Re: [Qemu-devel] [PATCH v4 5/5] migration: Use strnlen() for fixed-size string

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Philippe Mathieu-Daudé (philmd@redhat.com) wrote:
> GCC 8 introduced the -Wstringop-overflow, which detect buffer overflow
> by string-modifying functions declared in <string.h>, such strncpy(),
> used in global_state_store_running().
> 
> GCC indeed found an incorrect use of strlen(), because this array
> is loaded by VMSTATE_BUFFER(runstate, GlobalState) then parsed
> using qapi_enum_parse which does not get the buffer length.
> 
> Use strnlen() which returns sizeof(s->runstate) if the array is not
> NUL-terminated, assert the size is within range, and enforce the array
> to be NUL-terminated to avoid an overflow in qapi_enum_parse().
> 
> This fixes:
> 
>     CC      migration/global_state.o
>   qemu/migration/global_state.c: In function 'global_state_pre_save':
>   qemu/migration/global_state.c:109:15: error: 'strlen' argument 1 declared attribute 'nonstring' [-Werror=stringop-overflow=]
>        s->size = strlen((char *)s->runstate) + 1;
>                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~
>   qemu/migration/global_state.c:24:13: note: argument 'runstate' declared here
>        uint8_t runstate[100] QEMU_NONSTRING;
>                ^~~~~~~~
>   cc1: all warnings being treated as errors
>   make: *** [qemu/rules.mak:69: migration/global_state.o] Error 1
> 
> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  migration/global_state.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/global_state.c b/migration/global_state.c
> index 01805c567a..4f060a6dbd 100644
> --- a/migration/global_state.c
> +++ b/migration/global_state.c
> @@ -89,6 +89,16 @@ static int global_state_post_load(void *opaque, int version_id)
>      s->received = true;
>      trace_migrate_global_state_post_load(runstate);
>  
> +    if (strnlen((char *)s->runstate,
> +                sizeof(s->runstate)) == sizeof(s->runstate)) {
> +        /* This condition should never happen during migration, because
> +         * all runstate names are shorter than 100 bytes (the size of
> +         * s->runstate). However, a malicious stream could overflow
> +         * the qapi_enum_parse() call, so we force the last character
> +         * to a NUL byte.
> +         */
> +        s->runstate[sizeof(s->runstate) - 1] = '\0';
> +    }
>      r = qapi_enum_parse(&RunState_lookup, runstate, -1, &local_err);
>  
>      if (r == -1) {
> @@ -107,7 +117,8 @@ static int global_state_pre_save(void *opaque)
>      GlobalState *s = opaque;
>  
>      trace_migrate_global_state_pre_save((char *)s->runstate);
> -    s->size = strlen((char *)s->runstate) + 1;
> +    s->size = strnlen((char *)s->runstate, sizeof(s->runstate)) + 1;
> +    assert(s->size <= sizeof(s->runstate));
>  
>      return 0;
>  }
> -- 
> 2.17.2
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK