Re: [Qemu-devel] [PATCH] 9pfs: disallow / in path components

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 24, 2016 at 04:29:07PM +0200, Greg Kurz wrote:
> At various places in 9pfs, full paths are created by concatenating a guest
> originated string to the export path. A malicious guest could forge a
> relative path and access files outside the export path.
> 
> A tentative fix was sent recently by Prasad J Pandit, but it was only
> focused on the local backend and did not get a positive review. This patch
> tries to address the issue more globally, based on the official 9P spec.
> 
> The walk request described in the 9P spec [1] clearly shows that the client
> is supposed to send individual path components: the official linux client
> never sends portions of path containing the / character for example.
> 
> Moreover, the 9P spec [2] also states that a system can decide to restrict
> the set of supported characters used in path components, with an explicit
> mention "to remove slashes from name components".
> 
> This patch introduces a new name_has_illegal_characters() helper that looks
> for such unwanted characters in strings sent by the client. Since 9pfs is
> only supported on linux hosts, only the / character is checked at the
> moment. When support for other hosts (AKA. win32) is added, other chars
> may need to be blacklisted as well.
> 
> If a client sends a path component with an illegal character, the request
> will fail and EINVAL is returned to the client.
> 
> For the sake of simplicity and consistency, the check is done at top-level
> for all affected 9P requests:
> 
> - xattrwalk
> - xattrcreate
> - mknod
> - rename
> - renameat
> - unlinkat
> - mkdir
> - walk
> - link
> - symlink
> - create
> - lcreate
> 
> [1] http://man.cat-v.org/plan_9/5/walk
> [2] http://man.cat-v.org/plan_9/5/intro
> 
> Reported-by: Felix Wilhelm <fwilhelm@ernw.de>
> Suggested-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Greg Kurz <groug@kaod.org>


Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> 
> Since the linux client does not send / in path components and I don't
> have enough time to write an appropriate qtest, I choosed to do manual
> testing of the mkdir request with GDB:
> 
> [greg@vm66 host]$ mkdir ...foo
> (then turning ...foo into ../foo in QEMU with GDB)
> mkdir: cannot create directory ‘...foo’: Invalid argument
> 
> I also could run the POSIX file system test suite from TUXERA:
> 
> http://www.tuxera.com/community/open-source-posix/
> 
> and did not observe any regression with this patch.
> 
>  hw/9pfs/9p.c |   67 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 67 insertions(+)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index b6b02b46a9da..1c008814509c 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -1256,6 +1256,11 @@ static int v9fs_walk_marshal(V9fsPDU *pdu, uint16_t nwnames, V9fsQID *qids)
>      return offset;
>  }
>  
> +static bool name_has_illegal_characters(const char *name)
> +{
> +    return strchr(name, '/') != NULL;
> +}
> +
>  static void v9fs_walk(void *opaque)
>  {
>      int name_idx;
> @@ -1289,6 +1294,10 @@ static void v9fs_walk(void *opaque)
>              if (err < 0) {
>                  goto out_nofid;
>              }
> +            if (name_has_illegal_characters(wnames[i].data)) {
> +                err = -EINVAL;
> +                goto out_nofid;
> +            }
>              offset += err;
>          }
>      } else if (nwnames > P9_MAXWELEM) {
> @@ -1483,6 +1492,11 @@ static void v9fs_lcreate(void *opaque)
>      }
>      trace_v9fs_lcreate(pdu->tag, pdu->id, dfid, flags, mode, gid);
>  
> +    if (name_has_illegal_characters(name.data)) {
> +        err = -EINVAL;
> +        goto out_nofid;
