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

Borislav Petkov
2023-03-08T15:26:13Z

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

"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.