Re: [PATCH v7 30/41] x86/shstk: Handle thread shadow stack

[Borislav Petkov]

On Mon, Feb 27, 2023 at 02:29:46PM -0800, 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

"non-CET"

> 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

"POSIX"

...

> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index f851558b673f..bc3de4aeb661 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -552,8 +552,41 @@ static inline void fpu_inherit_perms(struct fpu *dst_fpu)
>  	}
>  }
>  
> +#ifdef CONFIG_X86_USER_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)
								      ^^^^^^^^^^^
ssp, like above.

Better yet:

static int update_fpu_shstk(struct task_struct *dst, unsigned long ssp)
{
#ifdef CONFIG_X86_USER_SHADOW_STACK
	...
#endif
	return 0;
}

and less ifdeffery.



> +{
> +	return 0;
> +}
> +#endif
> +
>  /* Clone current's FPU state on fork */
> -int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal)
> +int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal,
> +	      unsigned long ssp)
>  {
>  	struct fpu *src_fpu = &current->thread.fpu;
>  	struct fpu *dst_fpu = &dst->thread.fpu;
> @@ -613,6 +646,12 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal)
>  	if (use_xsave())
>  		dst_fpu->fpstate->regs.xsave.header.xfeatures &= ~XFEATURE_MASK_PASID;
>  
> +	/*
> +	 * Update shadow stack pointer, in case it changed during clone.
> +	 */
> +	if (update_fpu_shstk(dst, ssp))
> +		return 1;
> +
>  	trace_x86_fpu_copy_src(src_fpu);
>  	trace_x86_fpu_copy_dst(dst_fpu);
>  
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index b650cde3f64d..bf703f53fa49 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -48,6 +48,7 @@
>  #include <asm/frame.h>
>  #include <asm/unwind.h>
>  #include <asm/tdx.h>
> +#include <asm/shstk.h>
>  
>  #include "process.h"
>  
> @@ -119,6 +120,7 @@ void exit_thread(struct task_struct *tsk)
>  
>  	free_vm86(t);
>  
> +	shstk_free(tsk);
>  	fpu__drop(fpu);
>  }
>  
> @@ -140,6 +142,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>  	struct inactive_task_frame *frame;
>  	struct fork_frame *fork_frame;
>  	struct pt_regs *childregs;
> +	unsigned long shstk_addr = 0;
>  	int ret = 0;
>  
>  	childregs = task_pt_regs(p);
> @@ -174,7 +177,13 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>  	frame->flags = X86_EFLAGS_FIXED;
>  #endif
>  
> -	fpu_clone(p, clone_flags, args->fn);
> +	/* Allocate a new shadow stack for pthread if needed */
> +	ret = shstk_alloc_thread_stack(p, clone_flags, args->stack_size,
> +				       &shstk_addr);

That function will return 0 even if shstk_addr hasn't been written in it
and you will continue merrily and call

	fpu_clone(..., shstk_addr=0);

why don't you return the shadow stack address or negative on error
instead of adding an I/O parameter which is pretty much always nasty to
deal with.



> +	if (ret)
> +		return ret;
> +
> +	fpu_clone(p, clone_flags, args->fn, shstk_addr);
>  
>  	/* Kernel thread ? */
>  	if (unlikely(p->flags & PF_KTHREAD)) {

...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette