Re: No check of the size passed to unmap_single in swiotlb

From: Robin Murphy
Date: Mon Nov 20 2017 - 11:50:11 EST

On 20/11/17 16:26, Konrad Rzeszutek Wilk wrote:
On Mon, Nov 20, 2017 at 08:17:14AM +0000, Eric Yang wrote:
Hi all,


During debug a device only support 32bits DMA(Qualcomm Atheros AP) in our LS1043A 64bits ARM SOC, we found that the invoke of dma_unmap_single --> swiotlb_tbl_unmap_single will unmap the passed "size" anyway even when the "size" is incorrect.

If the size is larger than it should, the extra entries in io_tlb_orig_addr array will be refilled by INVALID_PHYS_ADDR, and it will cause the bounce buffer copy not happen when the one who really used the mis-freed entries doing DMA data transfers, and this will cause further unknow behaviors.

Here we just fix it temporarily by adding a judge of the "size" in the swiotlb_tbl_unmap_single, if it is larger than it deserves, just unmap the right size only. Like the code:

Did the DMA debug API (CONFIG_DMA_API_DEBUG) help in figuring this issue as well?

[yangyu@titan dash-lts]$ git diff ./lib/swiotlb.c
diff --git a/lib/swiotlb.c b/lib/swiotlb.c
index ad1d2962d129..58c97ede9d78 100644
--- a/lib/swiotlb.c
+++ b/lib/swiotlb.c
@@ -591,7 +591,10 @@ void swiotlb_tbl_unmap_single(struct device *hwdev, phys_addr_t tlb_addr,
for (i = index + nslots - 1; i >= index; i--) {
io_tlb_list[i] = ++count;
- io_tlb_orig_addr[i] = INVALID_PHYS_ADDR;
+ if(io_tlb_orig_addr[i] != orig_addr)
+ printk("======size wrong, ally down ally down!===\n");
+ else
+ io_tlb_orig_addr[i] = INVALID_PHYS_ADDR;
* Step 2: merge the returned slots with the preceding slots,

Although pass a right size of DMA buffer is the responsibility of the drivers, but Is it useful to add some size check code to prevent real damage happen?

There doesn't seem to be much good reason for SWIOTLB to be more special than other DMA API backends, and not all of them have enough internal state to be able to make such a check. It's also not necessarily possible to "prevent damage" anyway - if a driver does pass a bogus size for dma_unmap_single(..., DMA_FROM_DEVICE), SWIOTLB might be able to keep itself internally consistent, but it still can't prevent the arch code in the middle from invalidating the wrong cache lines and potentially corrupting adjacent memory.

In short, trying to work around broken drivers is a much worse idea than just fixing those drivers, and that's what we already have dma-debug for.