Re: [Qemu-devel] [PATCH] ivshmem: fix potential OOB r/w access

[Stefan Hajnoczi]

On Wed, Apr 16, 2014 at 09:33:02AM +0200, Sebastian Krahmer wrote:
> 
> Fix OOB access via malformed incoming_posn parameters
> and check that requested memory is actually alloced.
> 
> Signed-off-by: Sebastian Krahmer <address@hidden>
> 
> ---

Please use scripts/checkpatch.pl to check coding style.  QEMU always
uses {} even for a one-line if body.

> 
> diff --git a/hw/misc/ivshmem.c b/hw/misc/ivshmem.c
> index 8d144ba..f58356e 100644
> --- a/hw/misc/ivshmem.c
> +++ b/hw/misc/ivshmem.c
> @@ -28,6 +28,7 @@
>  
>  #include <sys/mman.h>
>  #include <sys/types.h>
> +#include <limits.h>
>  
>  #define PCI_VENDOR_ID_IVSHMEM   PCI_VENDOR_ID_REDHAT_QUMRANET
>  #define PCI_DEVICE_ID_IVSHMEM   0x1110
> @@ -401,23 +402,34 @@ static void close_guest_eventfds(IVShmemState *s, int 
> posn)
>  
>  /* this function increase the dynamic storage need to store data about other
>   * guests */
> -static void increase_dynamic_storage(IVShmemState *s, int new_min_size) {
> +static int increase_dynamic_storage(IVShmemState *s, int new_min_size) {
>  
>      int j, old_nb_alloc;
>  
> +    /* check for integer overflow */
> +    if (new_min_size >= INT_MAX/sizeof(Peer) - 1 || new_min_size <= 0)
> +        return -1;
> +
>      old_nb_alloc = s->nb_peers;
>  
> -    while (new_min_size >= s->nb_peers)
> -        s->nb_peers = s->nb_peers * 2;
> +    /* heap allocators already have good alloc strategies, no need
> +     * to re-implement here. +1 because #new_min_size is used as last array 
> index */
> +    if (new_min_size >= s->nb_peers)
> +        s->nb_peers = new_min_size + 1;
> +    else
> +        return 0;
>  
>      IVSHMEM_DPRINTF("bumping storage to %d guests\n", s->nb_peers);
>      s->peers = g_realloc(s->peers, s->nb_peers * sizeof(Peer));
> +    if (!s->peers)
> +        return -1;

This is not needed because g_realloc() does not return NULL on
out-of-memory:
https://developer.gnome.org/glib/unstable/glib-Memory-Allocation.html#g-realloc

>  
>      /* zero out new pointers */
>      for (j = old_nb_alloc; j < s->nb_peers; j++) {
>          s->peers[j].eventfds = NULL;
>          s->peers[j].nb_eventfds = 0;
>      }
> +    return 0;
>  }
>  
>  static void ivshmem_read(void *opaque, const uint8_t * buf, int flags)
> @@ -428,13 +440,19 @@ static void ivshmem_read(void *opaque, const uint8_t * 
> buf, int flags)
>      long incoming_posn;
>  
>      memcpy(&incoming_posn, buf, sizeof(long));
> +    if (incoming_posn < -1) {
> +        fprintf(stderr, "invalid posn index\n");
> +        return;
> +    }
>      /* pick off s->server_chr->msgfd and store it, posn should accompany msg 
> */
>      tmp_fd = qemu_chr_fe_get_msgfd(s->server_chr);
>      IVSHMEM_DPRINTF("posn is %ld, fd is %d\n", incoming_posn, tmp_fd);
> -
>      /* make sure we have enough space for this guest */
>      if (incoming_posn >= s->nb_peers) {
> -        increase_dynamic_storage(s, incoming_posn);
> +        if (increase_dynamic_storage(s, incoming_posn) < 0) {
> +            fprintf(stderr, "could not allocate memory\n");

This error message is wrong because the failure could also indicate
out-of-range incoming_posn.

> +            return;

If tmp_fd != -1 this return leaks it.