iommufd/selftest: Fix iommufd_dirty_tracking with large hugepage sizes

The hugepage test cases of iommufd_dirty_tracking have the 64MB and 128MB
coverages. Both of them are smaller than the default hugepage size 512MB,
when CONFIG_PAGE_SIZE_64KB=y. However, these test cases have a variant of
using huge pages, which would mmap(MAP_HUGETLB) using these smaller sizes
than the system hugepag size. This results in the kernel aligning up the
smaller size to 512MB. If a memory was located between the upper 64/128MB
size boundary and the hugepage 512MB boundary, it would get wiped out:
https://lore.kernel.org/all/aEoUhPYIAizTLADq@nvidia.com/

Given that this aligning up behavior is well documented, we have no choice
but to allocate a hugepage aligned size to avoid this unintended wipe out.
Instead of relying on the kernel's internal force alignment, pass the same
size to posix_memalign() and map().

Also, fix the FIXTURE_TEARDOWN() misusing munmap() to free the memory from
posix_memalign(), as munmap() doesn't destroy the allocator meta data. So,
call free() instead.

Fixes: a9af47e ("iommufd/selftest: Test IOMMU_HWPT_GET_DIRTY_BITMAP")
Link: https://patch.msgid.link/r/1ea8609ae6d523fdd4d8efb179ddee79c8582cb6.1750787928.git.nicolinc@nvidia.com
Cc: stable@vger.kernel.org
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Author: nicolinc

tools/testing/selftests/iommu/iommufd.c

@@ -2008,6 +2008,7 @@ FIXTURE_VARIANT(iommufd_dirty_tracking)
 
 FIXTURE_SETUP(iommufd_dirty_tracking)
 {
+	size_t mmap_buffer_size;
 	unsigned long size;
 	int mmap_flags;
 	void *vrc;
@@ -2022,22 +2023,33 @@ FIXTURE_SETUP(iommufd_dirty_tracking)
 	self->fd = open("/dev/iommu", O_RDWR);
 	ASSERT_NE(-1, self->fd);
 
-	rc = posix_memalign(&self->buffer, HUGEPAGE_SIZE, variant->buffer_size);
-	if (rc || !self->buffer) {
-		SKIP(return, "Skipping buffer_size=%lu due to errno=%d",
-			   variant->buffer_size, rc);
-	}
-
 	mmap_flags = MAP_SHARED | MAP_ANONYMOUS | MAP_FIXED;
+	mmap_buffer_size = variant->buffer_size;
 	if (variant->hugepages) {
 		/*
 		 * MAP_POPULATE will cause the kernel to fail mmap if THPs are
 		 * not available.
 		 */
 		mmap_flags |= MAP_HUGETLB | MAP_POPULATE;
+
+		/*
+		 * Allocation must be aligned to the HUGEPAGE_SIZE, because the
+		 * following mmap() will automatically align the length to be a
+		 * multiple of the underlying huge page size. Failing to do the
+		 * same at this allocation will result in a memory overwrite by
+		 * the mmap().
+		 */
+		if (mmap_buffer_size < HUGEPAGE_SIZE)
+			mmap_buffer_size = HUGEPAGE_SIZE;
+	}
+
+	rc = posix_memalign(&self->buffer, HUGEPAGE_SIZE, mmap_buffer_size);
+	if (rc || !self->buffer) {
+		SKIP(return, "Skipping buffer_size=%lu due to errno=%d",
+			   mmap_buffer_size, rc);
 	}
 	assert((uintptr_t)self->buffer % HUGEPAGE_SIZE == 0);
-	vrc = mmap(self->buffer, variant->buffer_size, PROT_READ | PROT_WRITE,
+	vrc = mmap(self->buffer, mmap_buffer_size, PROT_READ | PROT_WRITE,
 		   mmap_flags, -1, 0);
 	assert(vrc == self->buffer);
 
@@ -2066,8 +2078,8 @@ FIXTURE_SETUP(iommufd_dirty_tracking)
 
 FIXTURE_TEARDOWN(iommufd_dirty_tracking)
 {
-	munmap(self->buffer, variant->buffer_size);
-	munmap(self->bitmap, DIV_ROUND_UP(self->bitmap_size, BITS_PER_BYTE));
+	free(self->buffer);
+	free(self->bitmap);
 	teardown_iommufd(self->fd, _metadata);
 }