Re: [PATCH v2 25/39] x86/cet/shstk: Handle thread shadow stack

[Mike Rapoport]

On Thu, Sep 29, 2022 at 03:29:22PM -0700, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> 
> 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.
> 
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> 
> ---
> 
> v2:
>  - Have fpu_clone() take new shadow stack pointer and update SSP in
>    xsave buffer for new task. (tglx)
> 
> v1:
>  - Expand commit log.
>  - Add more comments.
>  - Switch to xsave helpers.
> 
> Yu-cheng v30:
>  - Update comments about clone()/clone3(). (Borislav Petkov)
> 
> Yu-cheng v29:
>  - WARN_ON_ONCE() when get_xsave_addr() returns NULL, and update comments.
>    (Dave Hansen)
> 
>  arch/x86/include/asm/cet.h         |  7 +++++
>  arch/x86/include/asm/fpu/sched.h   |  3 +-
>  arch/x86/include/asm/mmu_context.h |  2 ++
>  arch/x86/kernel/fpu/core.c         | 40 ++++++++++++++++++++++++-
>  arch/x86/kernel/process.c          | 17 ++++++++++-
>  arch/x86/kernel/shstk.c            | 48 +++++++++++++++++++++++++++++-
>  6 files changed, 113 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 778d3054ccc7..f332e9b42b6d 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -555,8 +555,40 @@ static inline void fpu_inherit_perms(struct fpu *dst_fpu)
>  	}
>  }
>  
> +#ifdef CONFIG_X86_SHADOW_STACK
> +static int update_fpu_shstk(struct task_struct *dst, unsigned long ssp)
> +{
> +	struct cet_user_state *xstate;
> +
> +	/* If ssp update is not needed. */
> +	if (!ssp)
> +		return 0;
> +
> +	xstate = get_xsave_addr(&dst->thread.fpu.fpstate->regs.xsave,
> +				XFEATURE_CET_USER);
> +
> +	/*
> +	 * If there is a non-zero ssp, then 'dst' must be configured with a shadow
> +	 * stack and the fpu state should be up to date since it was just copied
> +	 * from the parent in fpu_clone(). So there must be a valid non-init CET
> +	 * state location in the buffer.
> +	 */
> +	if (WARN_ON_ONCE(!xstate))
> +		return 1;
> +
> +	xstate->user_ssp = (u64)ssp;
> +
> +	return 0;
> +}
> +#else
> +static int update_fpu_shstk(struct task_struct *dst, unsigned long shstk_addr)
> +{

return 0; ?

> +}
> +#endif
> +

-- 
Sincerely yours,
Mike.