Re: [Qemu-devel] [PATCH] 9pfs: check the size of transport buffer before marshaling

[Greg Kurz <groug@kaod.org>]

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On Thu, 14 Sep 2017 19:31:36 +0300
Jan Dakinevich <jan.dakinevich@gmail.com> wrote:

> v9fs_do_readdir_with_stat() and v9fs_do_readdir() stores as much data in
> the buffer as can fit unless marshaling erorr occurs. However, after
> commit 23a006d the behavior pdu_marshal was changed, and on error the
> routine assumes that buffers are misconfigured and breaks communication.
> 

I agree and I could easily reproduce with a linux guest using 9p2000.u, ie, the
v9fs_do_readdir_with_stat() case, but I couldn't find a way to break transport
with 9p2000.L. Do you have a reproducer for the latter ?

I ask because I would appreciate to see some more details in the changelog,
for the records.

> Signed-off-by: Jan Dakinevich <jan.dakinevich@gmail.com>
> ---

Anyway the patch looks fine, but I'll wait for your answer before pushing
to 9p-next.

Cheers,

--
Greg

>  hw/9pfs/9p.c | 44 ++++++++++++++++++++++++++++++++------------
>  1 file changed, 32 insertions(+), 12 deletions(-)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index 1e38109..8e0b87e 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -1679,6 +1679,16 @@ static void v9fs_init_qiov_from_pdu(QEMUIOVector *qiov, V9fsPDU *pdu,
>      qemu_iovec_concat(qiov, &elem, skip, size);
>  }
>  
> +static size_t v9fs_marshal_size(V9fsPDU *pdu)
> +{
> +    struct iovec *iov;
> +    unsigned int niov;
> +
> +    pdu->s->transport->init_in_iov_from_pdu(pdu, &iov, &niov, 0);
> +
> +    return iov_size(iov, niov);
> +}
> +
>  static int v9fs_xattr_read(V9fsState *s, V9fsPDU *pdu, V9fsFidState *fidp,
>                             uint64_t off, uint32_t max_count)
>  {
> @@ -1725,6 +1735,10 @@ static int coroutine_fn v9fs_do_readdir_with_stat(V9fsPDU *pdu,
>      off_t saved_dir_pos;
>      struct dirent *dent;
>  
> +    /* 11 = 7 + 4 (7 = start offset, 4 = space for storing count) */
> +    size_t offset = 11;
> +    size_t marshal_size = v9fs_marshal_size(pdu);
> +
>      /* save the directory position */
>      saved_dir_pos = v9fs_co_telldir(pdu, fidp);
>      if (saved_dir_pos < 0) {
> @@ -1752,18 +1766,23 @@ static int coroutine_fn v9fs_do_readdir_with_stat(V9fsPDU *pdu,
>          if (err < 0) {
>              break;
>          }
> -        /* 11 = 7 + 4 (7 = start offset, 4 = space for storing count) */
> -        len = pdu_marshal(pdu, 11 + count, "S", &v9stat);
>  
> -        v9fs_readdir_unlock(&fidp->fs.dir);
> +        if (v9stat.size + 2 > MIN(marshal_size - offset, max_count - count)) {
> +            v9fs_readdir_unlock(&fidp->fs.dir);
>  
> -        if ((len != (v9stat.size + 2)) || ((count + len) > max_count)) {
>              /* Ran out of buffer. Set dir back to old position and return */
>              v9fs_co_seekdir(pdu, fidp, saved_dir_pos);
>              v9fs_stat_free(&v9stat);
>              v9fs_path_free(&path);
>              return count;
>          }
> +
> +        len = pdu_marshal(pdu, offset, "S", &v9stat);
> +        BUG_ON(len != v9stat.size + 2);
> +
> +        v9fs_readdir_unlock(&fidp->fs.dir);
> +
> +        offset += len;
>          count += len;
>          v9fs_stat_free(&v9stat);
>          v9fs_path_free(&path);
> @@ -1884,6 +1903,10 @@ static int coroutine_fn v9fs_do_readdir(V9fsPDU *pdu, V9fsFidState *fidp,
>      off_t saved_dir_pos;
>      struct dirent *dent;
>  
> +    /* 11 = 7 + 4 (7 = start offset, 4 = space for storing count) */
> +    size_t offset = 11;
> +    size_t marshal_size = v9fs_marshal_size(pdu);
> +
>      /* save the directory position */
>      saved_dir_pos = v9fs_co_telldir(pdu, fidp);
>      if (saved_dir_pos < 0) {
> @@ -1899,7 +1922,8 @@ static int coroutine_fn v9fs_do_readdir(V9fsPDU *pdu, V9fsFidState *fidp,
>          }
>          v9fs_string_init(&name);
>          v9fs_string_sprintf(&name, "%s", dent->d_name);
> -        if ((count + v9fs_readdir_data_size(&name)) > max_count) {
> +        if (v9fs_readdir_data_size(&name) > MIN(marshal_size - offset,
> +                                                max_count - count)) {
>              v9fs_readdir_unlock(&fidp->fs.dir);
>  
>              /* Ran out of buffer. Set dir back to old position and return */
> @@ -1918,18 +1942,14 @@ static int coroutine_fn v9fs_do_readdir(V9fsPDU *pdu, V9fsFidState *fidp,
>          qid.type = 0;
>          qid.version = 0;
>  
> -        /* 11 = 7 + 4 (7 = start offset, 4 = space for storing count) */
> -        len = pdu_marshal(pdu, 11 + count, "Qqbs",
> +        len = pdu_marshal(pdu, offset, "Qqbs",
>                            &qid, dent->d_off,
>                            dent->d_type, &name);
> +        BUG_ON(len != v9fs_readdir_data_size(&name));
>  
>          v9fs_readdir_unlock(&fidp->fs.dir);
>  
> -        if (len < 0) {
> -            v9fs_co_seekdir(pdu, fidp, saved_dir_pos);
> -            v9fs_string_free(&name);
> -            return len;
> -        }
> +        offset += len;
>          count += len;
>          v9fs_string_free(&name);
>          saved_dir_pos = dent->d_off;


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