On Tue, Aug 28, 2018 at 1:26 AM Andy Lutomirski luto@kernel.org wrote:
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.
Look in kernel/trace/bpf_trace.c, which defines a bunch of eBPF helpers that can only be called from privileged eBPF code. Ah, but I misremembered, the userspace write helper does have a guard against interrupts, just the arbitrary read helper doesn't.
BPF_CALL_3(bpf_probe_read, void *, dst, u32, size, const void *, unsafe_ptr) { int ret;
ret = probe_kernel_read(dst, unsafe_ptr, size); if (unlikely(ret < 0)) memset(dst, 0, size);
return ret; } [...] BPF_CALL_3(bpf_probe_write_user, void *, unsafe_ptr, const void *, src, u32, size) { /* * Ensure we're in user context which is safe for the helper to * run. This helper has no business in a kthread. * * access_ok() should prevent writing to non-user memory, but in * some situations (nommu, temporary switch, etc) access_ok() does * not provide enough validation, hence the check on KERNEL_DS. */
if (unlikely(in_interrupt() || current->flags & (PF_KTHREAD | PF_EXITING))) return -EPERM; if (unlikely(uaccess_kernel())) return -EPERM; if (!access_ok(VERIFY_WRITE, unsafe_ptr, size)) return -EPERM;
return probe_kernel_write(unsafe_ptr, src, size); }