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@xxxxxxxxxxxx> -- Kees Cook
