Re: [RFC v2 0/2] dmapool: Mitigate device-controllable mem. corruption

[Christoph Hellwig <hch@infradead.org>]

On Tue, Nov 19, 2024 at 09:55:27PM +0100, Brian Johannesmeyer wrote:
> 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.

You should probably Cc Keith as the person who most recently did major
work on the dmpool code and might still remember how it works.

> 
> **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
> 
> 
---end quoted text---