[PATCH v4 7/7] qemu-img: Deprecate use of -b without -F

[Eric Blake]

Creating an image that requires format probing of the backing image is
inherently unsafe (we've had several CVEs over the years based on
probes leaking information to the guest on a subsequent boot, although
these days tools like libvirt are aware of the issue enough to prevent
the worst effects).  However, if our probing algorithm ever changes,
or if other tools like libvirt determine a different probe result than
we do, then subsequent use of that backing file under a different
format will present corrupted data to the guest.  Start a deprecation
clock so that future qemu-img can refuse to create unsafe backing
chains that would rely on probing.  The warnings are intentionally
emitted from the block layer rather than qemu-img (thus, all paths
into image creation or rewriting perform the check).

However, there is one time where probing is safe: if we probe raw,
then it is safe to record that implicitly in the image (but we still
warn, as it's better to teach the user to supply -F always than to
make them guess when it is safe).

iotest 114 specifically wants to create an unsafe image for later
amendment rather than defaulting to our new default of recording a
probed format, so it needs an update.  While touching it, expand it to
cover all of the various warnings enabled by this patch.  iotest 290
also shows a change to qcow messages; note that the fact that we now
make a probed format of 'raw' explicit now results in a double
warning, but no one should be creating new qcow images so it is not
worth cleaning up.

Signed-off-by: Eric Blake <address@hidden>
---
 docs/system/deprecated.rst | 19 +++++++++++++++++++
 block.c                    | 21 ++++++++++++++++++++-
 qemu-img.c                 |  2 +-
 tests/qemu-iotests/114     | 11 +++++++++++
 tests/qemu-iotests/114.out |  8 ++++++++
 tests/qemu-iotests/290.out |  5 ++++-
 6 files changed, 63 insertions(+), 3 deletions(-)

diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
index 4261d5589e6a..b541d52c7dc0 100644
--- a/docs/system/deprecated.rst
+++ b/docs/system/deprecated.rst
@@ -382,6 +382,25 @@ image).  Rather, any changes to the backing chain should 
be performed
 with ``qemu-img rebase -u`` either before or after the remaining
 changes being performed by amend, as appropriate.

+qemu-img backing file without format (since 5.0.0)
+''''''''''''''''''''''''''''''''''''''''''''''''''
+
+The use of ``qemu-img create``, ``qemu-img rebase``, or ``qemu-img
+convert`` to create or modify an image that depends on a backing file
+now recommends that an explicit backing format be provided.  This is
+for safety: if qemu probes a different format than what you thought,
+the data presented to the guest will be corrupt; similarly, presenting
+a raw image to a guest allows a potential security exploit if a future
+probe sees a non-raw image based on guest writes.  To avoid the
+warning message, or even future refusal to create an unsafe image, you
+must pass ``-o backing_fmt=`` (or the shorthand ``-F`` during create)
+to specify the intended backing format.  You may use ``qemu-img rebase
+-u`` to retroactively add a backing format to an existing image.
+However, be aware that there are already potential security risks to
+blindly using ``qemu-img info`` to probe the format of an untrusted
+backing image, when deciding what format to add into an existing
+image.
+
 ``qemu-img convert -n -o`` (since 4.2.0)
 ''''''''''''''''''''''''''''''''''''''''

diff --git a/block.c b/block.c
index cd3277ab86fd..69955f53f35a 100644
--- a/block.c
+++ b/block.c
@@ -6064,6 +6064,20 @@ void bdrv_img_create(const char *filename, const char 
*fmt,
                               "Could not open backing image to determine 
size.\n");
             goto out;
         } else {
+            if (!backing_fmt) {
+                warn_report("Deprecated use of backing file without explicit "
+                            "backing format (detected format of %s)",
+                            bs->drv->format_name);
+                if (bs->drv == &bdrv_raw) {
+                    /*
+                     * A probe of raw is always correct, so in this one
+                     * case, we can write that into the image.
+                     */
+                    backing_fmt = bs->drv->format_name;
+                    qemu_opt_set(opts, BLOCK_OPT_BACKING_FMT, backing_fmt,
+                                 NULL);
+                }
+            }
             if (size == -1) {
                 /* Opened BS, have no size */
                 size = bdrv_getlength(bs);
@@ -6077,7 +6091,12 @@ void bdrv_img_create(const char *filename, const char 
*fmt,
             }
             bdrv_unref(bs);
         }
-    } /* (backing_file && !(flags & BDRV_O_NO_BACKING)) */
+        /* (backing_file && !(flags & BDRV_O_NO_BACKING)) */
+    } else if (backing_file && !backing_fmt) {
+        warn_report("Deprecated use of unopened backing file without "
+                    "explicit backing format, use of this image requires "
+                    "potentially unsafe format probing");
+    }

