[PATCH 042/104] virtiofsd: validate input buffer sizes in do_write_buf()

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: Stefan Hajnoczi <stefanha@redhat.com>

There is a small change in behavior: if fuse_write_in->size doesn't
match the input buffer size then the request is failed.  Previously
write requests with 1 fuse_buf element would truncate to
fuse_write_in->size.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 tools/virtiofsd/fuse_lowlevel.c | 49 ++++++++++++++++++++-------------
 1 file changed, 30 insertions(+), 19 deletions(-)

diff --git a/tools/virtiofsd/fuse_lowlevel.c b/tools/virtiofsd/fuse_lowlevel.c
index 7d33bbf539..4b1777b991 100644
--- a/tools/virtiofsd/fuse_lowlevel.c
+++ b/tools/virtiofsd/fuse_lowlevel.c
@@ -1005,8 +1005,8 @@ static void do_write(fuse_req_t req, fuse_ino_t nodeid, const void *inarg)
     }
 }
 
-static void do_write_buf(fuse_req_t req, fuse_ino_t nodeid, const void *inarg,
-                         struct fuse_bufvec *ibufv)
+static void do_write_buf(fuse_req_t req, fuse_ino_t nodeid,
+                         struct fuse_mbuf_iter *iter, struct fuse_bufvec *ibufv)
 {
     struct fuse_session *se = req->se;
     struct fuse_bufvec *pbufv = ibufv;
@@ -1014,28 +1014,27 @@ static void do_write_buf(fuse_req_t req, fuse_ino_t nodeid, const void *inarg,
         .buf[0] = ibufv->buf[0],
         .count = 1,
     };
-    struct fuse_write_in *arg = (struct fuse_write_in *)inarg;
+    struct fuse_write_in *arg;
+    size_t arg_size = sizeof(*arg);
     struct fuse_file_info fi;
 
     memset(&fi, 0, sizeof(fi));
+
+    arg = fuse_mbuf_iter_advance(iter, arg_size);
+    if (!arg) {
+        fuse_reply_err(req, EINVAL);
+        return;
+    }
+
+    fi.lock_owner = arg->lock_owner;
+    fi.flags = arg->flags;
     fi.fh = arg->fh;
     fi.writepage = arg->write_flags & FUSE_WRITE_CACHE;
 
     if (ibufv->count == 1) {
-        fi.lock_owner = arg->lock_owner;
-        fi.flags = arg->flags;
-        if (!(tmpbufv.buf[0].flags & FUSE_BUF_IS_FD)) {
-            tmpbufv.buf[0].mem = PARAM(arg);
-        }
-        tmpbufv.buf[0].size -=
-            sizeof(struct fuse_in_header) + sizeof(struct fuse_write_in);
-        if (tmpbufv.buf[0].size < arg->size) {
-            fuse_log(FUSE_LOG_ERR,
-                     "fuse: do_write_buf: buffer size too small\n");
-            fuse_reply_err(req, EIO);
-            return;
-        }
-        tmpbufv.buf[0].size = arg->size;
+        assert(!(tmpbufv.buf[0].flags & FUSE_BUF_IS_FD));
+        tmpbufv.buf[0].mem = ((char *)arg) + arg_size;
+        tmpbufv.buf[0].size -= sizeof(struct fuse_in_header) + arg_size;
         pbufv = &tmpbufv;
     } else {
         /*
@@ -1045,6 +1044,13 @@ static void do_write_buf(fuse_req_t req, fuse_ino_t nodeid, const void *inarg,
         ibufv->buf[0].size = 0;
     }
 
+    if (fuse_buf_size(pbufv) != arg->size) {
+        fuse_log(FUSE_LOG_ERR,
+                 "fuse: do_write_buf: buffer size doesn't match arg->size\n");
+        fuse_reply_err(req, EIO);
+        return;
+    }
+
     se->op.write_buf(req, nodeid, pbufv, arg->offset, &fi);
 }
 
@@ -2055,12 +2061,17 @@ void fuse_session_process_buf_int(struct fuse_session *se,
                                   struct fuse_chan *ch)
 {
     const struct fuse_buf *buf = bufv->buf;
+    struct fuse_mbuf_iter iter = FUSE_MBUF_ITER_INIT(buf);
     struct fuse_in_header *in;
     const void *inarg;
     struct fuse_req *req;
     int err;
 
-    in = buf->mem;
+    /* The first buffer must be a memory buffer */
+    assert(!(buf->flags & FUSE_BUF_IS_FD));
+
+    in = fuse_mbuf_iter_advance(&iter, sizeof(*in));
+    assert(in); /* caller guarantees the input buffer is large enough */
 
     if (se->debug) {
         fuse_log(FUSE_LOG_DEBUG,
@@ -2132,7 +2143,7 @@ void fuse_session_process_buf_int(struct fuse_session *se,
 
     inarg = (void *)&in[1];
     if (in->opcode == FUSE_WRITE && se->op.write_buf) {
-        do_write_buf(req, in->nodeid, inarg, bufv);
+        do_write_buf(req, in->nodeid, &iter, bufv);
     } else {
         fuse_ll_ops[in->opcode].func(req, in->nodeid, inarg);
     }
-- 
2.23.0