[PATCH 4.1 090/159] iommu/io-pgtable-arm: Unmap and free table when overwriting with block

Greg Kroah-Hartman
2015-09-26T21:39:59Z

From: Will Deacon <will.deacon@arm.com>

commit cf27ec930be906e142c752f9161197d69ca534d7 upstream.

When installing a block mapping, we unconditionally overwrite a non-leaf
PTE if we find one. However, this can cause a problem if the following
sequence of events occur:

(1) iommu_map called for a 4k (i.e. PAGE_SIZE) mapping at some address
- We initialise the page table all the way down to a leaf entry
- No TLB maintenance is required, because we're going from invalid
to valid.

(2) iommu_unmap is called on the mapping installed in (1)
- We walk the page table to the final (leaf) entry and zero it
- We only changed a valid leaf entry, so we invalidate leaf-only

(3) iommu_map is called on the same address as (1), but this time for
a 2MB (i.e. BLOCK_SIZE) mapping)
- We walk the page table down to the penultimate level, where we
find a table entry
- We overwrite the table entry with a block mapping and return
without any TLB maintenance and without freeing the memory used
by the now-orphaned table.

This last step can lead to a walk-cache caching the overwritten table
entry, causing unexpected faults when the new mapping is accessed by a
device. One way to fix this would be to collapse the page table when
freeing the last page at a given level, but this would require expensive
iteration on every map call. Instead, this patch detects the case when
we are overwriting a table entry and explicitly unmaps the table first,
which takes care of both freeing and TLB invalidation.

Reported-by: Brian Starkey <brian.starkey@arm.com>
Tested-by: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/iommu/io-pgtable-arm.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)

--- a/drivers/iommu/io-pgtable-arm.c
+++ b/drivers/iommu/io-pgtable-arm.c
@@ -200,6 +200,10 @@ typedef u64 arm_lpae_iopte;

static bool selftest_running = false;

+static int __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,
+ unsigned long iova, size_t size, int lvl,
+ arm_lpae_iopte *ptep);
+
static int arm_lpae_init_pte(struct arm_lpae_io_pgtable *data,
unsigned long iova, phys_addr_t paddr,
arm_lpae_iopte prot, int lvl,
@@ -207,10 +211,21 @@ static int arm_lpae_init_pte(struct arm_
{
arm_lpae_iopte pte = prot;

- /* We require an unmap first */
if (iopte_leaf(*ptep, lvl)) {
+ /* We require an unmap first */
WARN_ON(!selftest_running);
return -EEXIST;
+ } else if (iopte_type(*ptep, lvl) == ARM_LPAE_PTE_TYPE_TABLE) {
+ /*
+ * We need to unmap and free the old table before
+ * overwriting it with a block entry.
+ */
+ arm_lpae_iopte *tblp;
+ size_t sz = ARM_LPAE_BLOCK_SIZE(lvl, data);
+
+ tblp = ptep - ARM_LPAE_LVL_IDX(iova, lvl, data);
+ if (WARN_ON(__arm_lpae_unmap(data, iova, sz, lvl, tblp) != sz))
+ return -EINVAL;
}

if (data->iop.cfg.quirks & IO_PGTABLE_QUIRK_ARM_NS)