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CAVEATS-generic.md

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Known caveats in the seL4 API and implementation

Implementation Correctness

Only the ARMv7 version on the imx6 platform of seL4 has the full stack of correctness proofs. This proof covers the functional behaviour of the C code of the kernel. It does not cover machine code, compiler, linker, boot code, cache and TLB management. Compiler and linker can be removed from this list by additionally running the binary verification tool chain for seL4. The proof shows that the seL4 C code implements the abstract API specification of seL4, and that this specification satisfies the following high-level security properties:

  • integrity (no write without authority),
  • confidentiality (no read without authority), and
  • intransitive non-interference (isolation between adequately configured user-level components).

The security property proofs depend on additional assumptions on the correct configuration of the system. See the l4v repository on github for more details.

The x64 port of the kernel without VT-x and VT-d support has a functional correctness proof between abstract specification and C code, but without security theorems, and the ARMv7 version of the kernel with hypervisor extensions also has a functional correctness proofs, but without the security theorems. For the precise configuration of these three verified platforms, see the corresponding files in the config/ directory.

Proofs for the MCS version (mixed-criticality systems) and for seL4 on the RISC-V architecture are in progress.

Real Time

The default version of seL4 must be configured carefully for use in real-time requirements. It has a small number of potentially long-running kernel operations that are not preemptible (e.g., endpoint deletion, certain scheduling states, frame and CNode initialisation). These can (and must) be avoided by careful system configuration if low latency is required.

The MCS configuration of the kernel addresses many of these problems and provides principled access control for execution time, but its formal verification is currently still in progress.

IPC buffer in globals frame may be stale

When a thread invokes its own TCB object to (re-)register its IPC buffer and the thread is re-scheduled immediately, the userland IPC buffer pointer in the globals frame may still show the old value. It is updated on the next thread switch.

Re-using Address Spaces (ARM and x86):

Before an ASID/page directory/page table can be reused, all frame caps installed in it should be revoked. The kernel will not do this automatically for the user.

If, for instance, page cap c is installed in the address space denoted by a page directory under ASID A, and the page directory is subsequently revoked or deleted, and then a new page directory is installed under that same ASID A, the page cap c will still retain some authority in the new page directory, even though the user intention might be to run the new page directory under a new security context. The authority retained is to perform the unmap operation on the page the cap c refers to.