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

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

[Brian Johannesmeyer]

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

[RFC v2 1/2] dmapool: Move pool metadata into non-DMA memory

[Brian Johannesmeyer]

If a `struct dma_block` object resides in DMA memory, a malicious
peripheral device can corrupt its metadata --- specifically, its
`next_block` pointer, which links blocks in a DMA pool. By corrupting these
pointers, an attacker can manipulate `dma_pool_alloc()` into returning
attacker-controllable pointers, which can lead to kernel memory corruption
from a driver that calls it.

To prevent this, move the `struct dma_block` metadata into non-DMA memory,
ensuring that devices cannot tamper with the internal pointers of the DMA
pool allocator. Specifically:

- Add a `vaddr` field to `struct dma_block` to point to the actual
  DMA-accessible block.
- Maintain an array of `struct dma_block` objects in `struct dma_page` to
  track the metadata of each block within an allocated page.

This change secures the DMA pool allocator by keeping its metadata in
kernel memory, inaccessible to peripheral devices, thereby preventing
potential attacks that could corrupt kernel memory through DMA operations.

Co-developed-by: Raphael Isemann <teemperor@gmail.com>
Signed-off-by: Raphael Isemann <teemperor@gmail.com>
Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
---
 mm/dmapool.c | 60 +++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 52 insertions(+), 8 deletions(-)

diff --git a/mm/dmapool.c b/mm/dmapool.c
index a151a21e571b..25005a9fc201 100644
--- a/mm/dmapool.c
+++ b/mm/dmapool.c
@@ -43,6 +43,7 @@
 struct dma_block {
 	struct dma_block *next_block;
 	dma_addr_t dma;
+	void *vaddr;
 };
 
 struct dma_pool {		/* the pool */
@@ -64,6 +65,8 @@ struct dma_page {		/* cacheable header for 'allocation' bytes */
 	struct list_head page_list;
 	void *vaddr;
 	dma_addr_t dma;
+	struct dma_block *blocks;
+	size_t blocks_per_page;
 };
 
 static DEFINE_MUTEX(pools_lock);
@@ -91,14 +94,35 @@ static ssize_t pools_show(struct device *dev, struct device_attribute *attr, cha
 
 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;
+}
+
 #ifdef DMAPOOL_DEBUG
 static void pool_check_block(struct dma_pool *pool, struct dma_block *block,
 			     gfp_t mem_flags)
 {
-	u8 *data = (void *)block;
+	u8 *data = (void *)block->vaddr;
 	int i;
 
-	for (i = sizeof(struct dma_block); i < pool->size; i++) {
+	for (i = 0; i < pool->size; i++) {
 		if (data[i] == POOL_POISON_FREED)
 			continue;
 		dev_err(pool->dev, "%s %s, %p (corrupted)\n", __func__,
@@ -114,7 +138,7 @@ static void pool_check_block(struct dma_pool *pool, struct dma_block *block,
 	}
 
 	if (!want_init_on_alloc(mem_flags))
-		memset(block, POOL_POISON_ALLOCATED, pool->size);
+		memset(block->vaddr, POOL_POISON_ALLOCATED, pool->size);
 }
 
 static struct dma_page *pool_find_page(struct dma_pool *pool, dma_addr_t dma)
@@ -143,7 +167,7 @@ static bool pool_block_err(struct dma_pool *pool, void *vaddr, dma_addr_t dma)
 	}
 
 	while (block) {
-		if (block != vaddr) {
+		if (block->vaddr != vaddr) {
 			block = block->next_block;
 			continue;
 		}
@@ -301,6 +325,7 @@ static void pool_initialise_page(struct dma_pool *pool, struct dma_page *page)
 {
 	unsigned int next_boundary = pool->boundary, offset = 0;
 	struct dma_block *block, *first = NULL, *last = NULL;
+	size_t i = 0;
 
 	pool_init_page(pool, page);
 	while (offset + pool->size <= pool->allocation) {
@@ -310,7 +335,8 @@ static void pool_initialise_page(struct dma_pool *pool, struct dma_page *page)
 			continue;
 		}
 
-		block = page->vaddr + offset;
+		block = &page->blocks[i];
+		block->vaddr = page->vaddr + offset;
 		block->dma = page->dma + offset;
 		block->next_block = NULL;
 
@@ -322,6 +348,7 @@ static void pool_initialise_page(struct dma_pool *pool, struct dma_page *page)
 
 		offset += pool->size;
 		pool->nr_blocks++;
+		i++;
 	}
 
 	last->next_block = pool->next_block;
@@ -339,9 +366,18 @@ static struct dma_page *pool_alloc_page(struct dma_pool *pool, gfp_t mem_flags)
 	if (!page)
 		return NULL;
 
+	page->blocks_per_page = pool->allocation / pool->size;
+	page->blocks = kmalloc_array(page->blocks_per_page,
+				     sizeof(struct dma_block), GFP_KERNEL);
+	if (!page->blocks) {
+		kfree(page);
+		return NULL;
+	}
+
 	page->vaddr = dma_alloc_coherent(pool->dev, pool->allocation,
 					 &page->dma, mem_flags);
 	if (!page->vaddr) {
+		kfree(page->blocks);
 		kfree(page);
 		return NULL;
 	}
@@ -383,6 +419,7 @@ void dma_pool_destroy(struct dma_pool *pool)
 		if (!busy)
 			dma_free_coherent(pool->dev, pool->allocation,
 					  page->vaddr, page->dma);
+		kfree(page->blocks);
 		list_del(&page->page_list);
 		kfree(page);
 	}
@@ -432,9 +469,9 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
 	*handle = block->dma;
 	pool_check_block(pool, block, mem_flags);
 	if (want_init_on_alloc(mem_flags))
-		memset(block, 0, pool->size);
+		memset(block->vaddr, 0, pool->size);
 
