| Message ID | 20241122211144.4186080-1-bjohannesmeyer@gmail.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v3,1/3] dmapool: Move pool metadata into non-DMA memory | expand |
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. 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/without the patches applied. Here is its output *without* the patches applied: ``` dmapool test: size:16 align:16 blocks:8192 time:11860 dmapool test: size:64 align:64 blocks:8192 time:11951 dmapool test: size:256 align:256 blocks:8192 time:12287 dmapool test: size:1024 align:1024 blocks:2048 time:3134 dmapool test: size:4096 align:4096 blocks:1024 time:1686 dmapool test: size:68 align:32 blocks:8192 time:12050 ``` And here is its output *with* the patches applied: ``` dmapool test: size:16 align:16 blocks:8192 time:37954 dmapool test: size:64 align:64 blocks:8192 time:40036 dmapool test: size:256 align:256 blocks:8192 time:41942 dmapool test: size:1024 align:1024 blocks:2048 time:10964 dmapool test: size:4096 align:4096 blocks:1024 time:6101 dmapool test: size:68 align:32 blocks:8192 time:41307 ``` The patch set results in a 2.2x--2.6x runtime overhead, as demonstrated in the performance results. AFAICT, most of this overhead originates from the change in dma_pool_free()'s vaddr-to-block translation. Previously, the translation was a simple typecast, but with the patches applied, it now requires a lookup in an xarray. As Keith noted [1], achieving baseline performance would likely require changing the API. **Changes** - V2 -> V3: (i) Use an xarray for vaddr-to-block translations, which improves the performance of free operations. (ii) Remove the minimum DMA block size constraint, as it is no longer necessary. - V1 -> V2: Submit to public mailing lists. Thanks, Brian Johannesmeyer [0] Link: https://www.csl.sri.com/~neumann/ndss-iommu.pdf [1] Link: https://patchwork.kernel.org/project/linux-mm/cover/20241119205529.3871048-1-bjohannesmeyer@gmail.com/#26130533 Brian Johannesmeyer (3): dmapool: Move pool metadata into non-DMA memory dmapool: Use pool_find_block() in pool_block_err() Use xarray for efficient vaddr-to-block translation mm/dmapool.c | 92 ++++++++++++++++++++++++++++++++-------------------- 1 file changed, 57 insertions(+), 35 deletions(-)
diff --git a/mm/dmapool.c b/mm/dmapool.c index f2b96be25412..1cc2cc87ab93 100644 --- a/mm/dmapool.c +++ b/mm/dmapool.c @@ -35,6 +35,7 @@ #include <linux/string.h> #include <linux/types.h> #include <linux/wait.h> +#include <linux/xarray.h> #ifdef CONFIG_SLUB_DEBUG_ON #define DMAPOOL_DEBUG 1 @@ -59,6 +60,7 @@ struct dma_pool { /* the pool */ unsigned int boundary; char name[32]; struct list_head pools; + struct xarray block_map; }; struct dma_page { /* cacheable header for 'allocation' bytes */ @@ -96,23 +98,7 @@ static DEVICE_ATTR_RO(pools); static struct dma_block *pool_find_block(struct dma_pool *pool, void *vaddr) { - struct dma_page *page; - size_t offset, index; - - list_for_each_entry(page, &pool->page_list, page_list) { - if (vaddr < page->vaddr) - continue; - offset = vaddr - page->vaddr; - if (offset >= pool->allocation) - continue; - - index = offset / pool->size; - if (index >= page->blocks_per_page) - return NULL; - - return &page->blocks[index]; - } - return NULL; + return xa_load(&pool->block_map, (unsigned long)vaddr); } #ifdef DMAPOOL_DEBUG @@ -273,6 +259,7 @@ struct dma_pool *dma_pool_create(const char *name, struct device *dev, retval->boundary = boundary; retval->allocation = allocation; INIT_LIST_HEAD(&retval->pools); + xa_init(&retval->block_map); /* * pools_lock ensures that the ->dma_pools list does not get corrupted. @@ -324,6 +311,12 @@ static void pool_initialise_page(struct dma_pool *pool, struct dma_page *page) block->dma = page->dma + offset; block->next_block = NULL; + if (xa_err(xa_store(&pool->block_map, (unsigned long)block->vaddr, + block, GFP_KERNEL))) { + pr_err("dma_pool: Failed to store block in xarray\n"); + return; + } + if (last) last->next_block = block; else @@ -385,6 +378,7 @@ void dma_pool_destroy(struct dma_pool *pool) if (unlikely(!pool)) return; + xa_destroy(&pool->block_map); mutex_lock(&pools_reg_lock); mutex_lock(&pools_lock); list_del(&pool->pools);