[PATCH bpf-next] bpf: Make trampolines W^X

[Andy Lutomirski]

> On Jan 7, 2020, at 9:01 AM, Edgecombe, Rick P <rick.p.edgecombe at intel.com> wrote:
>
> CC Nadav and Jessica.
>
> On Mon, 2020-01-06 at 15:36 -1000, Andy Lutomirski wrote:
>>> On Jan 6, 2020, at 12:25 PM, Edgecombe, Rick P <rick.p.edgecombe at intel.com>
>>> wrote:
>>>
>>> On Sat, 2020-01-04 at 09:49 +0900, Andy Lutomirski wrote:
>>>>>>> On Jan 4, 2020, at 8:47 AM, KP Singh <kpsingh at chromium.org> wrote:
>>>>>>
>>>>>> From: KP Singh <kpsingh at google.com>
>>>>>>
>>>>>> The image for the BPF trampolines is allocated with
>>>>>> bpf_jit_alloc_exe_page which marks this allocated page executable. This
>>>>>> means that the allocated memory is W and X at the same time making it
>>>>>> susceptible to WX based attacks.
>>>>>>
>>>>>> Since the allocated memory is shared between two trampolines (the
>>>>>> current and the next), 2 pages must be allocated to adhere to W^X and
>>>>>> the following sequence is obeyed where trampolines are modified:
>>>>>
>>>>> Can we please do better rather than piling garbage on top of garbage?
>>>>>
>>>>>>
>>>>>> - Mark memory as non executable (set_memory_nx). While module_alloc for
>>>>>> x86 allocates the memory as PAGE_KERNEL and not PAGE_KERNEL_EXEC, not
>>>>>> all implementations of module_alloc do so
>>>>>
>>>>> How about fixing this instead?
>>>>>
>>>>>> - Mark the memory as read/write (set_memory_rw)
>>>>>
>>>>> Probably harmless, but see above about fixing it.
>>>>>
>>>>>> - Modify the trampoline
>>>>>
>>>>> Seems reasonable. It’s worth noting that this whole approach is
>>>>> suboptimal:
>>>>> the “module” allocator should really be returning a list of pages to be
>>>>> written (not at the final address!) with the actual executable mapping to
>>>>> be
>>>>> materialized later, but that’s a bigger project that you’re welcome to
>>>>> ignore
>>>>> for now.  (Concretely, it should produce a vmap address with backing pages
>>>>> but
>>>>> with the vmap alias either entirely unmapped or read-only. A subsequent
>>>>> healer
>>>>> would, all at once, make the direct map pages RO or not-present and make
>>>>> the
>>>>> vmap alias RX.)
>>>>>> - Mark the memory as read-only (set_memory_ro)
>>>>>> - Mark the memory as executable (set_memory_x)
>>>>>
>>>>> No, thanks. There’s very little excuse for doing two IPI flushes when one
>>>>> would suffice.
>>>>>
>>>>> As far as I know, all architectures can do this with a single flush
>>>>> without
>>>>> races  x86 certainly can. The module freeing code gets this sequence
>>>>> right.
>>>>> Please reuse its mechanism or, if needed, export the relevant interfaces.
>>>
>>> So if I understand this right, some trampolines have been added that are
>>> currently set as RWX at modification time AND left that way during runtime?
>>> The
>>> discussion on the order of set_memory_() calls in the commit message made me
>>> think that this was just a modification time thing at first.
>>
>> I’m not sure what the status quo is.
>>
>> We really ought to have a genuinely good API for allocation and initialization
>> of text.  We can do so much better than set_memory_blahblah.
>>
>> FWIW, I have some ideas about making kernel flushes cheaper. It’s currently
>> blocked on finding some time and on tglx’s irqtrace work.
>>
>
> Makes sense to me. I guess there are 6 types of text allocations now:
> - These two BPF trampolines
> - BPF JITs
> - Modules
> - Kprobes
> - Ftrace
>
> All doing (or should be doing) pretty much the same thing. I believe Jessica had
> said at one point that she didn't like all the other features using
> module_alloc() as it was supposed to be just for real modules. Where would the
> API live?

New header?  This shouldn’t matter that much.

Here are two strawman proposals.  All of this is very rough -- the
actual data structures and signatures are likely problematic for
multiple reasons.

--- First proposal ---

struct text_allocation {
  void *final_addr;
  struct page *pages;
  int npages;
};

int text_alloc(struct text_allocation *out, size_t size);

/* now final_addr is not accessible and pages is writable. */

int text_freeze(struct text_allocation *alloc);

/* now pages are not accessible and final_addr is RO.  Alternatively,
pages are RO and final_addr is unmapped. */

int text_finish(struct text_allocation *alloc);

/* now final_addr is RX.  All done. */

This gets it with just one flush and gives a chance to double-check in
case of race attacks from other CPUs.  Double-checking is annoying,
though.

--- Second proposal ---

struct text_allocation {
  void *final_addr;
  /* lots of opaque stuff including an mm_struct */
  /* optional: list of struct page, but this isn't obviously useful */
};

int text_alloc(struct text_allocation *out, size_t size);

/* Memory is allocated.  There is no way to access it at all right
now.  The memory is RO or not present in the direct map. */

void __user *text_activate_mapping(struct text_allocation *out);

/* Now the text is RW at *user* address given by return value.
Preemption is off if required by use_temporary_mm().  Real user memory
cannot be accessed. */

void text_deactivate_mapping(struct text_allocation *alloc);

/* Now the memory is inaccessible again. */

void text_finalize(struct text_allocation *alloc);

/* Now it's RX or XO at the final address. */


Pros of second approach:

 - Inherently immune to cross-CPU attack.  No double-check.

 - If we ever implement a cache of non-direct-mapped, unaliased pages,
then it works with no flushes at all.  We could even relax it a bit to
allow non-direct-mapped pages that may have RX / XO aliases but no W
aliases.

 - Can easily access without worrying about page boundaries.

Cons:

 - The use of a temporary mm is annoying -- you can't copy from user
memory, for example.