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

Hi Sergey,

On Tue, Oct 7, 2025 at 6:58 AM Sergey Matyukevich geomatsi@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@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