Re: [PATCH for-8.1 v10 04/14] linux-user: Use MAP_FIXED_NOREPLACE for initial image mmap

[Alex Bennée]

Richard Henderson <richard.henderson@linaro.org> writes:

> Use this as extra protection for the guest mapping over
> any qemu host mappings.
>
> Tested-by: Helge Deller <deller@gmx.de>
> Reviewed-by: Helge Deller <deller@gmx.de>
> Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  linux-user/elfload.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> index 36e4026f05..1b4bb2d5af 100644
> --- a/linux-user/elfload.c
> +++ b/linux-user/elfload.c
> @@ -3147,8 +3147,11 @@ static void load_elf_image(const char *image_name, int 
> image_fd,
>      /*
>       * Reserve address space for all of this.
>       *
> -     * In the case of ET_EXEC, we supply MAP_FIXED so that we get
> -     * exactly the address range that is required.
> +     * In the case of ET_EXEC, we supply MAP_FIXED_NOREPLACE so that we get
> +     * exactly the address range that is required.  Without reserved_va,
> +     * the guest address space is not isolated.  We have attempted to avoid
> +     * conflict with the host program itself via probe_guest_base, but using
> +     * MAP_FIXED_NOREPLACE instead of MAP_FIXED provides an extra check.
>       *
>       * Otherwise this is ET_DYN, and we are searching for a location
>       * that can hold the memory space required.  If the image is
> @@ -3160,7 +3163,7 @@ static void load_elf_image(const char *image_name, int 
> image_fd,
>       */
>      load_addr = target_mmap(loaddr, (size_t)hiaddr - loaddr + 1, PROT_NONE,
>                              MAP_PRIVATE | MAP_ANON | MAP_NORESERVE |
> -                            (ehdr->e_type == ET_EXEC ? MAP_FIXED : 0),
> +                            (ehdr->e_type == ET_EXEC ? MAP_FIXED_NOREPLACE : 
> 0),
>                              -1, 0);

We should probably also check the result == load_addr for the places
where MAP_FIXED_NOREPLACE isn't supported as we have this in osdep.h:

  #ifndef MAP_FIXED_NOREPLACE
  #define MAP_FIXED_NOREPLACE 0
  #endif

See 2667e069e7 (linux-user: don't use MAP_FIXED in pgd_find_hole_fallback)

>      if (load_addr == -1) {
>          goto exit_mmap;


-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro