Re: [PATCH net-next v3 1/2] bpf: Add bpf_copy_to_user BPF helper to be called in tracers (kprobes)

From: Alexei Starovoitov
Date: Tue Jul 19 2016 - 23:02:14 EST

On Wed, Jul 20, 2016 at 01:19:51AM +0200, Daniel Borkmann wrote:
> On 07/19/2016 06:34 PM, Alexei Starovoitov wrote:
> >On Tue, Jul 19, 2016 at 01:17:53PM +0200, Daniel Borkmann wrote:
> >>>+ return -EINVAL;
> >>>+
> >>>+ /* Is this a user address, or a kernel address? */
> >>>+ if (!access_ok(VERIFY_WRITE, to, size))
> >>>+ return -EINVAL;
> >>>+
> >>>+ return probe_kernel_write(to, from, size);
> >>
> >>I'm still worried that this can lead to all kind of hard to find
> >>bugs or races for user processes, if you make this writable to entire
> >>user address space (which is the only thing that access_ok() checks
> >>for). What if the BPF program has bugs and writes to wrong addresses
> >>for example, introducing bugs in some other, non-intended processes
> >>elsewhere? Can this be limited to syscalls only? And if so, to the
> >>passed memory only?
> >
> >my understanding that above code will write only to memory of current process,
> >so impact is contained and in that sense buggy kprobe program is no different
> >from buggy seccomp prorgram.
> Compared to seccomp, you might not notice that a race has happened,
> in seccomp case you might have killed your process, which is visible.
> But ok, in ptrace() case it might be similar issue perhaps ...
> The asm-generic version does __access_ok(..) { return 1; } for nommu
> case, I haven't checked closely enough whether there's actually an arch
> that uses this, but for example arm nommu with only one addr space would
> certainly result in access_ok() as 1, and then you could also go into
> probe_kernel_write(), no?

good point. how arm nommu handles copy_to_user? if there is nommu
then there is no user/kernel mm ? Crazy archs.
I guess we have to disable this helper on all such archs.

> Don't know that code well enough, but I believe the check would only
> ensure in normal use-cases that user process doesn't fiddle with kernel
> address space, but not necessarily guarantee that this really only
> belongs to the process address space.

why? on x86 that exactly what it does. access_ok=true means
it's user space address and since we're in _this user context_
probe_kernel_write can only affect this user.

> x86 code comments this with "note that, depending on architecture,
> this function probably just checks that the pointer is in the user
> space range - after calling this function, memory access functions may
> still return -EFAULT".

Yes. I've read that comment to :)
Certainly not an expert, but the archs I've looked at, access_ok
has the same meaning as on x86. They check the space range to
make sure address doesn't belong to kernel.
Could I have missed something? Certainly. Please double check :)

> Also, what happens in case of kernel thread?

my understanding if access_ok(addr)=true the addr will never point
to memory of kernel thread.
We need expert opinion. Whom should we ping?

> As it stands, it does ...
> if (unlikely(in_interrupt()))
> return -EINVAL;
> if (unlikely(!task || !task->pid))
> return -EINVAL;
> So up to here, irq/sirq, NULL current and that current is not the 'idle'
> process is being checked (still fail to see the point for the !task->pid,
> I believe the intend here is different).
> /* Is this a user address, or a kernel address? */
> if (!access_ok(VERIFY_WRITE, to, size))
> return -EINVAL;
> Now here. What if it's a kernel thread? You'll have KERNEL_DS segment,
> task->pid was non-zero as well for the kthread, so access_ok() will
> pass and you can still execute probe_kernel_write() ...

I think user_addr_max() should be zero for kthread, but
worth checking for sure.

> >Limiting this to syscalls will make it too limited.
> >I'm in favor of this change, because it allows us to experiment
> >with restartable sequences and lock-free algorithms that need ultrafast
> >access to cpuid without burdening the kernel with stable abi.
> >
> >>Have you played around with ptrace() to check whether you could
> >>achieve similar functionality (was thinking about things like [1],
> >>this be limited to a similar functionality for only the current task.
> >>ptrace() utilizes helpers like access_process_vm(), maybe this can
> >>similarly be adapted here, too (under the circumstances that sleeping
> >>is not allowed)?
> >
> >If we hack access_process_vm I think at the end it will look like
> >probe_kernel_write. Walking mm requires semaphore, so we would only
> >be able to do it in task_work and there we can do normal copy_to_user
> >just as well, but it will complicate the programs quite a bit, since
> >writes will be asynchronous and batched.
> >Looks like with access_ok+probe_write we can achieve the same
> >with a lot less code.
> I believe it may not quite be the same as it currently stands. No
> fundamental objection, just that this needs to be made "safe" to the
> limits you state above yourself. ;)

completely agree :) the more eyes on the patch the better.

> >As far as races between user and bpf program, yeah, if process
> >is multithreaded, the other threads may access the same mem that
> >bpf is writing to, but that's no different from reading.
> >For tracing we walk complex datastructures and pointers. They
> >can be changed by user space on the fly and bpf will see garbage.
> >Like we have uprobe+bpf that walks clang c++ internal datastructures
> >to figure out how long it takes clang to process .h headers.
> >There is a lot of fragility in the bpf script, yet it's pretty
> >useful to quickly analyze compile times.
> >I see bpf_copy_to_user to be hugely valuable too, not as a stable
> >interface, but rather as a tool to quickly debug and experiment.
> Right, so maybe there should be a warn once to the dmesg log that this
> is just experimental?

good point. The tracing experience showed that trace_printk warning
was quite effective on steering user space assumptions. Let's do
something here as well.