     if (size == -1) {
         error_setg(errp, "Image creation needs a size parameter");
diff --git a/qemu-img.c b/qemu-img.c
index 4ca1d2db7307..e977dd7e1e62 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3639,7 +3639,7 @@ static int img_rebase(int argc, char **argv)
      * doesn't change when we switch the backing file.
      */
     if (out_baseimg && *out_baseimg) {
-        ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt, false);
+        ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt, true);
     } else {
         ret = bdrv_change_backing_file(bs, NULL, NULL, false);
     }
diff --git a/tests/qemu-iotests/114 b/tests/qemu-iotests/114
index 26104fff6c67..5b06eab0ceee 100755
--- a/tests/qemu-iotests/114
+++ b/tests/qemu-iotests/114
@@ -42,9 +42,15 @@ _unsupported_proto vxhs
 # qcow2.py does not work too well with external data files
 _unsupported_imgopts data_file

+# Intentionally specify backing file without backing format; demonstrate
+# the difference in warning messages when backing file could be probed.
+# Note that only a raw probe result will affect the resulting image.
+truncate --size=64M "$TEST_IMG.orig"
+_make_test_img -b "$TEST_IMG.orig" 64M

 TEST_IMG="$TEST_IMG.base" _make_test_img 64M
 _make_test_img -b "$TEST_IMG.base" 64M
+_make_test_img -u -b "$TEST_IMG.base" 64M

 # Set an invalid backing file format
 $PYTHON qcow2.py "$TEST_IMG" add-header-ext 0xE2792ACA "foo"
@@ -55,6 +61,11 @@ _img_info
 $QEMU_IO -c "open $TEST_IMG" -c "read 0 4k" 2>&1 | _filter_qemu_io | 
_filter_testdir
 $QEMU_IO -c "open -o backing.driver=$IMGFMT $TEST_IMG" -c "read 0 4k" | 
_filter_qemu_io

+# Rebase the image, to show that omitting backing format triggers a warning,
+# but probing now lets us use the backing file.
+$QEMU_IMG rebase -u -b "$TEST_IMG.base" "$TEST_IMG"
+$QEMU_IO -c "open $TEST_IMG" -c "read 0 4k" 2>&1 | _filter_qemu_io | 
_filter_testdir
+
 # success, all done
 echo '*** done'
 rm -f $seq.full
diff --git a/tests/qemu-iotests/114.out b/tests/qemu-iotests/114.out
index 67adef37a4f6..59673abcd5e3 100644
--- a/tests/qemu-iotests/114.out
+++ b/tests/qemu-iotests/114.out
@@ -1,5 +1,10 @@
 QA output created by 114
+qemu-img: warning: Deprecated use of backing file without explicit backing 
format (detected format of raw)
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
backing_file=TEST_DIR/t.IMGFMT.orig backing_fmt=raw
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
+qemu-img: warning: Deprecated use of backing file without explicit backing 
format (detected format of IMGFMT)
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
backing_file=TEST_DIR/t.IMGFMT.base
+qemu-img: warning: Deprecated use of unopened backing file without explicit 
backing format, use of this image requires potentially unsafe format probing
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
backing_file=TEST_DIR/t.IMGFMT.base
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
@@ -11,4 +16,7 @@ qemu-io: can't open device TEST_DIR/t.qcow2: Could not open 
backing file: Unknow
 no file open, try 'help open'
 read 4096/4096 bytes at offset 0
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+qemu-img: warning: Deprecated use of backing file without explicit backing 
format, use of this image requires potentially unsafe format probing
+read 4096/4096 bytes at offset 0
+4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 *** done
diff --git a/tests/qemu-iotests/290.out b/tests/qemu-iotests/290.out
index 5b4ae3196d04..f80a5d982726 100644
--- a/tests/qemu-iotests/290.out
+++ b/tests/qemu-iotests/290.out
@@ -2,6 +2,7 @@ QA output created by 290

 == qcow backed by qcow ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
+qemu-img: warning: Deprecated use of backing file without explicit backing 
format (detected format of IMGFMT)
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 
backing_file=TEST_DIR/t.IMGFMT.base
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
@@ -26,7 +27,9 @@ cluster_size: 512
 backing file: TEST_DIR/t.IMGFMT.base

 == qcow backed by raw ==
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 
backing_file=TEST_DIR/t.IMGFMT.base
+qemu-img: warning: Deprecated use of backing file without explicit backing 
format (detected format of raw)
+qemu-img: TEST_DIR/t.IMGFMT: IMGFMT cannot store backing format; an explicit 
backing format of raw is unsafe
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 
backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=raw
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
 virtual size: 128 MiB (134217728 bytes)
-- 
2.25.1