[PATCH v3 2/3] hv: vmbus: Add vmbus_alloc_buffer()/vmbus_free_buffer() for CoCo VMs

[Kameron Carr <kameroncarr@linux.microsoft.com>]

On CoCo VMs without confidential VMBus, the netvsc send and receive buffers
must be made host-visible by decrypting them. These buffers are vmalloc'ed,
but set_memory_decrypted()/encrypted() do not work on vmalloc'ed memory.
This use case is (so far) unique to netvsc, so solve it locally rather than
changing the set_memory() or allocation APIs.

Add vmbus_alloc_buffer()/vmbus_free_buffer() to the VMBus core. When the
guest's isolation model requires it, allocate the buffer as a list of
physically-contiguous chunks via alloc_pages_node(), starting at
MAX_PAGE_ORDER and falling back to smaller orders so the allocation still
succeeds under memory fragmentation. Each chunk is decrypted in place via
set_memory_decrypted() on its direct-map address, and the chunks are then
stitched into a single virtually-contiguous range with vmap(). Buffers that
do not need decryption keep using vzalloc().

To free the buffer, vmbus_free_buffer() calls vunmap() on the range then
re-encrypts and frees each chunk individually; any chunk that fails
re-encryption is leaked to prevent accidentally freeing decrypted memory.

This approach minimizes scattering of decrypted 4 KiB pages through the
kernel direct map and the resulting shattering of large page mappings.

Signed-off-by: Kameron Carr <kameroncarr@linux.microsoft.com>
---
 drivers/hv/channel.c   | 155 +++++++++++++++++++++++++++++++++++++++++
 include/linux/hyperv.h |   7 ++
 2 files changed, 162 insertions(+)

diff --git a/drivers/hv/channel.c b/drivers/hv/channel.c
index 4782f5070bba9..f4370617deacb 100644
--- a/drivers/hv/channel.c
+++ b/drivers/hv/channel.c
@@ -13,11 +13,13 @@
 #include <linux/wait.h>
 #include <linux/mm.h>
 #include <linux/slab.h>
+#include <linux/log2.h>
 #include <linux/module.h>
 #include <linux/hyperv.h>
 #include <linux/uio.h>
 #include <linux/interrupt.h>
 #include <linux/set_memory.h>
+#include <linux/vmalloc.h>
 #include <linux/export.h>
 #include <asm/page.h>
 #include <asm/mshyperv.h>
@@ -608,6 +610,159 @@ int vmbus_establish_gpadl_caller_decrypted(struct vmbus_channel *channel,
 }
 EXPORT_SYMBOL_GPL(vmbus_establish_gpadl_caller_decrypted);
 
