[PATCH v3 kvm/queue 02/16] 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.

Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
---
 include/uapi/linux/memfd.h |  1 +
 mm/memfd.c                 | 12 +++++++++++-
 2 files changed, 12 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..c898a007fb76 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -245,7 +245,8 @@ 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,
@@ -267,6 +268,10 @@ 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 +320,11 @@ SYSCALL_DEFINE2(memfd_create,
                *file_seals &= ~F_SEAL_SEAL;
        }
 
+       if (flags & MFD_INACCESSIBLE) {
+               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