Re: [PATCH-for-5.0 v2 1/4] Revert "prevent crash when executing guest-file-read with large count"

[Daniel P . Berrangé]

On Tue, Mar 31, 2020 at 04:06:35PM +0200, Philippe Mathieu-Daudé wrote:
> By using g_try_malloc() instead of g_malloc() the qemu-guest-agent
> Denial-of-Service attack referred in commit 807e2b6fce is reduced,
> but still triggerable:

As explained previously, I believe there is *no* denial of service
attack here. The described scenario is just a user hurting themselves
by intentionally telling QEMU not to limit the amount of data returned.

> 
>   - bisect file size S until g_try_malloc(S) fails,
>   - use S - 1:
>     g_try_malloc(S - 1) succeeds, but g_new0() few lines later will
>     fail.
> 
>  346     buf = g_try_malloc0(count + 1);
>  347     if (!buf) {
>  348         error_setg(errp,
>  349                    "failed to allocate sufficient memory "
>  350                    "to complete the requested service");
>  351         return NULL;
>  352     }
>  353     is_ok = ReadFile(fh, buf, count, &read_count, NULL);
>  354     if (!is_ok) {
>  355         error_setg_win32(errp, GetLastError(), "failed to read file");
>  356         slog("guest-file-read failed, handle %" PRId64, handle);
>  357     } else {
>  358         buf[read_count] = 0;
>  359         read_data = g_new0(GuestFileRead, 1);
>                          ^^^^^^
> 
> Instead we are going to put a low hard limit on 'count' in the next
> commits.
> This reverts commit 807e2b6fce022707418bc8f61c069d91c613b3d2.
> 
> Suggested-by: Daniel P. Berrangé <address@hidden>
> Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> ---
>  qga/commands-win32.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/qga/commands-win32.c b/qga/commands-win32.c
> index b49920e201..46cea7d1d9 100644
> --- a/qga/commands-win32.c
> +++ b/qga/commands-win32.c
> @@ -343,13 +343,7 @@ GuestFileRead *qmp_guest_file_read(int64_t handle, bool 
> has_count,
>      }
>  
>      fh = gfh->fh;
> -    buf = g_try_malloc0(count + 1);
> -    if (!buf) {
> -        error_setg(errp,
> -                   "failed to allocate sufficient memory "
> -                   "to complete the requested service");
> -        return NULL;
> -    }
> +    buf = g_malloc0(count + 1);
>      is_ok = ReadFile(fh, buf, count, &read_count, NULL);
>      if (!is_ok) {
>          error_setg_win32(errp, GetLastError(), "failed to read file");
> -- 
> 2.21.1
> 

Regards,
Daniel
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