CVE-2021-47040

What it actually does

Missing overflow and sign-extension checks in io_provide_buffers_prep allow an attacker-supplied length value to wrap or extend incorrectly, resulting in incorrect buffer accounting and potential out-of-bounds memory access inside the kernel.

Why it matters to the business

Systems that expose io_uring to untrusted workloads risk kernel memory corruption, which can produce crashes, denial of service, or local privilege escalation affecting server availability and data confidentiality.

How an attack unfolds

An attacker opens an io_uring instance, submits a PROVIDE_BUFFERS operation with a crafted length that triggers integer wrap-around or sign extension, bypasses the intended bounds check, and causes the kernel to reference or write beyond allocated memory.

How to fix it

Upgrade the Linux kernel to a version containing commit d81269fecb8ce and the subsequent check_add_overflow / check_mul_overflow hardening; rebuild and reboot any affected systems or containers.

How to confirm the fix worked

Confirm the running kernel contains the referenced commit or later equivalent; exercise io_provide_buffers_prep with boundary values (maximum unsigned, negative, and overflow cases) under a fuzzer or unit test to verify rejection.

Typical attack path

1. Obtain local execution or unprivileged access on a Linux host that uses io_uring
2. Create an io_uring ring and register file descriptors
3. Prepare a struct io_uring_sqe with opcode IORING_OP_PROVIDE_BUFFERS and a malicious len field
4. Submit the request and trigger the vulnerable preparation path
5. Observe or exploit resulting memory corruption for further attacks

How to detect it

Review kernel audit logs and io_uring tracepoints for PROVIDE_BUFFERS submissions with unusually large or negative length fields; correlate with subsequent oops or page-fault events in the io_uring worker threads.

What your security team should do

• Prioritize kernel updates on any host that permits untrusted io_uring use
• Where immediate patching is impossible, restrict io_uring via seccomp or LSM policies
• Monitor for new kernel releases and test in staging before broad deployment