[PATCH v2 6/6] riscv: vector: initialize vlenb on the first context switch

[Andy Chiu]

Hi Sergey,

On Tue, Oct 7, 2025 at 6:58 AM Sergey Matyukevich <geomatsi at gmail.com> wrote:
>
> The vstate in thread_struct is zeroed when the vector context is
> initialized. That includes read-only register vlenb, which holds
> the vector register length in bytes. This zeroed state persists
> until mstatus.VS becomes 'dirty' and a context switch saves the
> actual hardware values.
>
> This can expose the zero vlenb value to the user-space in early
> debug scenarios, e.g. when ptrace attaches to a traced process
> early, before any vector instruction except the first one was
> executed.
>
> Fix this by forcing the vector context save on the first context switch.
>
> Signed-off-by: Sergey Matyukevich <geomatsi at gmail.com>
> ---
>  arch/riscv/kernel/vector.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/arch/riscv/kernel/vector.c b/arch/riscv/kernel/vector.c
> index 901e67adf576..3dd22a71aa18 100644
> --- a/arch/riscv/kernel/vector.c
> +++ b/arch/riscv/kernel/vector.c
> @@ -120,6 +120,7 @@ static int riscv_v_thread_zalloc(struct kmem_cache *cache,
>
>         ctx->datap = datap;
>         memset(ctx, 0, offsetof(struct __riscv_v_ext_state, datap));
> +
>         return 0;
>  }
>
> @@ -216,8 +217,11 @@ bool riscv_v_first_use_handler(struct pt_regs *regs)
>                 force_sig(SIGBUS);
>                 return true;
>         }
> +
>         riscv_v_vstate_on(regs);
>         riscv_v_vstate_set_restore(current, regs);
> +       set_tsk_thread_flag(current, TIF_RISCV_V_FORCE_SAVE);
> +

I am afraid that this approach can result in a security issue where a
context switch happens before the v-restore part of the current
process, cheating the kernel to store stale v-regs onto the current
context memory. Please note that this handler is run with irq enabled
so preemption is allowed.

I would expect simply initializing the vleb in riscv_v_thread_zalloc,
perhaps dropping the "z" in the name to prevent confusion.

>         return true;
>  }
>
> --
> 2.51.0
>

Thanks,
Andy