摘要:

In the Linux kernel, the following vulnerability has been resolved:swiotlb: fix info leak with DMA_FROM_DEVICEThe problem I m addressing was discovered by the LTP test coveringcve-2018-1000204.A short description of what happens follows:1) The test case issues a command code 00 (TEST UNIT READY) via the SG_IO interface with: dxfer_len == 524288, dxdfer_dir == SG_DXFER_FROM_DEV and a corresponding dxferp. The peculiar thing about this is that TUR is not reading from the device.2) In sg_start_req() the invocation of blk_rq_map_user() effectively bounces the user-space buffer. As if the device was to transfer into it. Since commit a45b599ad808 ( scsi: sg: allocate with __GFP_ZERO in sg_build_indirect() ) we make sure this first bounce buffer is allocated with GFP_ZERO.3) For the rest of the story we keep ignoring that we have a TUR, so the device won t touch the buffer we prepare as if the we had a DMA_FROM_DEVICE type of situation. My setup uses a virtio-scsi device and the buffer allocated by SG is mapped by the function virtqueue_add_split() which uses DMA_FROM_DEVICE for the in sgs (here scatter-gather and not scsi generics). This mapping involves bouncing via the swiotlb (we need swiotlb to do virtio in protected guest like s390 Secure Execution, or AMD SEV).4) When the SCSI TUR is done, we first copy back the content of the second (that is swiotlb) bounce buffer (which most likely contains some previous IO data), to the first bounce buffer, which contains all zeros. Then we copy back the content of the first bounce buffer to the user-space buffer.5) The test case detects that the buffer, which it zero-initialized, ain t all zeros and fails.One can argue that this is an swiotlb problem, because without swiotlbwe leak all zeros, and the swiotlb should be transparent in a sense thatit does not affect the outcome (if all other participants are wellbehaved).Copying the content of the original buffer into the swiotlb buffer isthe only way I can think of to make swiotlb transparent in suchscenarios. So let s do just that if in doubt, but allow the driverto tell us that the whole mapped buffer is going to be overwritten,in which case we can preserve the old behavior and avoid the performanceimpact of the extra bounce.

安全等级: Low

公告ID: KylinSec-SA-2024-3294

发布日期: 2024年8月20日

关联CVE: CVE-2022-48853