Am 06.11.2024 um 16:45 hat Denis V. Lunev geschrieben: > On 11/6/24 10:53, Kevin Wolf wrote: > > [ Cc: qemu-block ] > > > > Am 06.11.2024 um 09:04 hat Dmitry Frolov geschrieben: > > > The sum "cluster_index + count" may overflow uint32_t. > > > > > > Found by Linux Verification Center (linuxtesting.org) with SVACE. > > > > > > Signed-off-by: Dmitry Frolov <fro...@swemel.ru> > > Thanks, applied to the block branch. > > > > While trying to check if this can be triggered in practice, I found this > > line in parallels_fill_used_bitmap(): > > > > s->used_bmap_size = DIV_ROUND_UP(payload_bytes, s->cluster_size); > > > > s->used_bmap_size is unsigned long, payload_bytes is the int64_t result > > of bdrv_getlength() for the image file, which could certainly be made > > more than 4 GB * cluster_size. I think we need an overflow check there, > > too. > > > > When allocate_clusters() calculates new_usedsize, it doesn't seem to > > consider the overflow case either. > > > > Denis, can you take a look? > > > > Kevin > > > Hi, Kevin, Dmitry! > > In general, the situation is the following. > > On-disk format heavily uses offsets from the beginning of the disk > denominated in clusters. These offsets are saved in uint32 on disk. > This means that the image with 4T virtual size and 1M cluster size > will use offsets from 0 to 4 * 2^10 in different tables on disk. > > There is existing problem in the format specification that we > can not easily apply limits to the virtual size of the disk as > we also can have arbitrary size growing metadata like CBT, which > is kept in the same address space (cluster offsets). > > Though in reality I have never seen images with non-1 Mb cluster > size and in order to nearly overflow them we would need really > allocated images of 4 PB. > > Theoretically the problem is possible but it looks impractical > to me in the real life so far.
It probably won't happen with normal images, but we need to consider malicious images, and I think they could be constructed in a way that causes integer overflows here. At least the one that directly takes bdrv_getlength() should be trivial to trigger, you just need to extend the file size enough outside of QEMU. Kevin
