Re: [Qemu-devel] [PATCH] translate-all: Bugfix for user-mode self-modifying code in 2 page long TB

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 05/07/16 13:45, Stanislav Shmarov wrote:
> In user-mode emulation Translation Block can consist of 2 guest pages.
> In that case QEMU also mprotects 2 host pages that are dedicated for
> guest memory, containing instructions. QEMU detects self-modifying code
> with SEGFAULT signal processing.
>
> In case if instruction in 1st page is modifying memory of 2nd
> page (or vice versa) QEMU will mark 2nd page with PAGE_WRITE,
> invalidate TB, generate new TB contatining 1 guest instruction and
> exit to CPU loop. QEMU won't call mprotect, and new TB will cause
> same SEGFAULT. Page will have both PAGE_WRITE_ORG and PAGE_WRITE
> flags, so QEMU will handle the signal as guest binary problem,
> and exit with guest SEGFAULT.
>
> Solution is retranslate TB before marking pages as PAGE_WRITE,
> and remove protection with mprotect on second SEGFAULT.
>
> Signed-off-by: Stanislav Shmarov <snarpix@gmail.com>
> ---
>  translate-all.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/translate-all.c b/translate-all.c
> index eaa95e4..1e2ac84 100644
> --- a/translate-all.c
> +++ b/translate-all.c
> @@ -2022,11 +2022,7 @@ int page_unprotect(target_ulong address, uintptr_t pc)
>  
>          prot = 0;
>          for (addr = host_start ; addr < host_end ; addr += TARGET_PAGE_SIZE) {
> -            p = page_find(addr >> TARGET_PAGE_BITS);
> -            p->flags |= PAGE_WRITE;
> -            prot |= p->flags;
> -
> -            /* and since the content will be modified, we must invalidate
> +            /* Since the content will be modified, we must invalidate
>                 the corresponding translated code. */
>              if (tb_invalidate_phys_page(addr, pc)) {
>                  mmap_unlock();
> @@ -2035,6 +2031,10 @@ int page_unprotect(target_ulong address, uintptr_t pc)
>  #ifdef DEBUG_TB_CHECK
>              tb_invalidate_check(addr);
>  #endif
> +
> +            p = page_find(addr >> TARGET_PAGE_BITS);
> +            p->flags |= PAGE_WRITE;
> +            prot |= p->flags;

I'm afraid you need to do this even later, out of the loop for guest
pages, to ensure that mprotect() will be eventually called. That will
require a separate loop through all the guest pages which make up the
host page.

Kind regards,
Sergey

>          }
>          mprotect((void *)g2h(host_start), qemu_host_page_size,
>                   prot & PAGE_BITS);