[PATCH v3 01/15] mm/shmem: Introduce F_SEAL_INACCESSIBLE

[Chao Peng <chao.p.peng@linux.intel.com>]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

Introduce a new seal F_SEAL_INACCESSIBLE indicating the content of
the file is inaccessible from userspace in any possible ways like
read(),write() or mmap() etc.

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@linux.intel.com>
Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
---
 include/uapi/linux/fcntl.h |  1 +
 mm/shmem.c                 | 37 +++++++++++++++++++++++++++++++++++--
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 2f86b2ad6d7e..e2bad051936f 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 file from accessing */
 /* (1U << 31) is reserved for signed error codes */
 
 /*
diff --git a/mm/shmem.c b/mm/shmem.c
index 18f93c2d68f1..faa7e9b1b9bc 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1098,6 +1098,10 @@ static int shmem_setattr(struct user_namespace *mnt_userns,
 		    (newsize > oldsize && (info->seals & F_SEAL_GROW)))
 			return -EPERM;
 
+		if ((info->seals & F_SEAL_INACCESSIBLE) &&
+		    (newsize & ~PAGE_MASK))
+			return -EINVAL;
+
 		if (newsize != oldsize) {
 			error = shmem_reacct_size(SHMEM_I(inode)->flags,
 					oldsize, newsize);
@@ -1364,6 +1368,8 @@ 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 +2268,9 @@ 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 +2468,15 @@ 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 +2550,21 @@ 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 +2690,12 @@ 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