We discovered a security-related issue in the DMA pool allocator. V1 of our RFC was submitted to the Linux kernel security team. They recommended submitting it to the relevant subsystem maintainers and the hardening mailing list instead, as they did not consider this an explicit security issue. Their rationale was that Linux implicitly assumes hardware can be trusted. **Threat Model**: While Linux drivers typically trust their hardware, there may be specific drivers that do not operate under this assumption. Hence, this threat model assumes a malicious peripheral device capable of corrupting DMA data to exploit the kernel. In this scenario, the device manipulates kernel-initialized data (similar to the attack described in the Thunderclap paper [0]) to achieve arbitrary kernel memory corruption. **DMA pool background**. A DMA pool aims to reduce the overhead of DMA allocations by creating a large DMA buffer --- the "pool" --- from which smaller buffers are allocated as needed. Fundamentally, a DMA pool functions like a heap: it is a structure composed of linked memory "blocks", which, in this context, are DMA buffers. When a driver employs a DMA pool, it grants the device access not only to these blocks but also to the pointers linking them. **Vulnerability**. Similar to traditional heap corruption vulnerabilities --- where a malicious program corrupts heap metadata to e.g., hijack control flow --- a malicious device may corrupt DMA pool metadata. This corruption can trivially lead to arbitrary kernel memory corruption from any driver that uses it. Indeed, because the DMA pool API is extensively used, this vulnerability is not confined to a single instance. In fact, every usage of the DMA pool API is potentially vulnerable. An exploit proceeds with the following steps: 1. The DMA `pool` initializes its list of blocks, then points to the first block. 2. The malicious device overwrites the first 8 bytes of the first block --- which contain its `next_block` pointer --- to an arbitrary kernel address, `kernel_addr`. 3. The driver makes its first call to `dma_pool_alloc()`, after which, the pool should point to the second block. However, it instead points to `kernel_addr`. 4. The driver again calls `dma_pool_alloc()`, which incorrectly returns `kernel_addr`. Therefore, anytime the driver writes to this "block", it may corrupt sensitive kernel data. I have a PDF document that illustrates how these steps work. Please let me know if you would like me to share it with you. **Proposed mitigation**. To mitigate the corruption of DMA pool metadata (i.e., the pointers linking the blocks), the metadata should be moved into non-DMA memory, ensuring it cannot be altered by a device. I have included a patch series that implements this change. Since I am not deeply familiar with the DMA pool internals, I would appreciate any feedback on the patches. I have tested the patches with the `DMAPOOL_TEST` test and my own basic unit tests that ensure the DMA pool allocator is not vulnerable. **Performance**. I evaluated the patch set's performance by running the `DMAPOOL_TEST` test with `DMAPOOL_DEBUG` enabled and with/without the patches applied. Here is its output *without* the patches applied: ``` dmapool test: size:16 align:16 blocks:8192 time:3194110 dmapool test: size:64 align:64 blocks:8192 time:4730440 dmapool test: size:256 align:256 blocks:8192 time:5489630 dmapool test: size:1024 align:1024 blocks:2048 time:517150 dmapool test: size:4096 align:4096 blocks:1024 time:399616 dmapool test: size:68 align:32 blocks:8192 time:6156527 ``` And here is its output *with* the patches applied: ``` dmapool test: size:16 align:16 blocks:8192 time:3541031 dmapool test: size:64 align:64 blocks:8192 time:4227262 dmapool test: size:256 align:256 blocks:8192 time:4890273 dmapool test: size:1024 align:1024 blocks:2048 time:515775 dmapool test: size:4096 align:4096 blocks:1024 time:523096 dmapool test: size:68 align:32 blocks:8192 time:3450830 ``` Based on my interpretation of the output, the patch set does not appear to negatively impact performance. In fact, it shows slight performance improvements in some tests (i.e., for sizes 64, 256, 1024, and 68). I speculate that these performance gains may be due to improved spatial locality of the `next_block` pointers. With the patches applied, the `next_block` pointers are consistently spaced 24 bytes apart, matching the new size of `struct dma_block`. Previously, the spacing between `next_block` pointers depended on the block size, so for 1024-byte blocks, the pointers were spaced 1024 bytes apart. However, I am still unsure why the performance improvement for 68-byte blocks is so significant. [0] Link: https://www.csl.sri.com/~neumann/ndss-iommu.pdf Brian Johannesmeyer (2): dmapool: Move pool metadata into non-DMA memory dmapool: Use pool_find_block() in pool_block_err() mm/dmapool.c | 96 ++++++++++++++++++++++++++++++++++------------------ 1 file changed, 63 insertions(+), 33 deletions(-) -- 2.34.1
