Re: [RFC PATCH 1/2] x86: WARN() when uaccess helpers fault on kernel addresses

[Andy Lutomirski]

> On Aug 7, 2018, at 2:17 PM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> 
> On Tue, Aug 7, 2018 at 4:55 AM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> On Aug 6, 2018, at 6:22 PM, Jann Horn <jannh@xxxxxxxxxx> wrote:
>>> 
>>> There have been multiple kernel vulnerabilities that permitted userspace to
>>> pass completely unchecked pointers through to userspace accessors:
>>> 
>>> - the waitid() bug - commit 96ca579a1ecc ("waitid(): Add missing
>>> access_ok() checks")
>>> - the sg/bsg read/write APIs
>>> - the infiniband read/write APIs
>>> 
>>> These don't happen all that often, but when they do happen, it is hard to
>>> test for them properly; and it is probably also hard to discover them with
>>> fuzzing. Even when an unmapped kernel address is supplied to such buggy
>>> code, it just returns -EFAULT instead of doing a proper BUG() or at least
>>> WARN().
>>> 
>>> This patch attempts to make such misbehaving code a bit more visible by
>>> WARN()ing in the pagefault handler code when a userspace accessor causes
>>> #PF on a kernel address and the current context isn't whitelisted.
>> 
>> I like this a lot, and, in fact, I once wrote a patch to do something similar. It was before the fancy extable code, though, so it was a mess.  Here are some thoughts:
>> 
>> - It should be three patches. One patch to add the _UA annotations, one to improve the info passes to the handlers, and one to change behavior.
>> 
>> - You should pass the vector, the error code, and the address to the handler.
>> 
>> - The uaccess handler should IMO WARN if the vector is anything other than #PF (which mainly means warning if itâs #GP). I think it should pr_emerg() and return false if the vector is #PF and the address is too high.
> 
> What about #MC? do_machine_check() sometimes invokes fixup handlers.
> It looks like fixup_exception() is basically reached for anything that
> can't be restarted (either MCG_STATUS_RIPV isn't set or the worst
> severity is MCE_AR_SEVERITY), but doesn't reach MCE_PANIC_SEVERITY? In
> particular for "Action required: data load in error recoverable area
> of kernel", if I'm reading the code correctly. It seems like the code
> is intentionally preventing memory errors during user access from
> being treated as kernel memory errors? So perhaps #MC in user access
> should not WARN()?

Agreed. 

> 
>> - Arguably most non-uaccess fixups should at least warn for anything other than #GP and #UD.