Re: [Qemu-devel] [PATCH v2 03/20] 9p: xattr: Fix crash due to free of uninitialized value

[Greg Kurz <groug@kaod.org>]

On Thu, 31 May 2018 21:25:58 -0400
Keno Fischer <keno@juliacomputing.com> wrote:

> If the size returned from llistxattr is 0, we skipped the malloc
> call, leaving xattr.value uninitialized. However, this value is
> later passed to `g_free` without any further checks, causing an

Ouch, good catch.

> error. Fix that by always calling g_malloc unconditionally. If
> `size` is 0, it will return a pointer that is safe to pass to g_free,
> likely NULL.
> 

"Allocates n_bytes bytes of memory, initialized to 0's. If n_bytes is 0 it
 returns NULL."

https://developer.gnome.org/glib/unstable/glib-Memory-Allocation.html#g-malloc

The fix is good, but it seems the same can also happen if v9fs_co_lgetxattr()
returns 0 a few lines below. Can you check this out and fix it if needed ?

> Signed-off-by: Keno Fischer <keno@juliacomputing.com>
> ---
> 
> Changes since v1: New patch
> 
>  hw/9pfs/9p.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index d74302d..b80db65 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -3256,8 +3256,8 @@ static void coroutine_fn v9fs_xattrwalk(void *opaque)
>          xattr_fidp->fs.xattr.len = size;
>          xattr_fidp->fid_type = P9_FID_XATTR;
>          xattr_fidp->fs.xattr.xattrwalk_fid = true;
> +        xattr_fidp->fs.xattr.value = g_malloc0(size);
>          if (size) {
> -            xattr_fidp->fs.xattr.value = g_malloc0(size);
>              err = v9fs_co_llistxattr(pdu, &xattr_fidp->path,
>                                       xattr_fidp->fs.xattr.value,
>                                       xattr_fidp->fs.xattr.len);