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

[Sergey Matyukevich <geomatsi@gmail.com>]

On Wed, Oct 15, 2025 at 02:54:39PM -0500, Andy Chiu wrote:
> 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.

Ok, so we can just set 'ctx->vlenb = riscv_v_vsize / 32' in the renamed
riscv_v_thread_alloc function. But note, that w/o forced context save
we implicitly reset the vector configuration to 'all zeros', overwriting
the hardware defaults.

By the way, could you please elaborate a little bit more about your security
concerns with the TIF_RISCV_V_FORCE_SAVE approach ? The atomic and per-process
flag modification looks safe to me, so I'd like to understand what I am
missing.

Thanks,
Sergey