Re: [PATCH v2 15/22] x86/fpu/xstate: Support ptracer-induced xstate area expansion

[Andy Lutomirski]

On Thu, Nov 19, 2020 at 3:37 PM Chang S. Bae <chang.seok.bae@xxxxxxxxx> wrote:
>
> ptrace() may request an update to task->fpu that has not yet been
> allocated. Detect this case and allocate task->fpu to support the request.
> Also, disable the (now unnecessary) associated first-use fault.
>
> No functional change until the kernel supports dynamic user states.
>
> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
> Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  arch/x86/kernel/fpu/regset.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
>
> diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
> index 8d863240b9c6..6b9d0c0a266d 100644
> --- a/arch/x86/kernel/fpu/regset.c
> +++ b/arch/x86/kernel/fpu/regset.c
> @@ -125,6 +125,35 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
>
>         xsave = __xsave(fpu);
>
> +       /*
> +        * When a ptracer attempts to write any state in task->fpu but not allocated,
> +        * it dynamically expands the xstate area of fpu->state_ptr.
> +        */
> +       if (count > get_xstate_size(fpu->state_mask)) {
> +               unsigned int offset, size;
> +               struct xstate_header hdr;
> +               u64 mask;
> +
> +               offset = offsetof(struct xregs_state, header);
> +               size = sizeof(hdr);
> +
> +               /* Retrieve XSTATE_BV */
> +               if (kbuf) {
> +                       memcpy(&hdr, kbuf + offset, size);
> +               } else {
> +                       ret = __copy_from_user(&hdr, ubuf + offset, size);
> +                       if (ret)
> +                               return ret;
> +               }
> +
> +               mask = hdr.xfeatures & xfeatures_mask_user_dynamic;
> +               if (!mask) {
> +                       ret = alloc_xstate_area(fpu, mask, NULL);
> +                       if (ret)
> +                               return ret;
> +               }
> +       }
> +

This whole function is garbage.  The count parameter is entirely
ignored except that the beginning of the function compares it to the
constant known size.  Now that it's dynamic, you need to actually
validate the count.  Right now, you will happily overrun the buffer if
the mask in the buffer isn't consistent with count.

Please add some real bounds checking here.