CVE-2024-44943 mm: gup: stop abusing try_grab_folio
A kernel warning triggered during SEV VM startup with CMA memory can lead to a denial of service condition.
- 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-2024-44943?
CVE-2024-44943 is a medium-severity denial of service vulnerability in Linux kernel, published on 28 Aug 2024. 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:
mm: gup: stop abusing try_grab_folio
A kernel warning was reported when pinning folio in CMA memory when launching SEV virtual machine. The splat looks like:
[ 464.325306] WARNING: CPU: 13 PID: 6734 at mm/gup.c:1313 __get_user_pages+0x423/0x520 [ 464.325464] CPU: 13 PID: 6734 Comm: qemu-kvm Kdump: loaded Not tainted 6.6.33+ #6 [ 464.325477] RIP: 0010:__get_user_pages+0x423/0x520 [ 464.325515] Call Trace: [ 464.325520] <TASK> [ 464.325523] ? __get_user_pages+0x423/0x520 [ 464.325528] ? __warn+0x81/0x130 [ 464.325536] ? __get_user_pages+0x423/0x520 [ 464.325541] ? report_bug+0x171/0x1a0 [ 464.325549] ? handle_bug+0x3c/0x70 [ 464.325554] ? exc_invalid_op+0x17/0x70 [ 464.325558] ? asm_exc_invalid_op+0x1a/0x20 [ 464.325567] ? __get_user_pages+0x423/0x520 [ 464.325575] __gup_longterm_locked+0x212/0x7a0 [ 464.325583] internal_get_user_pages_fast+0xfb/0x190 [ 464.325590] pin_user_pages_fast+0x47/0x60 [ 464.325598] sev_pin_memory+0xca/0x170 [kvm_amd] [ 464.325616] sev_mem_enc_register_region+0x81/0x130 [kvm_amd]
Per the analysis done by yangge, when starting the SEV virtual machine, it will call pin_user_pages_fast(..., FOLL_LONGTERM, ...) to pin the memory. But the page is in CMA area, so fast GUP will fail then fallback to the slow path due to the longterm pinnalbe check in try_grab_folio().
The slow path will try to pin the pages then migrate them out of CMA area. But the slow path also uses try_grab_folio() to pin the page, it will also fail due to the same check then the above warning is triggered.
In addition, the try_grab_folio() is supposed to be used in fast path and it elevates folio refcount by using add ref unless zero. We are guaranteed to have at least one stable reference in slow path, so the simple atomic add could be used. The performance difference should be trivial, but the misuse may be confusing and misleading.
Redefined try_grab_folio() to try_grab_folio_fast(), and try_grab_page() to try_grab_folio(), and use them in the proper paths. This solves both the abuse and the kernel warning.
The proper naming makes their usecase more clear and should prevent from abusing in the future.
peterx said:
: The user will see the pin fails, for gpu-slow it further triggers the WARN : right below that failure (as in the original report): : : folio = try_grab_folio(page, page_increm - 1, : foll_flags); : if (WARN_ON_ONCE(!folio)) { <------------------------ here : /* : * Release the 1st page ref if the : * folio is problematic, fail hard. : */ : gup_put_folio(page_folio(page), 1, : foll_flags); : ret = -EFAULT; : goto out; : }
[1] https://lore.kernel.org/linux-mm/[email protected]/
[[email protected]: fix implicit declaration of function try_grab_folio_fast]
Which versions are affected by CVE-2024-44943?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2024-44943?
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-2024-44943 being exploited?
There is no known exploitation of CVE-2024-44943 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2024-44943, DEBIAN-CVE-2024-44943, GHSA-fgc5-qhj8-xm5g, UBUNTU-CVE-2024-44943.