Re: [RFCv2 00/10] Linear Address Masking enabling

Matthew Wilcox
2022-05-13T14:45:08Z

On Thu, May 12, 2022 at 11:24:27PM +0200, Thomas Gleixner wrote:
> On Thu, May 12 2022 at 21:39, Thomas Gleixner wrote:
> > On Thu, May 12 2022 at 10:22, Dave Hansen wrote:
> >> One of the stated reasons for adding LAM hardware is that folks want to
> >> use sanitizers outside of debugging environments. To me, that means
> >> that LAM is something that the same binary might run with or without.
> >
> > On/off yes, but is there an actual use case where such a mechanism would
> > at start time dynamically chose the number of bits?
>
> This would need cooperation from the application because it has to tell
> the magic facility whether it intends to utilize the large VA space on a
> 5-level paging system or not.

Taking a step back ...

Consider an application like 'grep'. It operates on streams of data,
and has a fairly bounded amount of virtual memory that it will use.
Let's say it's 1GB (*handwave*). So it has 33 bits of address space that
it can use for "security" of one form or another. ASLR is one thing which
is vying for bits, and now ASAN is another. Somehow it is supposed to
tell the kernel "I want to use 6 bits for ASAN and 27 bits for ASLR".
Or "I want to use 15 bits for ASAN and 18 bits for ASLR".

So how does grep make that decision? How does it find out what the
kernel supports? Deciding which tradeoff is more valuable to grep is
clearly not something the kernel can help with, but I do think that the
kernel needs to have an API to query what's available.

Something like Libreoffice or Firefox is obviously going to be much more
complex; it doesn't really have a bounded amount of virtual memory to
work with. But if we can't even solve this problem for applications
with bounded address space, we stand no chance of solving it for hard
cases.