Re: [PATCH v4 27/39] x86/shstk: Handle thread shadow stack

Kees Cook
2022-12-03T02:44:28Z

On Fri, Dec 02, 2022 at 04:35:54PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> When a process is duplicated, but the child shares the address space with
> the parent, there is potential for the threads sharing a single stack to
> cause conflicts for each other. In the normal non-cet case this is handled
> in two ways.
>
> With regular CLONE_VM a new stack is provided by userspace such that the
> parent and child have different stacks.
>
> For vfork, the parent is suspended until the child exits. So as long as
> the child doesn't return from the vfork()/CLONE_VFORK calling function and
> sticks to a limited set of operations, the parent and child can share the
> same stack.
>
> For shadow stack, these scenarios present similar sharing problems. For the
> CLONE_VM case, the child and the parent must have separate shadow stacks.
> Instead of changing clone to take a shadow stack, have the kernel just
> allocate one and switch to it.
>
> Use stack_size passed from clone3() syscall for thread shadow stack size. A
> compat-mode thread shadow stack size is further reduced to 1/4. This
> allows more threads to run in a 32-bit address space. The clone() does not
> pass stack_size, which was added to clone3(). In that case, use
> RLIMIT_STACK size and cap to 4 GB.
>
> For shadow stack enabled vfork(), the parent and child can share the same
> shadow stack, like they can share a normal stack. Since the parent is
> suspended until the child terminates, the child will not interfere with
> the parent while executing as long as it doesn't return from the vfork()
> and overwrite up the shadow stack. The child can safely overwrite down
> the shadow stack, as the parent can just overwrite this later. So CET does
> not add any additional limitations for vfork().
>
> Userspace implementing posix vfork() can actually prevent the child from
> returning from the vfork() calling function, using CET. Glibc does this
> by adjusting the shadow stack pointer in the child, so that the child
> receives a #CP if it tries to return from vfork() calling function.
>
> Free the shadow stack on thread exit by doing it in mm_release(). Skip
> this when exiting a vfork() child since the stack is shared in the
> parent.
>
> During this operation, the shadow stack pointer of the new thread needs
> to be updated to point to the newly allocated shadow stack. Since the
> ability to do this is confined to the FPU subsystem, change
> fpu_clone() to take the new shadow stack pointer, and update it
> internally inside the FPU subsystem. This part was suggested by Thomas
> Gleixner.
>
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

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

--
Kees Cook