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