Re: [RFC PATCH 0/7] x86: introduce system calls addess space isolation

[Mike Rapoport]

On Fri, Apr 26, 2019 at 07:41:09AM -0700, Dave Hansen wrote:
> On 4/25/19 2:45 PM, Mike Rapoport wrote:
> > The idea behind the prevention is that if we fault in pages in the
> > execution path, we can compare target address against the kernel symbol
> > table.  So if we're in a function, we allow local jumps (and simply falling
> > of the end of a page) but if we're jumping to a new function it must be to
> > an external label in the symbol table.  Since ROP attacks are all about
> > jumping to gadget code which is effectively in the middle of real
> > functions, the jumps they induce are to code that doesn't have an external
> > symbol, so it should mostly detect when they happen.
> 
> This turns the problem from: "attackers can leverage any data/code that
> the kernel has mapped (anything)" to "attackers can leverage any
> code/data that the current syscall has faulted in".
> 
> That seems like a pretty restrictive change.
> 
> > At this time we are not suggesting any API that will enable the system
> > calls isolation. Because of the overhead required for this, it should only
> > be activated for processes or containers we know should be untrusted. We
> > still have no actual numbers, but surely forcing page faults during system
> > call execution will not come for free.
> 
> What's the minimum number of faults that have to occur to handle the
> simplest dummy fault?
 
For the current implementation it's 3.

Here is the example trace of #PF's produced by a dummy get_answer
system call from patch 7:

[   12.012906] #PF: DATA: do_syscall_64+0x26b/0x4c0 fault at 0xffffffff82000bb8
[   12.012918] #PF: INSN: __x86_indirect_thunk_rax+0x0/0x20 fault at __x86_indirect_thunk_rax+0x0/0x20
[   12.012929] #PF: INSN: __x64_sys_get_answer+0x0/0x10 fault at__x64_sys_get_answer+0x0/0x10

For the sci_write_dmesg syscall that does copy_from_user() and printk() its
between 35 and 60 depending on console and /proc/sys/kernel/printk values.

This includes both code and data accesses. The data page faults can be
avoided if we pre-populate SCI page tables with data.

-- 
Sincerely yours,
Mike.