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>
