[v6,25/41] x86/mm: Introduce MAP_ABOVE4G

  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.

Since the default search behavior is top down, the normal kaslr base can
be used for MAP_ABOVE4G. This is unlike MAP_32BIT which has to add it's
own randomization in the bottom up case.

For MAP_32BIT, only the bottom up search path is used. For MAP_ABOVE4G
both are potentially valid, so both are used. In the bottomup search
path, the default behavior is already consistent with MAP_ABOVE4G since
mmap base should be above 4GB.

Without MAP_ABOVE4G, the shadow stack will already normally be above 4GB.
So without introducing MAP_ABOVE4G, trying to transition to 32 bit mode
with shadow stack enabled would usually segfault anyway. This is already
pretty decent guard rails. But the addition of MAP_ABOVE4G is some small
complexity spent to make it make it more complete.

Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

---
v5:
 - New patch
---
 arch/x86/include/uapi/asm/mman.h | 1 +
 arch/x86/kernel/sys_x86_64.c     | 6 +++++-
 include/linux/mman.h             | 4 ++++
 3 files changed, 10 insertions(+), 1 deletion(-)
  

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?

Reviewed-by: Kees Cook <keescook@chromium.org>
  

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@chromium.org>