Introduce a new memfd_create() flag indicating the content of the
created memfd 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 flag 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.
The flag can not coexist with MFD_ALLOW_SEALING, future sealing is
also impossible for a memfd created with this flag.
Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
---
include/uapi/linux/memfd.h | 1 +
mm/memfd.c | 15 ++++++++++++++-
2 files changed, 15 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 2afd898798e4..72d7139ccced 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -18,6 +18,7 @@
#include <linux/hugetlb.h>
#include <linux/shmem_fs.h>
#include <linux/memfd.h>
+#include <linux/memfile_notifier.h>
#include <uapi/linux/memfd.h>
/*
@@ -262,7 +263,8 @@ 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,
@@ -284,6 +286,10 @@ 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)
@@ -330,12 +336,19 @@ SYSCALL_DEFINE2(memfd_create,
if (flags & MFD_ALLOW_SEALING) {
file_seals = memfd_file_seals_ptr(file);
*file_seals &= ~F_SEAL_SEAL;
+ } else if (flags & MFD_INACCESSIBLE) {
+ error = memfile_node_set_flags(file,
+ MEMFILE_F_USER_INACCESSIBLE);
+ if (error)
+ goto err_file;
}
fd_install(fd, file);
kfree(name);
return fd;
+err_file:
+ fput(file);
err_fd:
put_unused_fd(fd);
err_name:
--
2.25.1