+/**
+ * vmbus_free_buffer - release a buffer allocated by vmbus_alloc_buffer().
+ *
+ * @addr: buffer address, or NULL if none was allocated (e.g. cleanup from a
+ *        failed allocation)
+ * @chunks: chunks array from vmbus_alloc_buffer(), or NULL
+ * @chunk_cnt: number of entries in @chunks
+ *
+ * When @chunks is NULL the buffer is a plain vzalloc() allocation.
+ *
+ * Otherwise tear down the vmap, and for each chunk re-encrypt and free
+ * the underlying pages. Any chunk that cannot be re-encrypted is leaked.
+ */
+void vmbus_free_buffer(void *addr, struct page **chunks, u32 chunk_cnt)
+{
+	u32 i;
+
+	if (!chunks) {
+		vfree(addr);
+		return;
+	}
+
+	vunmap(addr);
+
+	for (i = 0; i < chunk_cnt; i++) {
+		unsigned long vaddr =
+			(unsigned long)page_address(chunks[i]);
+		unsigned int order = folio_order(page_folio(chunks[i]));
+
+		if (set_memory_encrypted(vaddr, 1U << order))
+			continue;
+		__free_pages(chunks[i], order);
+	}
+
+	kvfree(chunks);
+}
+EXPORT_SYMBOL_GPL(vmbus_free_buffer);
+
+/**
+ * vmbus_alloc_buffer - allocate a host-visible, virtually-contiguous buffer.
+ *
+ * @channel: the channel the buffer will be attached to
+ * @size: requested buffer size in bytes (will be rounded up to PAGE_SIZE)
+ * @chunks_out: on success, set to the array of underlying chunks, or NULL when
+ *              the buffer was allocated with vzalloc()
+ * @chunk_cnt_out: on success, set to the number of chunks
+ *
+ * Buffers not requiring decryption are allocated with vzalloc().
+ *
+ * Buffers requiring decryption are allocated as a series of
+ * physically-contiguous chunks, starting at MAX_PAGE_ORDER and falling back to
+ * smaller orders on allocation failure. Each chunk is transitioned to
+ * host-visible via set_memory_decrypted() on its direct-map address, then all
+ * chunks are combined into a virtually-contiguous range via vmap().
+ *
+ * Return: the buffer's virtual address, or NULL on failure.
+ */
+void *vmbus_alloc_buffer(struct vmbus_channel *channel,
+			 u32 size,
+			 struct page ***chunks_out,
+			 u32 *chunk_cnt_out)
+{
+	unsigned long nr_pages = PFN_UP(size);
+	unsigned long remaining = nr_pages;
+	unsigned long page_idx = 0;
+	struct page **chunks = NULL;
+	struct page **pages = NULL;
+	int order = MAX_PAGE_ORDER;
+	u32 chunk_cnt = 0;
+	void *addr;
+	u32 i;
+	int ret;
+
+	*chunks_out = NULL;
+	*chunk_cnt_out = 0;
+
+	if (!nr_pages)
+		return NULL;
+
+	/* If the buffer does not need to be decrypted, just use vzalloc() */
+	if (!hv_is_isolation_supported() || channel->co_external_memory)
+		return vzalloc(nr_pages << PAGE_SHIFT);
+
+	/* Worst case: every chunk is a single page. */
+	chunks = kvmalloc_array(nr_pages, sizeof(*chunks),
+				GFP_KERNEL | __GFP_ZERO);
+	if (!chunks)
+		goto err;
+
+	pages = kvmalloc_array(nr_pages, sizeof(*pages), GFP_KERNEL);
+	if (!pages)
+		goto err;
+
+	while (remaining) {
+		struct page *page;
+		gfp_t gfp;
+
+		order = min(order, ilog2(remaining));
+
+		/*
+		 * Use __GFP_NORETRY | __GFP_NOWARN to avoid OOM-killing,
+		 * but try harder at order 0 since that is the final
+		 * fallback.
+		 * __GFP_COMP stores order information in the page folio.
+		 */
+		gfp = GFP_KERNEL | __GFP_ZERO;
+		if (order)
+			gfp |= __GFP_COMP | __GFP_NORETRY | __GFP_NOWARN;
+
+		page = alloc_pages_node(cpu_to_node(channel->target_cpu),
+					gfp, order);
+		if (!page) {
+			if (!order--)
+				goto err;
+			continue;
+		}
+
+		ret = set_memory_decrypted((unsigned long)page_address(page),
+					   1U << order);
+		if (ret) {
+			/*
+			 * set_memory_decrypted() failed; the page state is
+			 * unknown so it must be leaked rather than freed.
+			 */
+			goto err;
+		}
+
+		chunks[chunk_cnt++] = page;
+
+		for (i = 0; i < (1U << order); i++)
+			pages[page_idx++] = page + i;
+
+		remaining -= 1U << order;
+	}
+
+	addr = vmap(pages, nr_pages, VM_MAP, pgprot_decrypted(PAGE_KERNEL));
+	if (!addr)
+		goto err;
+
+	memset(addr, 0, nr_pages << PAGE_SHIFT);
+
+	kvfree(pages);
+	*chunks_out = chunks;
+	*chunk_cnt_out = chunk_cnt;
+	return addr;
+
+err:
+	kvfree(pages);
+	vmbus_free_buffer(NULL, chunks, chunk_cnt);
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(vmbus_alloc_buffer);
+
 /**
  * request_arr_init - Allocates memory for the requestor array. Each slot
  * keeps track of the next available slot in the array. Initially, each
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 1146addbb42c4..f843ee0efa22f 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -1214,6 +1214,13 @@ extern int vmbus_establish_gpadl_caller_decrypted(struct vmbus_channel *channel,
 extern int vmbus_teardown_gpadl(struct vmbus_channel *channel,
 				     struct vmbus_gpadl *gpadl);
 
+extern void *vmbus_alloc_buffer(struct vmbus_channel *channel,
+				u32 size,
+				struct page ***chunks_out,
+				u32 *chunk_cnt_out);
+
+extern void vmbus_free_buffer(void *addr, struct page **chunks, u32 chunk_cnt);
+
 void vmbus_reset_channel_cb(struct vmbus_channel *channel);
 
 extern int vmbus_recvpacket(struct vmbus_channel *channel,
-- 
2.45.4