memfd_secret(2) System Calls Manual memfd_secret(2)
NAME
memfd_secret - create an anonymous RAM-based file to access secret mem-
ory regions
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_memfd_secret, unsigned int flags);
Note: glibc provides no wrapper for memfd_secret(), necessitating the
use of syscall(2).
DESCRIPTION
memfd_secret() creates an anonymous RAM-based file and returns a file
descriptor that refers to it. The file provides a way to create and
access memory regions with stronger protection than usual RAM-based
files and anonymous memory mappings. Once all open references to the
file are closed, it is automatically released. The initial size of the
file is set to 0. Following the call, the file size should be set us-
ing ftruncate(2).
The memory areas backing the file created with memfd_secret(2) are vis-
ible only to the processes that have access to the file descriptor.
The memory region is removed from the kernel page tables and only the
page tables of the processes holding the file descriptor map the corre-
sponding physical memory. (Thus, the pages in the region can't be ac-
cessed by the kernel itself, so that, for example, pointers to the re-
gion can't be passed to system calls.)
The following values may be bitwise ORed in flags to control the behav-
ior of memfd_secret():
FD_CLOEXEC
Set the close-on-exec flag on the new file descriptor, which
causes the region to be removed from the process on execve(2).
See the description of the O_CLOEXEC flag in open(2)
As its return value, memfd_secret() returns a new file descriptor that
refers to an anonymous file. This file descriptor is opened for both
reading and writing (O_RDWR) and O_LARGEFILE is set for the file de-
scriptor.
With respect to fork(2) and execve(2), the usual semantics apply for
the file descriptor created by memfd_secret(). A copy of the file de-
scriptor is inherited by the child produced by fork(2) and refers to
the same file. The file descriptor is preserved across execve(2), un-
less the close-on-exec flag has been set.
The memory region is locked into memory in the same way as with
mlock(2), so that it will never be written into swap, and hibernation
is inhibited for as long as any memfd_secret() descriptions exist.
However the implementation of memfd_secret() will not try to populate
the whole range during the mmap(2) call that attaches the region into
the process's address space; instead, the pages are only actually allo-
cated as they are faulted in. The amount of memory allowed for memory
mappings of the file descriptor obeys the same rules as mlock(2) and
cannot exceed RLIMIT_MEMLOCK.
RETURN VALUE
On success, memfd_secret() returns a new file descriptor. On error, -1
is returned and errno is set to indicate the error.
ERRORS
EINVAL flags included unknown bits.
EMFILE The per-process limit on the number of open file descriptors has
been reached.
EMFILE The system-wide limit on the total number of open files has been
reached.
ENOMEM There was insufficient memory to create a new anonymous file.
ENOSYS memfd_secret() is not implemented on this architecture, or has
not been enabled on the kernel command-line with secretmem_en-
able=1.
VERSIONS
The memfd_secret() system call first appeared in Linux 5.14.
STANDARDS
The memfd_secret() system call is Linux-specific.
NOTES
The memfd_secret() system call is designed to allow a user-space
process to create a range of memory that is inaccessible to anybody
else - kernel included. There is no 100% guarantee that kernel won't
be able to access memory ranges backed by memfd_secret() in any circum-
stances, but nevertheless, it is much harder to exfiltrate data from
these regions.
memfd_secret() provides the following protections:
• Enhanced protection (in conjunction with all the other in-kernel at-
tack prevention systems) against ROP attacks. Absence of any in-
kernel primitive for accessing memory backed by memfd_secret() means
that one-gadget ROP attack can't work to perform data exfiltration.
The attacker would need to find enough ROP gadgets to reconstruct
the missing page table entries, which significantly increases diffi-
culty of the attack, especially when other protections like the ker-
nel stack size limit and address space layout randomization are in
place.
• Prevent cross-process user-space memory exposures. Once a region
for a memfd_secret() memory mapping is allocated, the user can't ac-
cidentally pass it into the kernel to be transmitted somewhere. The
memory pages in this region cannot be accessed via the direct map
and they are disallowed in get_user_pages.
• Harden against exploited kernel flaws. In order to access memory
areas backed by memfd_secret(), a kernel-side attack would need to
either walk the page tables and create new ones, or spawn a new
privileged user-space process to perform secrets exfiltration using
ptrace(2).
The way memfd_secret() allocates and locks the memory may impact over-
all system performance, therefore the system call is disabled by de-
fault and only available if the system administrator turned it on using
"secretmem.enable=y" kernel parameter.
To prevent potential data leaks of memory regions backed by memfd_se-
cret() from a hybernation image, hybernation is prevented when there
are active memfd_secret() users.
SEE ALSO
fcntl(2), ftruncate(2), mlock(2), memfd_create(2), mmap(2), setr-
limit(2)
Linux man-pages 6.03 2023-02-05 memfd_secret(2)
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