CVE-2022-48827

In the Linux kernel, the following vulnerability has been resolved:

NFSD: Fix the behavior of READ near OFFSET_MAX

Dan Aloni reports:

Due to commit 8cfb9015280d (NFS: Always provide aligned buffers to the RPC read layers) on the client, a read of 0xfff is aligned up to server rsize of 0x1000.

As a result, in a test where the server has a file of size 0x7fffffffffffffff, and the client tries to read from the offset 0x7ffffffffffff000, the read causes loff_t overflow in the server and it returns an NFS code of EINVAL to the client. The client as a result indefinitely retries the request.

The Linux NFS client does not handle NFS?ERR_INVAL, even though all NFS specifications permit servers to return that status code for a READ.

Instead of NFS?ERR_INVAL, have out-of-range READ requests succeed and return a short result. Set the EOF flag in the result to prevent the client from retrying the READ request. This behavior appears to be consistent with Solaris NFS servers.

Note that NFSv3 and NFSv4 use u64 offset values on the wire. These must be converted to loff_t internally before use – an implicit type cast is not adequate for this purpose. Otherwise VFS checks against sb->s_maxbytes do not work properly.

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

References

• https://git.kernel.org/stable/c/0cb4d23ae08c48f6bf3c29a8e5c4a74b8388b960
• https://git.kernel.org/stable/c/1726a39b0879acfb490b22dca643f26f4f907da9
• https://git.kernel.org/stable/c/44502aca8e02ab32d6b0eb52e006a5ec9402719b
• https://git.kernel.org/stable/c/c6eff5c4277146a78b4fb8c9b668dd64542c41b0
• https://git.kernel.org/stable/c/0cb4d23ae08c48f6bf3c29a8e5c4a74b8388b960
• https://git.kernel.org/stable/c/1726a39b0879acfb490b22dca643f26f4f907da9
• https://git.kernel.org/stable/c/44502aca8e02ab32d6b0eb52e006a5ec9402719b
• https://git.kernel.org/stable/c/c6eff5c4277146a78b4fb8c9b668dd64542c41b0