CVE-2022-49771

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

dm ioctl: fix misbehavior if list_versions races with module loading

__list_versions will first estimate the required space using the dm_target_iterate(list_version_get_needed, &needed) call and then will fill the space using the dm_target_iterate(list_version_get_info, &iter_info) call. Each of these calls locks the targets using the down_read(&_lock) and up_read(&_lock) calls, however between the first and second dm_target_iterate there is no lock held and the target modules can be loaded at this point, so the second dm_target_iterate call may need more space than what was the first dm_target_iterate returned.

The code tries to handle this overflow (see the beginning of list_version_get_info), however this handling is incorrect.

The code sets param->data_size = param->data_start + needed and iter_info.end = (char *)vers+len - needed is the size returned by the first dm_target_iterate call; len is the size of the buffer allocated by userspace.

len may be greater than needed; in this case, the code will write up to len bytes into the buffer, however param->data_size is set to needed, so it may write data past the param->data_size value. The ioctl interface copies only up to param->data_size into userspace, thus part of the result will be truncated.

Fix this bug by setting iter_info.end = (char *)vers + needed; - this guarantees that the second dm_target_iterate call will write only up to the needed buffer and it will exit with DM_BUFFER_FULL_FLAG if it overflows the needed space - in this case, userspace will allocate a larger buffer and retry.

Note that there is also a bug in list_version_get_needed - we need to add strlen(tt->name) + 1 to the needed size, not strlen(tt->name).

Weakness

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/0c8d4112df329bf3dfbf27693f918c3b08676538
• https://git.kernel.org/stable/c/3a1c35d72dc0b34d1e746ed705790c0f630aa427
• https://git.kernel.org/stable/c/4fe1ec995483737f3d2a14c3fe1d8fe634972979
• https://git.kernel.org/stable/c/5398b8e275bf81a2517b327d216c0f37ac9ac5ae
• https://git.kernel.org/stable/c/6a818db0d5aecf80d4ba9e10ac153f60adc629ca
• https://git.kernel.org/stable/c/6ffce7a92ef5c68f7e5d6f4d722c2f96280c064b
• https://git.kernel.org/stable/c/b545c0e1e4094d4de2bdfe9a3823f9154b0c0005
• https://git.kernel.org/stable/c/f59f5a269ca5e43c567aca7f1f52500a0186e9b7