CVE-2021-46988

MEDIUM
Published Feb 28, 2024 Modified Dec 26, 2024 CWE-416

Description

In the Linux kernel, the following vulnerability has been resolved: userfaultfd: release page in error path to avoid BUG_ON Consider the following sequence of events: 1. Userspace issues a UFFD ioctl, which ends up calling into shmem_mfill_atomic_pte(). We successfully account the blocks, we shmem_alloc_page(), but then the copy_from_user() fails. We return -ENOENT. We don't release the page we allocated. 2. Our caller detects this error code, tries the copy_from_user() after dropping the mmap_lock, and retries, calling back into shmem_mfill_atomic_pte(). 3. Meanwhile, let's say another process filled up the tmpfs being used. 4. So shmem_mfill_atomic_pte() fails to account blocks this time, and immediately returns - without releasing the page. This triggers a BUG_ON in our caller, which asserts that the page should always be consumed, unless -ENOENT is returned. To fix this, detect if we have such a "dangling" page when accounting fails, and if so, release it before returning.

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CVSS v3.1 Score

5.5
MEDIUM
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Weakness Type (CWE)

CWE-416 Use After Free

Affected Products

Vendor Product
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel

References

Advisories & Patches

https://git.kernel.org/stable/c/07c9b834c97d0fa3402fb7f3f3b32df370a6ff1f https://git.kernel.org/stable/c/140cfd9980124aecb6c03ef2e69c72d0548744de https://git.kernel.org/stable/c/2d59a0ed8b26b8f3638d8afc31f839e27759f1f6 https://git.kernel.org/stable/c/319116227e52d49eee671f0aa278bac89b3c1b69 https://git.kernel.org/stable/c/7ed9d238c7dbb1fdb63ad96a6184985151b0171c https://git.kernel.org/stable/c/ad53127973034c63b5348715a1043d0e80ceb330 https://git.kernel.org/stable/c/b3f1731c6d7fbc1ebe3ed8eff6d6bec56d76ff43 https://git.kernel.org/stable/c/07c9b834c97d0fa3402fb7f3f3b32df370a6ff1f https://git.kernel.org/stable/c/140cfd9980124aecb6c03ef2e69c72d0548744de https://git.kernel.org/stable/c/2d59a0ed8b26b8f3638d8afc31f839e27759f1f6 https://git.kernel.org/stable/c/319116227e52d49eee671f0aa278bac89b3c1b69 https://git.kernel.org/stable/c/7ed9d238c7dbb1fdb63ad96a6184985151b0171c https://git.kernel.org/stable/c/ad53127973034c63b5348715a1043d0e80ceb330 https://git.kernel.org/stable/c/b3f1731c6d7fbc1ebe3ed8eff6d6bec56d76ff43

Frequently Asked Questions

What is CVE-2021-46988? +
In the Linux kernel, the following vulnerability has been resolved: userfaultfd: release page in error path to avoid BUG_ON Consider the following sequence of events: 1. Userspace issues a UFFD ioctl, which ends up calling into shmem_mfill_atomic_pte(). We successfully account the blocks, we shmem_alloc_page(), but then the copy_from_user() fails. We return -ENOENT. We don't release the page we allocated. 2. Our caller detects this error code, tries the copy_from_user() after dropping the mmap_lock, and retries, calling back into shmem_mfill_atomic_pte(). 3. Meanwhile, let's say another process filled up the tmpfs being used. 4. So shmem_mfill_atomic_pte() fails to account blocks this time, and immediately returns - without releasing the page. This triggers a BUG_ON in our caller, which asserts that the page should always be consumed, unless -ENOENT is returned. To fix this, detect if we have such a "dangling" page when accounting fails, and if so, release it before returning. It has a CVSS v3.1 base score of 5.5 (MEDIUM).
How severe is CVE-2021-46988? +
CVE-2021-46988 has a CVSS v3.1 score of 5.5 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance.
What products are affected by CVE-2021-46988? +
CVE-2021-46988 affects products from linux, specifically: linux_kernel. Check the affected products table above for specific version ranges.
How do I check if I'm vulnerable to CVE-2021-46988? +
You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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