linux-kernel - [PATCH v3] iommu/vt-d: calculate mask for non-aligned flushes

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Message-Id: <20220401022430.1262215-1-stevensd@google.com>
Date:   Fri,  1 Apr 2022 11:24:30 +0900
From:   David Stevens <stevensd@...omium.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Kevin Tian <kevin.tian@...el.com>
Cc:     Tina Zhang <tina.zhang@...el.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        David Stevens <stevensd@...omium.org>, stable@...r.kernel.org
Subject: [PATCH v3] iommu/vt-d: calculate mask for non-aligned flushes

From: David Stevens <stevensd@...omium.org>

Calculate the appropriate mask for non-size-aligned page selective
invalidation. Since psi uses the mask value to mask out the lower order
bits of the target address, properly flushing the iotlb requires using a
mask value such that [pfn, pfn+pages) all lie within the flushed
size-aligned region.  This is not normally an issue because iova.c
always allocates iovas that are aligned to their size. However, iovas
which come from other sources (e.g. userspace via VFIO) may not be
aligned.

To properly flush the IOTLB, both the start and end pfns need to be
equal after applying the mask. That means that the most efficient mask
to use is the index of the lowest bit that is equal where all higher
bits are also equal. For example, if pfn=0x17f and pages=3, then
end_pfn=0x181, so the smallest mask we can use is 8. Any differences
above the highest bit of pages are due to carrying, so by xnor'ing pfn
and end_pfn and then masking out the lower order bits based on pages, we
get 0xffffff00, where the first set bit is the mask we want to use.

Fixes: 6fe1010d6d9c ("vfio/type1: DMA unmap chunking")
Cc: stable@...r.kernel.org
Signed-off-by: David Stevens <stevensd@...omium.org>
Reviewed-by: Kevin Tian <kevin.tian@...el.com>
---
The seeds of the bug were introduced by f76aec76ec7f6, which
simultaniously added the alignement requirement to the iommu driver and
made the iova allocator return aligned iovas. However, I don't think
there was any way to trigger the bug at that time. The tagged VFIO
change is one that actually introduced a code path that could trigger
the bug. There may also be other ways to trigger the bug that I am not
aware of.

v1 -> v2:
 - Calculate an appropriate mask for non-size-aligned iovas instead
   of falling back to domain selective flush.
v2 -> v3:
 - Add more detail to commit message.

 drivers/iommu/intel/iommu.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 5b196cfe9ed2..ab2273300346 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -1717,7 +1717,8 @@ static void iommu_flush_iotlb_psi(struct intel_iommu *iommu,
 				  unsigned long pfn, unsigned int pages,
 				  int ih, int map)
 {
-	unsigned int mask = ilog2(__roundup_pow_of_two(pages));
+	unsigned int aligned_pages = __roundup_pow_of_two(pages);
+	unsigned int mask = ilog2(aligned_pages);
 	uint64_t addr = (uint64_t)pfn << VTD_PAGE_SHIFT;
 	u16 did = domain->iommu_did[iommu->seq_id];
 
@@ -1729,10 +1730,30 @@ static void iommu_flush_iotlb_psi(struct intel_iommu *iommu,
 	if (domain_use_first_level(domain)) {
 		domain_flush_piotlb(iommu, domain, addr, pages, ih);
 	} else {
+		unsigned long bitmask = aligned_pages - 1;
+
+		/*
+		 * PSI masks the low order bits of the base address. If the
+		 * address isn't aligned to the mask, then compute a mask value
+		 * needed to ensure the target range is flushed.
+		 */
+		if (unlikely(bitmask & pfn)) {
+			unsigned long end_pfn = pfn + pages - 1, shared_bits;
+
+			/*
+			 * Since end_pfn <= pfn + bitmask, the only way bits
+			 * higher than bitmask can differ in pfn and end_pfn is
+			 * by carrying. This means after masking out bitmask,
+			 * high bits starting with the first set bit in
+			 * shared_bits are all equal in both pfn and end_pfn.
+			 */
+			shared_bits = ~(pfn ^ end_pfn) & ~bitmask;
+			mask = shared_bits ? __ffs(shared_bits) : BITS_PER_LONG;
+		}
+
 		/*
 		 * Fallback to domain selective flush if no PSI support or
-		 * the size is too big. PSI requires page size to be 2 ^ x,
-		 * and the base address is naturally aligned to the size.
+		 * the size is too big.
 		 */
 		if (!cap_pgsel_inv(iommu->cap) ||
 		    mask > cap_max_amask_val(iommu->cap))
-- 
2.35.1.1094.g7c7d902a7c-goog