> +    }
> +
>      fidp = get_fid(pdu, dfid);
>      if (fidp == NULL) {
>          err = -ENOENT;
> @@ -2077,6 +2091,11 @@ static void v9fs_create(void *opaque)
>      }
>      trace_v9fs_create(pdu->tag, pdu->id, fid, name.data, perm, mode);
>  
> +    if (name_has_illegal_characters(name.data)) {
> +        err = -EINVAL;
> +        goto out_nofid;
> +    }
> +
>      fidp = get_fid(pdu, fid);
>      if (fidp == NULL) {
>          err = -EINVAL;
> @@ -2242,6 +2261,11 @@ static void v9fs_symlink(void *opaque)
>      }
>      trace_v9fs_symlink(pdu->tag, pdu->id, dfid, name.data, symname.data, gid);
>  
> +    if (name_has_illegal_characters(symname.data)) {
> +        err = -EINVAL;
> +        goto out_nofid;
> +    }
> +
>      dfidp = get_fid(pdu, dfid);
>      if (dfidp == NULL) {
>          err = -EINVAL;
> @@ -2316,6 +2340,11 @@ static void v9fs_link(void *opaque)
>      }
>      trace_v9fs_link(pdu->tag, pdu->id, dfid, oldfid, name.data);
>  
> +    if (name_has_illegal_characters(name.data)) {
> +        err = -EINVAL;
> +        goto out_nofid;
> +    }
> +
>      dfidp = get_fid(pdu, dfid);
>      if (dfidp == NULL) {
>          err = -ENOENT;
> @@ -2398,6 +2427,12 @@ static void v9fs_unlinkat(void *opaque)
>      if (err < 0) {
>          goto out_nofid;
>      }
> +
> +    if (name_has_illegal_characters(name.data)) {
> +        err = -EINVAL;
> +        goto out_nofid;
> +    }
> +
>      dfidp = get_fid(pdu, dfid);
>      if (dfidp == NULL) {
>          err = -EINVAL;
> @@ -2504,6 +2539,12 @@ static void v9fs_rename(void *opaque)
>      if (err < 0) {
>          goto out_nofid;
>      }
> +
> +    if (name_has_illegal_characters(name.data)) {
> +        err = -EINVAL;
> +        goto out_nofid;
> +    }
> +
>      fidp = get_fid(pdu, fid);
>      if (fidp == NULL) {
>          err = -ENOENT;
> @@ -2616,6 +2657,12 @@ static void v9fs_renameat(void *opaque)
>          goto out_err;
>      }
>  
> +    if (name_has_illegal_characters(old_name.data) ||
> +        name_has_illegal_characters(new_name.data)) {
> +        err = -EINVAL;
> +        goto out_err;
> +    }
> +
>      v9fs_path_write_lock(s);
>      err = v9fs_complete_renameat(pdu, olddirfid,
>                                   &old_name, newdirfid, &new_name);
> @@ -2826,6 +2873,11 @@ static void v9fs_mknod(void *opaque)
>      }
>      trace_v9fs_mknod(pdu->tag, pdu->id, fid, mode, major, minor);
>  
> +    if (name_has_illegal_characters(name.data)) {
> +        err = -EINVAL;
> +        goto out_nofid;
> +    }
> +
>      fidp = get_fid(pdu, fid);
>      if (fidp == NULL) {
>          err = -ENOENT;
> @@ -2977,6 +3029,11 @@ static void v9fs_mkdir(void *opaque)
>      }
>      trace_v9fs_mkdir(pdu->tag, pdu->id, fid, name.data, mode, gid);
>  
> +    if (name_has_illegal_characters(name.data)) {
> +        err = -EINVAL;
> +        goto out_nofid;
> +    }
> +
>      fidp = get_fid(pdu, fid);
>      if (fidp == NULL) {
>          err = -ENOENT;
> @@ -3020,6 +3077,11 @@ static void v9fs_xattrwalk(void *opaque)
>      }
>      trace_v9fs_xattrwalk(pdu->tag, pdu->id, fid, newfid, name.data);
>  
> +    if (name_has_illegal_characters(name.data)) {
> +        err = -EINVAL;
> +        goto out_nofid;
> +    }
> +
>      file_fidp = get_fid(pdu, fid);
>      if (file_fidp == NULL) {
>          err = -ENOENT;
> @@ -3126,6 +3188,11 @@ static void v9fs_xattrcreate(void *opaque)
>      }
>      trace_v9fs_xattrcreate(pdu->tag, pdu->id, fid, name.data, size, flags);
>  
> +    if (name_has_illegal_characters(name.data)) {
> +        err = -EINVAL;
> +        goto out_nofid;
> +    }
> +
>      file_fidp = get_fid(pdu, fid);
>      if (file_fidp == NULL) {
>          err = -EINVAL;