Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

[Alexei Starovoitov]

On Thu, Jan 04, 2018 at 02:15:53AM +0000, Alan Cox wrote:
> 
> > > Elena has done the work of auditing static analysis reports to a dozen
> > > or so locations that need some 'nospec' handling.  
> > 
> > How exactly is that related (especially in longer-term support terms) to 
> > BPF anyway?
> 
> If you read the papers you need a very specific construct in order to not
> only cause a speculative load of an address you choose but also to then
> manage to cause a second operation that in some way reveals bits of data
> or allows you to ask questions.
> 
> BPF allows you to construct those sequences relatively easily and it's
> the one case where a user space application can fairly easily place code
> it wants to execute in the kernel. Without BPF you have to find the right
> construct in the kernel, prime all the right predictions and measure the
> result without getting killed off. There are places you can do that but
> they are not so easy and we don't (at this point) think there are that
> many.

for BPF in particular we're thinking to do a different fix.
Instead of killing speculation we can let cpu speculate.
The fix will include rounding up bpf maps to nearest power of 2 and
inserting bpf_and operation on the index after bounds check,
so cpu can continue speculate beyond bounds check, but it will
load from zero-ed memory.
So this nospec arch dependent hack won't be necessary for bpf side,
but may still be needed in other parts of the kernel.

Also note that variant 1 is talking about exploiting prog_array
bpf feature which had 64-bit access prior to
commit 90caccdd8cc0 ("bpf: fix bpf_tail_call() x64 JIT")
That was a fix for JIT and not related to this cpu issue, but
I believe it breaks the existing exploit.

Since it's not clear whether it's still possible to use bpf
with 32-bit speculation only, we're going to do this rounding fix
for unpriv just in case.