[PATCH] cxl: Fix possible deadlock when processing page faults from cxllib

[Frederic Barrat]

cxllib_handle_fault() is called by an external driver when it needs to
have the host process page faults for a buffer which may cover several
pages. Currently the function holds the mm->mmap_sem semaphore with
read access while iterating over the buffer, since it could spawn
several VMAs. When calling a lower-level function to handle the page
fault for a single page, the semaphore is accessed again in read
mode. That is wrong and can lead to deadlocks if a writer tries to
sneak in while a buffer of several pages is being processed.

The fix is to release the semaphore once cxllib_handle_fault() got the
information it needs from the current vma. The address space/VMAs
could evolve while we iterate over the full buffer, but in the
unlikely case where we miss a page, the driver will raise a new page
fault when retrying.

Fixes: 3ced8d730063 ("cxl: Export library to support IBM XSL")
Cc: stable at vger.kernel.org # 4.13+
Signed-off-by: Frederic Barrat <fbarrat at linux.vnet.ibm.com>
---
 drivers/misc/cxl/cxllib.c | 85 ++++++++++++++++++++++++++++++-----------------
 1 file changed, 55 insertions(+), 30 deletions(-)

diff --git a/drivers/misc/cxl/cxllib.c b/drivers/misc/cxl/cxllib.c
index 30ccba436b3b..55cd35d1a9cc 100644
--- a/drivers/misc/cxl/cxllib.c
+++ b/drivers/misc/cxl/cxllib.c
@@ -208,49 +208,74 @@ int cxllib_get_PE_attributes(struct task_struct *task,
 }
 EXPORT_SYMBOL_GPL(cxllib_get_PE_attributes);
 
-int cxllib_handle_fault(struct mm_struct *mm, u64 addr, u64 size, u64 flags)
+static int get_vma_info(struct mm_struct *mm, u64 addr,
+			u64 *vma_start, u64 *vma_end,
+			unsigned long *page_size)
 {
-	int rc;
-	u64 dar;
 	struct vm_area_struct *vma = NULL;
-	unsigned long page_size;
-
-	if (mm == NULL)
-		return -EFAULT;
+	int rc = 0;
 
 	down_read(&mm->mmap_sem);
 
 	vma = find_vma(mm, addr);
 	if (!vma) {
-		pr_err("Can't find vma for addr %016llx\n", addr);
 		rc = -EFAULT;
 		goto out;
 	}
-	/* get the size of the pages allocated */
-	page_size = vma_kernel_pagesize(vma);
-
-	for (dar = (addr & ~(page_size - 1)); dar < (addr + size); dar += page_size) {
-		if (dar < vma->vm_start || dar >= vma->vm_end) {
-			vma = find_vma(mm, addr);
-			if (!vma) {
-				pr_err("Can't find vma for addr %016llx\n", addr);
-				rc = -EFAULT;
-				goto out;
-			}
-			/* get the size of the pages allocated */
-			page_size = vma_kernel_pagesize(vma);
+	*page_size = vma_kernel_pagesize(vma);
+	*vma_start = vma->vm_start;
+	*vma_end = vma->vm_end;
+out:
+	up_read(&mm->mmap_sem);
+	return rc;
+}
+
+int cxllib_handle_fault(struct mm_struct *mm, u64 addr, u64 size, u64 flags)
+{
+	int rc;
+	u64 dar, vma_start, vma_end;
+	unsigned long page_size;
+
+	if (mm == NULL)
+		return -EFAULT;
+
+	/*
+	 * The buffer we have to process can extend over several pages
+	 * and may also cover several VMAs.
+	 * We iterate over all the pages. The page size could vary
+	 * between VMAs.
+	 */
+	rc = get_vma_info(mm, addr, &vma_start, &vma_end, &page_size);
+	if (rc)
+		return rc;
+
+	for (dar = (addr & ~(page_size - 1)); dar < (addr + size);
+	     dar += page_size) {
+		if (dar < vma_start || dar >= vma_end) {
+			/*
+			 * We don't hold the mm->mmap_sem semaphore
+			 * while iterating, since the semaphore is
+			 * required by one of the lower-level page
+			 * fault processing functions and it could
+			 * create a deadlock.
+			 *
+			 * It means the VMAs can be altered between 2
+			 * loop iterations and we could theoretically
+			 * miss a page (however unlikely). But that's
+			 * not really a problem, as the driver will
+			 * retry access, get another page fault on the
+			 * missing page and call us again.
+			 */
+			rc = get_vma_info(mm, dar, &vma_start, &vma_end,
+					&page_size);
+			if (rc)
+				return rc;
 		}
 
 		rc = cxl_handle_mm_fault(mm, flags, dar);
-		if (rc) {
-			pr_err("cxl_handle_mm_fault failed %d", rc);
-			rc = -EFAULT;
-			goto out;
-		}
+		if (rc)
+			return -EFAULT;
 	}
-	rc = 0;
-out:
-	up_read(&mm->mmap_sem);
-	return rc;
+	return 0;
 }
 EXPORT_SYMBOL_GPL(cxllib_handle_fault);
-- 
2.14.1