-DEBIAN-12-CVE-2025-21872, ROOT-OS-UBUNTU-2204-CVE-2025-21872 and 2 more.