CVE-2026-68388

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

smb/client: handle overlapping allocated ranges in fallocate

smb3_simple_fallocate_range() can skip holes when an allocated range returned by the server starts before the current fallocate offset. The skipped hole is not zero-filled, but fallocate still returns success. A later write to that hole may therefore fail with ENOSPC.

The function queries allocated ranges so that it can preserve existing contents and write zeroes only into holes. However, the server may return a range that starts before the current fallocate offset.

For example, assume the fallocate request is [100, 400) and the only allocated range returned by the server is [0, 200):

    Request:      [100, 400)
    Server range: [  0, 200)  allocated

    Correct:
    [100, 200)    allocated data, skip
    [200, 400)    hole, zero-fill

    Current:
    [100, 300)    skipped
    [300, 400)    zero-filled afterwards

The current code adds the full server range length, 200, to the current offset 100 and moves to 300. As a result, the hole in [200, 300) is skipped without being zero-filled.

Fix this by advancing only over the part of the allocated range that overlaps the current fallocate offset. Ignore ranges that end before the current offset and reject ranges whose end offset overflows.

This also prevents a malformed range length from causing an out-of-bounds zero-buffer read.

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/01719883235507b1585e4c51e320d9a7113dc698
• https://git.kernel.org/stable/c/377fe3e583e46369ee1004d5cfe12271d6589a68
• https://git.kernel.org/stable/c/437637f5ff3f573b2edf8571de91fb00a21eb4e6
• https://git.kernel.org/stable/c/7e08ab7a061b17ac1989a225c6afb53f44a86808
• https://git.kernel.org/stable/c/a4a09e5142835633fffbde68bd0a039ba4d4bf97
• https://git.kernel.org/stable/c/aeb58a4eb39a7ff4d7782b4f4ada0fda5e0675d2
• https://git.kernel.org/stable/c/b09ae45d85dc816987a71db9eebc54b0ae288e94
• https://git.kernel.org/stable/c/f47c7277c03a636fcc3a57969f2dc09567b3c050