[v3] migration: fix the memory overwriting risk in add_to_iovec

[Lin Feng <linfeng23@huawei.com>]

From: Feng Lin <linfeng23@huawei.com>

When testing migration, a Segmentation fault qemu core is generated.
0  error_free (err=0x1)
1  0x00007f8b862df647 in qemu_fclose (f=f@entry=0x55e06c247640)
2  0x00007f8b8516d59a in migrate_fd_cleanup (s=s@entry=0x55e06c0e1ef0)
3  0x00007f8b8516d66c in migrate_fd_cleanup_bh (opaque=0x55e06c0e1ef0)
4  0x00007f8b8626a47f in aio_bh_poll (ctx=ctx@entry=0x55e06b5a16d0)
5  0x00007f8b8626e71f in aio_dispatch (ctx=0x55e06b5a16d0)
6  0x00007f8b8626a33d in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>)
7  0x00007f8b866bdba4 in g_main_context_dispatch ()
8  0x00007f8b8626cde9 in glib_pollfds_poll ()
9  0x00007f8b8626ce62 in os_host_main_loop_wait (timeout=<optimized out>)
10 0x00007f8b8626cffd in main_loop_wait (nonblocking=nonblocking@entry=0)
11 0x00007f8b862ef01f in main_loop ()
Using gdb print the struct QEMUFile f = {
  ...,
  iovcnt = 65, last_error = 21984,
  last_error_obj = 0x1, shutdown = true
}
Well iovcnt is overflow, because the max size of MAX_IOV_SIZE is 64.
struct QEMUFile {
    ...;
    struct iovec iov[MAX_IOV_SIZE];
    unsigned int iovcnt;
    int last_error;
    Error *last_error_obj;
    bool shutdown;
};
iovcnt and last_error is overwrited by add_to_iovec().
Right now, add_to_iovec() increase iovcnt before check the limit.
And it seems that add_to_iovec() assumes that iovcnt will set to zero
in qemu_fflush(). But qemu_fflush() will directly return when f->shutdown
is true.

The situation may occur when libvirtd restart during migration, after
f->shutdown is set, before calling qemu_file_set_error() in
qemu_file_shutdown().

So the safiest way is checking the iovcnt before increasing it.

Signed-off-by: Feng Lin <linfeng23@huawei.com>
---
 migration/qemu-file.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/migration/qemu-file.c b/migration/qemu-file.c
index d6e03dbc0e..f6486cf7bc 100644
--- a/migration/qemu-file.c
+++ b/migration/qemu-file.c
@@ -416,6 +416,9 @@ static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
     {
         f->iov[f->iovcnt - 1].iov_len += size;
     } else {
+        if (f->iovcnt >= MAX_IOV_SIZE) {
+            goto fflush;
+        }
         if (may_free) {
             set_bit(f->iovcnt, f->may_free);
         }
@@ -423,12 +426,12 @@ static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
         f->iov[f->iovcnt++].iov_len = size;
     }
 
-    if (f->iovcnt >= MAX_IOV_SIZE) {
-        qemu_fflush(f);
-        return 1;
+    if (f->iovcnt < MAX_IOV_SIZE) {
+        return 0;
     }
-
-    return 0;
+fflush:
+    qemu_fflush(f);
+    return 1;
 }
 
 static void add_buf_to_iovec(QEMUFile *f, size_t len)
-- 
2.23.0