[PATCH v4 01/12] mm/shmem: Introduce F_SEAL_INACCESSIBLE

[Chao Peng]

From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>

Introduce a new seal F_SEAL_INACCESSIBLE indicating the content of
the file is inaccessible from userspace through ordinary MMU access
(e.g., read/write/mmap). However, the file content can be accessed
via a different mechanism (e.g. KVM MMU) indirectly.

It provides semantics required for KVM guest private memory support
that a file descriptor with this seal set is going to be used as the
source of guest memory in confidential computing environments such
as Intel TDX/AMD SEV but may not be accessible from host userspace.

At this time only shmem implements this seal.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
---
 include/uapi/linux/fcntl.h |  1 +
 mm/shmem.c                 | 40 ++++++++++++++++++++++++++++++++++++--
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 2f86b2ad6d7e..09ef34754dfa 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -43,6 +43,7 @@
 #define F_SEAL_GROW	0x0004	/* prevent file from growing */
 #define F_SEAL_WRITE	0x0008	/* prevent writes */
 #define F_SEAL_FUTURE_WRITE	0x0010  /* prevent future writes while mapped */
+#define F_SEAL_INACCESSIBLE	0x0020  /* prevent ordinary MMU access (e.g. read/write/mmap) to file content */
 /* (1U << 31) is reserved for signed error codes */
 
 /*
diff --git a/mm/shmem.c b/mm/shmem.c
index 18f93c2d68f1..72185630e7c4 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1098,6 +1098,13 @@ static int shmem_setattr(struct user_namespace *mnt_userns,
 		    (newsize > oldsize && (info->seals & F_SEAL_GROW)))
 			return -EPERM;
 
+		if (info->seals & F_SEAL_INACCESSIBLE) {
+			if(i_size_read(inode))
+				return -EPERM;
+			if (newsize & ~PAGE_MASK)
+				return -EINVAL;
+		}
+
 		if (newsize != oldsize) {
 			error = shmem_reacct_size(SHMEM_I(inode)->flags,
 					oldsize, newsize);
@@ -1364,6 +1371,8 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 		goto redirty;
 	if (!total_swap_pages)
 		goto redirty;
+	if (info->seals & F_SEAL_INACCESSIBLE)
+		goto redirty;
 
 	/*
 	 * Our capabilities prevent regular writeback or sync from ever calling
@@ -2262,6 +2271,9 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 	if (ret)
 		return ret;
 
+	if (info->seals & F_SEAL_INACCESSIBLE)
+		return -EPERM;
+
 	/* arm64 - allow memory tagging on RAM-based files */
 	vma->vm_flags |= VM_MTE_ALLOWED;
 
@@ -2459,12 +2471,15 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
 	pgoff_t index = pos >> PAGE_SHIFT;
 
 	/* i_rwsem is held by caller */
-	if (unlikely(info->seals & (F_SEAL_GROW |
-				   F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) {
+	if (unlikely(info->seals & (F_SEAL_GROW | F_SEAL_WRITE |
+				    F_SEAL_FUTURE_WRITE |
+				    F_SEAL_INACCESSIBLE))) {
 		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))
 			return -EPERM;
 		if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
 			return -EPERM;
+		if (info->seals & F_SEAL_INACCESSIBLE)
+			return -EPERM;
 	}
 
 	return shmem_getpage(inode, index, pagep, SGP_WRITE);
@@ -2538,6 +2553,21 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 		end_index = i_size >> PAGE_SHIFT;
 		if (index > end_index)
 			break;
+
+		/*
+		 * inode_lock protects setting up seals as well as write to
+		 * i_size. Setting F_SEAL_INACCESSIBLE only allowed with
+		 * i_size == 0.
+		 *
+		 * Check F_SEAL_INACCESSIBLE after i_size. It effectively
+		 * serialize read vs. setting F_SEAL_INACCESSIBLE without
+		 * taking inode_lock in read path.
+		 */
+		if (SHMEM_I(inode)->seals & F_SEAL_INACCESSIBLE) {
+			error = -EPERM;
+			break;
+		}
+
 		if (index == end_index) {
 			nr = i_size & ~PAGE_MASK;
 			if (nr <= offset)
@@ -2663,6 +2693,12 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 			goto out;
 		}
 
+		if ((info->seals & F_SEAL_INACCESSIBLE) &&
+		    (offset & ~PAGE_MASK || len & ~PAGE_MASK)) {
+			error = -EINVAL;
+			goto out;
+		}
+
 		shmem_falloc.waitq = &shmem_falloc_waitq;
 		shmem_falloc.start = (u64)unmap_start >> PAGE_SHIFT;
 		shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
-- 
2.17.1