CVE-2022-50103

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

sched, cpuset: Fix dl_cpu_busy() panic due to empty cs->cpus_allowed

With cgroup v2, the cpusets cpus_allowed mask can be empty indicating that the cpuset will just use the effective CPUs of its parent. So cpuset_can_attach() can call task_can_attach() with an empty mask. This can lead to cpumask_any_and() returns nr_cpu_ids causing the call to dl_bw_of() to crash due to percpu value access of an out of bound CPU value. For example:

[80468.182258] BUG: unable to handle page fault for address: ffffffff8b6648b0
  :
[80468.191019] RIP: 0010:dl_cpu_busy+0x30/0x2b0
  :
[80468.207946] Call Trace:
[80468.208947]  cpuset_can_attach+0xa0/0x140
[80468.209953]  cgroup_migrate_execute+0x8c/0x490
[80468.210931]  cgroup_update_dfl_csses+0x254/0x270
[80468.211898]  cgroup_subtree_control_write+0x322/0x400
[80468.212854]  kernfs_fop_write_iter+0x11c/0x1b0
[80468.213777]  new_sync_write+0x11f/0x1b0
[80468.214689]  vfs_write+0x1eb/0x280
[80468.215592]  ksys_write+0x5f/0xe0
[80468.216463]  do_syscall_64+0x5c/0x80
[80468.224287]  entry_SYSCALL_64_after_hwframe+0x44/0xae

Fix that by using effective_cpus instead. For cgroup v1, effective_cpus is the same as cpus_allowed. For v2, effective_cpus is the real cpumask to be used by tasks within the cpuset anyway.

Also update task_can_attach()s 2nd argument name to cs_effective_cpus to reflect the change. In addition, a check is added to task_can_attach() to guard against the possibility that cpumask_any_and() may return a value >= nr_cpu_ids.

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/147f66d22f58712dce7ccdd6a1f6cb3ee8042df4
• https://git.kernel.org/stable/c/336626564b58071b8980a4e6a31a8f5d92705d9b
• https://git.kernel.org/stable/c/357f3f0e522a6ce1ce4a571cb780d9861d53bec7
• https://git.kernel.org/stable/c/b6e8d40d43ae4dec00c8fea2593eeea3114b8f44
• https://git.kernel.org/stable/c/f56607b44c9896e51678a7e8cdd3a5479f4b4548