On Mon, Aug 27, 2018 at 4:12 PM, Jann Horn jannh@google.com wrote:
On Tue, Aug 28, 2018 at 1:04 AM Andy Lutomirski luto@kernel.org wrote:
In NMI context, we might be in the middle of context switching or in the middle of switch_mm_irqs_off(). In either case, CR3 might not match current->mm, which could cause copy_from_user_nmi() and friends to read the wrong memory.
Fix it by adding a new nmi_uaccess_okay() helper and checking it in copy_from_user_nmi() and in __copy_from_user_nmi()'s callers.
What about eBPF probes (which I think can be attached to kprobe points / tracepoints / perf events) that perform userspace reads / userspace writes / kernel reads? Can those run in NMI context, and if so, do they also need special handling?
I assume they can run in NMI context, which might be problematic in and of themselves. For example, does BPF adequately protect against a BPF program accessing a map while bpf(2) is modifying it? It seems like bpf_prog_active is intended to serve this purpose.
But I don't see any obvious mechanism for eBPF programs to read user memory.