Re: [PATCH] linux-user: Ensure mmap_min_addr is non-zero

[John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>]

Hi!

On 7/24/20 11:23 PM, Richard Henderson wrote:
> When the chroot does not have /proc mounted, we can read neither
> /proc/sys/vm/mmap_min_addr nor /proc/sys/maps.
> 
> The enforcement of mmap_min_addr in the host kernel is done by
> the security module, and so does not apply to processes owned
> by root.  Which leads pgd_find_hole_fallback to succeed in probing
> a reservation at address 0.  Which confuses pgb_reserved_va to
> believe that guest_base has not actually been initialized.
> 
> We don't actually want NULL addresses to become accessible, so
> make sure that mmap_min_addr is initialized with a non-zero value.
> 
> Buglink: https://bugs.launchpad.net/qemu/+bug/1888728
> Reported-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  linux-user/main.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/linux-user/main.c b/linux-user/main.c
> index 3597e99bb1..75c9785157 100644
> --- a/linux-user/main.c
> +++ b/linux-user/main.c
> @@ -758,14 +758,26 @@ int main(int argc, char **argv, char **envp)
>  
>          if ((fp = fopen("/proc/sys/vm/mmap_min_addr", "r")) != NULL) {
>              unsigned long tmp;
> -            if (fscanf(fp, "%lu", &tmp) == 1) {
> +            if (fscanf(fp, "%lu", &tmp) == 1 && tmp != 0) {
>                  mmap_min_addr = tmp;
> -                qemu_log_mask(CPU_LOG_PAGE, "host mmap_min_addr=0x%lx\n", mmap_min_addr);
> +                qemu_log_mask(CPU_LOG_PAGE, "host mmap_min_addr=0x%lx\n",
> +                              mmap_min_addr);
>              }
>              fclose(fp);
>          }
>      }
>  
> +    /*
> +     * We prefer to not make NULL pointers accessible to QEMU.
> +     * If we're in a chroot with no /proc, fall back to 1 page.
> +     */
> +    if (mmap_min_addr == 0) {
> +        mmap_min_addr = qemu_host_page_size;
> +        qemu_log_mask(CPU_LOG_PAGE,
> +                      "host mmap_min_addr=0x%lx (fallback)\n",
> +                      mmap_min_addr);
> +    }
> +
>      /*
>       * Prepare copy of argv vector for target.
>       */
> 

This fixes the problem for me, therefore:

Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>

Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer - glaubitz@debian.org
`. `'   Freie Universitaet Berlin - glaubitz@physik.fu-berlin.de
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