Hi Sergey, On Tue, Oct 7, 2025 at 6:58 AM Sergey Matyukevich <[email protected]> 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 <[email protected]> > --- > 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
