[PATCH RFC v3 30/35] arm64: mte: ptrace: Handle pages with missing tag storage

[Alexandru Elisei]

A page can end up mapped in a MTE enabled VMA without the corresponding tag
storage block reserved. Tag accesses made by ptrace in this case can lead
to the wrong tags being read or memory corruption for the process that is
using the tag storage memory as data.

Reserve tag storage by treating ptrace accesses like a fault.

Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
---

Changes since rfc v2:

* New patch, issue reported by Peter Collingbourne.

 arch/arm64/kernel/mte.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index faf09da3400a..b1fa02dad4fd 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -412,10 +412,13 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
 	while (len) {
 		struct vm_area_struct *vma;
 		unsigned long tags, offset;
+		unsigned int fault_flags;
+		struct page *page;
+		vm_fault_t ret;
 		void *maddr;
-		struct page *page = get_user_page_vma_remote(mm, addr,
-							     gup_flags, &vma);
 
+get_page:
+		page = get_user_page_vma_remote(mm, addr, gup_flags, &vma);
 		if (IS_ERR(page)) {
 			err = PTR_ERR(page);
 			break;
@@ -433,6 +436,25 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
 			put_page(page);
 			break;
 		}
+
+		if (tag_storage_enabled() && !page_tag_storage_reserved(page)) {
+			fault_flags = FAULT_FLAG_DEFAULT | \
+				      FAULT_FLAG_USER | \
+				      FAULT_FLAG_REMOTE | \
+				      FAULT_FLAG_ALLOW_RETRY | \
+				      FAULT_FLAG_RETRY_NOWAIT;
+			if (write)
+				fault_flags |= FAULT_FLAG_WRITE;
+
+			put_page(page);
+			ret = handle_mm_fault(vma, addr, fault_flags, NULL);
+			if (ret & VM_FAULT_ERROR) {
+				err = -EFAULT;
+				break;
+			}
+			goto get_page;
+		}
+
 		WARN_ON_ONCE(!page_mte_tagged(page));
 
 		/* limit access to the end of the page */
-- 
2.43.0