Re: [PATCH v5] accel/qaic: tighten integer overflow checking in map_user_pages()

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On 8/7/2023 8:43 AM, Dan Carpenter wrote:
On Mon, Aug 07, 2023 at 05:09:34PM +0300, Dan Carpenter wrote:
The encode_dma() function has some validation on in_trans->size but it's
not complete and it would be more clear to move those checks to
find_and_map_user_pages().

The encode_dma() had two checks:

	if (in_trans->addr + in_trans->size < in_trans->addr || !in_trans->size)
		return -EINVAL;

It's not sufficeint to just check if in_trans->size is zero.  The

sufficient

resources->xferred_dma_size variable represent the number of bytes
already transferred.  If we have already transferred more bytes than
in_trans->size then there are negative bytes remaining which doesn't
make sense.  Check for that as well.

I introduced a new variable "remaining" which represents the amount

introduce (commit text should be present tense per my understanding)

we want to transfer (in_trans->size) minus the ammount we have already

amount

transferred (resources->xferred_dma_size).

The check in encode_dma() checked that "addr + size" could not overflow
however we may already have transferred some bytes so the real starting
address is "xfer_start_addr" so check that "xfer_start_addr + size"
cannot overflow instead.  Also check that "addr +
resources->xferred_dma_size cannot overflow.

My other concern was that we are dealing with u64 values but on 32bit
systems the kmalloc() function will truncate the sizes to 32 bits.  So
I calculated "total = in_trans->size + offset_in_page(xfer_start_addr);"
and returned -EINVAL if it were >= SIZE_MAX.  This will not affect 64bit
systems.

Fixes: 129776ac2e38 ("accel/qaic: Add control path")
Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
---
This is re-write re-write of the previous version.

I am not necessarily sure it is correct.  Please review carefully.  In
particular, please check how "total" is calculated.  Maybe it would make
more sense to write that as:

	total = remaining + offset_in_page(xfer_start_addr);

I think this makes more sense.


The other question I had is should we add a check:

	if (remaining == 0)
		return 0;

I don't see why adding this would hurt anything. I don't believe it is necessary. Remaining is a function of of the driver code and not an external input. The driver transfers as much as it can, and stops when everything is sent (remaining == 0). If we hit this check, it is a driver bug.


  drivers/accel/qaic/qaic_control.c | 23 +++++++++++++++--------
  1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/drivers/accel/qaic/qaic_control.c b/drivers/accel/qaic/qaic_control.c
index cfbc92da426f..d64505bcf4ae 100644
--- a/drivers/accel/qaic/qaic_control.c
+++ b/drivers/accel/qaic/qaic_control.c
@@ -392,18 +392,28 @@ static int find_and_map_user_pages(struct qaic_device *qdev,
  				   struct qaic_manage_trans_dma_xfer *in_trans,
  				   struct ioctl_resources *resources, struct dma_xfer *xfer)
  {
+	u64 xfer_start_addr, remaining, end, total;
  	unsigned long need_pages;
  	struct page **page_list;
  	unsigned long nr_pages;
  	struct sg_table *sgt;
-	u64 xfer_start_addr;
  	int ret;
  	int i;
- xfer_start_addr = in_trans->addr + resources->xferred_dma_size;
+	if (check_add_overflow(in_trans->addr, resources->xferred_dma_size, &xfer_start_addr))
+		return -EINVAL;
+
+	if (in_trans->size == 0 ||
+	    in_trans->size < resources->xferred_dma_size ||
+	    check_add_overflow(xfer_start_addr, in_trans->size, &end))
                                                 ^^^^^^^^^^^^^^
This should be remaining.  So maybe it should be something like this
with a return 0 for no bytes remaining and total calculated differently.

Yep, should be remaining.  Your below proposed version looks good to me.


	if (check_add_overflow(in_trans->addr, resources->xferred_dma_size, &xfer_start_addr))
		return -EINVAL;

	if (in_trans->size < resources->xferred_dma_size)
		return -EINVAL;

This check should never hit unless we have a driver bug. I don't object to having it though.

	remaining = in_trans->size - resources->xferred_dma_size;
	if (remaining == 0)
		return 0;

	if (check_add_overflow(xfer_start_addr, remaining, &end))
		return -EINVAL;

	/* Still not really sure why total is calculated this way */

Physical memory layout.

Lets say remaining is 4k, and PAGE_SIZE is 4k.

4k / 4k = 1 so we need 1 page.

Except, if that 4k remaining is some remainder of the transfer and not the complete transfer, then where we start the transfer matters.

If the remaining 4k starts right at a page boundary, then we just need a single page. However, if the remaining 4k starts X bytes into a page (where X is non-zero), we would actually be transferring data from two physical pages, even though 4k can be fully represented by one page.

This representation might make a bit more sense (although I argue it is wrong) -

total = remaining;
need_pages = DIV_ROUND_UP(total, PAGE_SIZE);
if (offset_in_page(xfer_state_addr))
	need_pages++;

Where this breaks down is when the start addr and the remaining combine to fit into a page.

Assume remaining == 5k and PAGE_SIZE == 4k. offset_in_page() is going to return 1k.

The right answer is 2 pages. The first page will contain 3k (4k - 1k from the offset) and the second page will contain 2k.

DIV_ROUND_UP(remaining, PAGE_SIZE) == 5k / 4k == 2

DIV_ROUND_UP(remaining + offset_in_page(), PAGE_SIZE) == (5k + 1k) / 4k == 2

Seems like we need a comment explaining this.

	total = remaining + offset_in_page(xfer_start_addr);
	if (total >= SIZE_MAX)
		return -EINVAL;

	need_pages = DIV_ROUND_UP(total, PAGE_SIZE);

regards,
dan carpenter

+		return -EINVAL;
- need_pages = DIV_ROUND_UP(in_trans->size + offset_in_page(xfer_start_addr) -
-				  resources->xferred_dma_size, PAGE_SIZE);
+	remaining = in_trans->size - resources->xferred_dma_size;
+	total = in_trans->size + offset_in_page(xfer_start_addr);
+	if (total >= SIZE_MAX)
+		return -EINVAL;
+
+	need_pages = DIV_ROUND_UP(total - resources->xferred_dma_size, PAGE_SIZE);
nr_pages = need_pages; @@ -435,7 +445,7 @@ static int find_and_map_user_pages(struct qaic_device *qdev, ret = sg_alloc_table_from_pages(sgt, page_list, nr_pages,
  					offset_in_page(xfer_start_addr),
-					in_trans->size - resources->xferred_dma_size, GFP_KERNEL);
+					remaining, GFP_KERNEL);
  	if (ret) {
  		ret = -ENOMEM;
  		goto free_sgt;
@@ -566,9 +576,6 @@ static int encode_dma(struct qaic_device *qdev, void *trans, struct wrapper_list
  	    QAIC_MANAGE_EXT_MSG_LENGTH)
  		return -ENOMEM;
- if (in_trans->addr + in_trans->size < in_trans->addr || !in_trans->size)
-		return -EINVAL;
-
  	xfer = kmalloc(sizeof(*xfer), GFP_KERNEL);
  	if (!xfer)
  		return -ENOMEM;
--
2.39.2




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