[PATCH v4 02/12] mm/memfd: Introduce MFD_INACCESSIBLE flag

[Chao Peng]

Introduce a new memfd_create() flag indicating the content of the
created memfd is inaccessible from userspace. It does this by force
setting F_SEAL_INACCESSIBLE seal when the file is created. It also set
F_SEAL_SEAL to prevent future sealing, which means, it can not coexist
with MFD_ALLOW_SEALING.

The pages backed by such memfd will be used as guest private memory in
confidential computing environments such as Intel TDX/AMD SEV. Since
page migration/swapping is not yet supported for such usages so these
pages are currently marked as UNMOVABLE and UNEVICTABLE which makes
them behave like long-term pinned pages.

Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
---
 include/uapi/linux/memfd.h |  1 +
 mm/memfd.c                 | 20 +++++++++++++++++++-
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/memfd.h b/include/uapi/linux/memfd.h
index 7a8a26751c23..48750474b904 100644
--- a/include/uapi/linux/memfd.h
+++ b/include/uapi/linux/memfd.h
@@ -8,6 +8,7 @@
 #define MFD_CLOEXEC		0x0001U
 #define MFD_ALLOW_SEALING	0x0002U
 #define MFD_HUGETLB		0x0004U
+#define MFD_INACCESSIBLE	0x0008U
 
 /*
  * Huge page size encoding when MFD_HUGETLB is specified, and a huge page
diff --git a/mm/memfd.c b/mm/memfd.c
index 9f80f162791a..26998d96dc11 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -245,16 +245,19 @@ long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
 
-#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
+#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | \
+		       MFD_INACCESSIBLE)
 
 SYSCALL_DEFINE2(memfd_create,
 		const char __user *, uname,
 		unsigned int, flags)
 {
+	struct address_space *mapping;
 	unsigned int *file_seals;
 	struct file *file;
 	int fd, error;
 	char *name;
+	gfp_t gfp;
 	long len;
 
 	if (!(flags & MFD_HUGETLB)) {
@@ -267,6 +270,10 @@ SYSCALL_DEFINE2(memfd_create,
 			return -EINVAL;
 	}
 
+	/* Disallow sealing when MFD_INACCESSIBLE is set. */
+	if (flags & MFD_INACCESSIBLE && flags & MFD_ALLOW_SEALING)
+		return -EINVAL;
+
 	/* length includes terminating zero */
 	len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
 	if (len <= 0)
@@ -315,6 +322,17 @@ SYSCALL_DEFINE2(memfd_create,
 		*file_seals &= ~F_SEAL_SEAL;
 	}
 
+	if (flags & MFD_INACCESSIBLE) {
+		mapping = file_inode(file)->i_mapping;
+		gfp = mapping_gfp_mask(mapping);
+		gfp &= ~__GFP_MOVABLE;
+		mapping_set_gfp_mask(mapping, gfp);
+		mapping_set_unevictable(mapping);
+
+		file_seals = memfd_file_seals_ptr(file);
+		*file_seals &= F_SEAL_SEAL | F_SEAL_INACCESSIBLE;
+	}
+
 	fd_install(fd, file);
 	kfree(name);
 	return fd;
-- 
2.17.1