Re: [PATCH] afs: Fix accidental truncation when storing data

[Marc Dionne]

On Wed, Jun 28, 2023 at 10:47 AM David Howells <dhowells@xxxxxxxxxx> wrote:
>
>
> When an AFS FS.StoreData RPC call is made, amongst other things it is given
> the resultant file size to be.  On the server, this is processed by
> truncating the file to new size and then writing the data.
>
> Now, kafs has a lock (vnode->io_lock) that serves to serialise operations
> against a specific vnode (ie. inode), but the parameters for the op are set
> before the lock is taken.  This allows two writebacks (say sync and kswapd)
> to race - and if writes are ongoing the writeback for a later write could
> occur before the writeback for an earlier one if the latter gets
> interrupted.
>
> Note that afs_writepages() cannot take i_mutex and only takes a shared lock
> on vnode->validate_lock.
>
> Also note that the server does the truncation and the write inside a lock,
> so there's no problem at that end.
>
> Fix this by moving the calculation for the proposed new i_size inside the
> vnode->io_lock.  Also reset the iterator (which we might have read from)
> and update the mtime setting there.
>
> Fixes: bd80d8a80e12 ("afs: Use ITER_XARRAY for writing")
> Reported-by: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> cc: linux-afs@xxxxxxxxxxxxxxxxxxx
> cc: linux-fsdevel@xxxxxxxxxxxxxxx
> ---
>  fs/afs/write.c |    8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/fs/afs/write.c b/fs/afs/write.c
> index 8750b99c3f56..c1f4391ccd7c 100644
> --- a/fs/afs/write.c
> +++ b/fs/afs/write.c
> @@ -413,17 +413,19 @@ static int afs_store_data(struct afs_vnode *vnode, struct iov_iter *iter, loff_t
>         afs_op_set_vnode(op, 0, vnode);
>         op->file[0].dv_delta = 1;
>         op->file[0].modification = true;
> -       op->store.write_iter = iter;
>         op->store.pos = pos;
>         op->store.size = size;
> -       op->store.i_size = max(pos + size, vnode->netfs.remote_i_size);
>         op->store.laundering = laundering;
> -       op->mtime = vnode->netfs.inode.i_mtime;
>         op->flags |= AFS_OPERATION_UNINTR;
>         op->ops = &afs_store_data_operation;
>
>  try_next_key:
>         afs_begin_vnode_operation(op);
> +
> +       op->store.write_iter = iter;
> +       op->store.i_size = max(pos + size, vnode->netfs.remote_i_size);
> +       op->mtime = vnode->netfs.inode.i_mtime;
> +
>         afs_wait_for_operation(op);
>
>         switch (op->error) {

Looks good to me; the traces where I got a failure indicate that an
extending store occurred in a different thread while waiting for the
io lock, so this looks like the right fix.

Reviewed-by: Marc Dionne <marc.dionne@xxxxxxxxxxxx>

Marc