-	return block;
+	return block->vaddr;
 }
 EXPORT_SYMBOL(dma_pool_alloc);
 
@@ -449,9 +486,16 @@ EXPORT_SYMBOL(dma_pool_alloc);
  */
 void dma_pool_free(struct dma_pool *pool, void *vaddr, dma_addr_t dma)
 {
-	struct dma_block *block = vaddr;
+	struct dma_block *block;
 	unsigned long flags;
 
+	block = pool_find_block(pool, vaddr);
+	if (!block) {
+		dev_err(pool->dev, "%s %s, invalid vaddr %p\n",
+			__func__, pool->name, vaddr);
+		return;
+	}
+
 	spin_lock_irqsave(&pool->lock, flags);
 	if (!pool_block_err(pool, vaddr, dma)) {
 		pool_block_push(pool, block, dma);
-- 
2.34.1

[RFC v2 2/2] dmapool: Use pool_find_block() in pool_block_err()

[Brian Johannesmeyer]

In the previous patch, the `pool_find_block()` function was added to
translate a virtual address into the corresponding `struct dma_block`. The
existing `pool_find_page()` function performs a similar role by translating
a DMA address into the `struct dma_page` containing it.

To reduce redundant code and improve consistency, remove the
`pool_find_page()` function and update `pool_block_err()` to use
`pool_find_block()` instead. Doing so eliminates duplicate functionality
and consolidates the block lookup process.

Co-developed-by: Raphael Isemann <teemperor@gmail.com>
Signed-off-by: Raphael Isemann <teemperor@gmail.com>
Signed-off-by: Brian Johannesmeyer <bjohannesmeyer@gmail.com>
---
 mm/dmapool.c | 38 ++++++++++++--------------------------
 1 file changed, 12 insertions(+), 26 deletions(-)

diff --git a/mm/dmapool.c b/mm/dmapool.c
index 25005a9fc201..267fe13347bd 100644
--- a/mm/dmapool.c
+++ b/mm/dmapool.c
@@ -141,39 +141,25 @@ static void pool_check_block(struct dma_pool *pool, struct dma_block *block,
 		memset(block->vaddr, POOL_POISON_ALLOCATED, pool->size);
 }
 
-static struct dma_page *pool_find_page(struct dma_pool *pool, dma_addr_t dma)
-{
-	struct dma_page *page;
-
-	list_for_each_entry(page, &pool->page_list, page_list) {
-		if (dma < page->dma)
-			continue;
-		if ((dma - page->dma) < pool->allocation)
-			return page;
-	}
-	return NULL;
-}
-
 static bool pool_block_err(struct dma_pool *pool, void *vaddr, dma_addr_t dma)
 {
-	struct dma_block *block = pool->next_block;
-	struct dma_page *page;
+	struct dma_block *block = pool_find_block(pool, vaddr);
 
-	page = pool_find_page(pool, dma);
-	if (!page) {
-		dev_err(pool->dev, "%s %s, %p/%pad (bad dma)\n",
-			__func__, pool->name, vaddr, &dma);
+	if (!block) {
+		dev_err(pool->dev, "%s %s, invalid block %p\n",
+			__func__, pool->name, vaddr);
 		return true;
 	}
 
-	while (block) {
-		if (block->vaddr != vaddr) {
-			block = block->next_block;
-			continue;
+	struct dma_block *iter = pool->next_block;
+
+	while (iter) {
+		if (iter == block) {
+			dev_err(pool->dev, "%s %s, dma %pad already free\n",
+				__func__, pool->name, &dma);
+			return true;
 		}
-		dev_err(pool->dev, "%s %s, dma %pad already free\n",
-			__func__, pool->name, &dma);
-		return true;
+		iter = iter->next_block;
 	}
 
 	memset(vaddr, POOL_POISON_FREED, pool->size);
-- 
2.34.1

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

[Greg KH]

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.
> 
> **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.

I know I said it privately, but I'll say it here in public, very cool
finding, this is nice work!

> **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
> ```

You had mentioned that the size:68 numbers were going to be re-run, has
that happened and this really is that much of a boost to that size?  Or
is this the original numbers?

thanks,

greg k-h

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

[Brian Johannesmeyer]

On Tue, Nov 19, 2024 at 3:15 PM Greg KH <gregkh@linuxfoundation.org> wrote:
> I know I said it privately, but I'll say it here in public, very cool
> finding, this is nice work!

Thanks! I appreciate your earlier feedback as well.

> You had mentioned that the size:68 numbers were going to be re-run, has
> that happened and this really is that much of a boost to that size?  Or
> is this the original numbers?

I re-ran the test, and the numbers are consistent across multiple
runs. I’m also surprised by how significant the improvement is for the
68-byte block size.

Thanks,

Brian Johannesmeyer

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

[Christoph Hellwig]

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---

Re: [RFC v2 1/2] dmapool: Move pool metadata into non-DMA memory

[Christoph Hellwig]

On Tue, Nov 19, 2024 at 09:55:28PM +0100, Brian Johannesmeyer wrote:

> +	page->blocks_per_page = pool->allocation / pool->size;
> +	page->blocks = kmalloc_array(page->blocks_per_page,
> +				     sizeof(struct dma_block), GFP_KERNEL);

Given that you now need an array of the blocks anyway, it might make
sense to switch from a linked list to a bitmap for tracking free state,
which would be a lot more efficient as you only need a bit per block
as tracking overhead instead of a two pointers and a dma_addr_t.

e.g. do a find_first_zero_bit() to find the ffree slot, then calculate
the dma_addr and virt address by simple offseting into the dma_page
ones with bitnr * pool->size.