The backing store string location offset 0 determines that the file isn't present. The string size shouldn't be then checked: from qemu.git/docs/specs/qcow2.txt == Header == The first cluster of a qcow2 image contains the file header: Byte 0 - 3: magic QCOW magic string ("QFI\xfb") 4 - 7: version Version number (valid values are 2 and 3) 8 - 15: backing_file_offset Offset into the image file at which the backing file name is stored (NB: The string is not null terminated). 0 if the image doesn't have a backing file. 16 - 19: backing_file_size Length of the backing file name in bytes. Must not be longer than 1023 bytes. Undefined if the image doesn't have a backing file. ^^^^^^^^^ This patch intentionally leaves the backing file string size check in place in case a malformatted file would be presented to libvirt. Also according to the docs the string size is maximum 1023 bytes, thus this patch adds a check to verify that. I was also able to verify that the check was done the same way in the legacy qcow fromat (in qemu's code). --- src/util/virstoragefile.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/src/util/virstoragefile.c b/src/util/virstoragefile.c index 5db9184..13056a7 100644 --- a/src/util/virstoragefile.c +++ b/src/util/virstoragefile.c @@ -385,15 +385,22 @@ qcowXGetBackingStore(char **res, offset = virReadBufInt64BE(buf + QCOWX_HDR_BACKING_FILE_OFFSET); if (offset > buf_size) return BACKING_STORE_INVALID; + + if (offset == 0) { + if (format) + *format = VIR_STORAGE_FILE_NONE; + return BACKING_STORE_OK; + } + size = virReadBufInt32BE(buf + QCOWX_HDR_BACKING_FILE_SIZE); if (size == 0) { if (format) *format = VIR_STORAGE_FILE_NONE; return BACKING_STORE_OK; } - if (offset + size > buf_size || offset + size < offset) + if (size > 1023) return BACKING_STORE_INVALID; - if (size + 1 == 0) + if (offset + size > buf_size || offset + size < offset) return BACKING_STORE_INVALID; if (VIR_ALLOC_N(*res, size + 1) < 0) return BACKING_STORE_ERROR;