Re: [PATCH v6 25/41] x86/mm: Introduce MAP_ABOVE4G

[Edgecombe, Rick P]

On Sun, 2023-02-19 at 12:43 -0800, Kees Cook wrote:
> On Sat, Feb 18, 2023 at 01:14:17PM -0800, Rick Edgecombe wrote:
> > The x86 Control-flow Enforcement Technology (CET) feature includes
> > a new
> > type of memory called shadow stack. This shadow stack memory has
> > some
> > unusual properties, which require some core mm changes to function
> > properly.
> > 
> > One of the properties is that the shadow stack pointer (SSP), which
> > is a
> > CPU register that points to the shadow stack like the stack pointer
> > points
> > to the stack, can't be pointing outside of the 32 bit address space
> > when
> > the CPU is executing in 32 bit mode. It is desirable to prevent
> > executing
> > in 32 bit mode when shadow stack is enabled because the kernel
> > can't easily
> > support 32 bit signals.
> > 
> > On x86 it is possible to transition to 32 bit mode without any
> > special
> > interaction with the kernel, by doing a "far call" to a 32 bit
> > segment.
> > So the shadow stack implementation can use this address space
> > behavior
> > as a feature, by enforcing that shadow stack memory is always
> > crated
> > outside of the 32 bit address space. This way userspace will
> > trigger a
> > general protection fault which will in turn trigger a segfault if
> > it
> > tries to transition to 32 bit mode with shadow stack enabled.
> > 
> > This provides a clean error generating border for the user if they
> > try
> > attempt to do 32 bit mode shadow stack, rather than leave the
> > kernel in a
> > half working state for userspace to be surprised by.
> > 
> > So to allow future shadow stack enabling patches to map shadow
> > stacks
> > out of the 32 bit address space, introduce MAP_ABOVE4G. The
> > behavior
> > is pretty much like MAP_32BIT, except that it has the opposite
> > address
> > range. The are a few differences though.
> > 
> > If both MAP_32BIT and MAP_ABOVE4G are provided, the kernel will use
> > the
> > MAP_ABOVE4G behavior. Like MAP_32BIT, MAP_ABOVE4G is ignored in a
> > 32 bit
> > syscall.
> 
> Should the interface refuse to accept both set instead?

I guess that might be less surprising. But I think to do this would
either require adding logic to core mm or a new arch breakout. I
actually kind of wish there was an easy way to keep this flag from
being used from userspace and just be a kernel only thing. It is only
used internally in this series and there isn't any know use for
userspace.

> 
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>