Re: [PATCH] mini-os: Avoid segfaults in tc{g,s}etattr

[Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>]

Jason Andryuk, le dim. 26 avril 2020 23:40:19 -0400, a ecrit:
> Commit c96c22f1d94 "mini-os: minimal implementations of some termios
> functions" introduced implementations of tcgetattr and tcsetattr.
> However, they do not check if files[fildes].cons.dev is non-NULL before
> dereferencing.  This is not a problem for FDs allocated through
> alloc_fd, but the files array pre-allocates FDs 0-2 for stdio.  Those
> entries have a NULL .dev, so tc{g,s}etattr on them segfault.
> 
> ioemu-stubdom segfaults when term_init() calls tcgetattr on FD 0.
> 
> Restore tcgetattr and tcsetattr behavior when .dev is NULL equivalent to
> unsupported_function as it was before c96c22f1d94.
> 
> Signed-off-by: Jason Andryuk <jandryuk@xxxxxxxxx>

Reviewed-by: Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>

Thanks!

> ---
> I can't get ioemu-stubdom to start without this.  With this, the guest
> just reboots immediately, but it does that with a non-stubdom
> device_model_version="qemu-xen-traditional" .  The same guest disk image
> (cirros 0.5.1) boots with a linux stubdom or non-stubdom Ubuntu
> qemu-system-x86_64.
> 
>  lib/sys.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/lib/sys.c b/lib/sys.c
> index da434fc..c6a7b9f 100644
> --- a/lib/sys.c
> +++ b/lib/sys.c
> @@ -1472,6 +1472,11 @@ int tcsetattr(int fildes, int action, const struct 
> termios *tios)
>              return -1;
>      }
>  
> +    if (files[fildes].cons.dev == NULL) {
> +        errno = ENOSYS;
> +        return -1;
> +    }
> +
>      if (tios->c_oflag & OPOST)
>          files[fildes].cons.dev->is_raw = false;
>      else
> @@ -1492,6 +1497,11 @@ int tcgetattr(int fildes, struct termios *tios)
>          return -1;
>      }
>  
> +    if (files[fildes].cons.dev == NULL) {
> +        errno = ENOSYS;
> +        return 0;
> +    }
> +
>      if (tios == NULL) {
>          errno = EINVAL;
>          return -1;
> -- 
> 2.20.1
>