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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | SEE ALSO |
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memfd_secret(2) System Calls Manual memfd_secret(2)
memfd_secret - create an anonymous RAM-based file to access
secret memory regions
Standard C library (libc, -lc)
#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).
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 using ftruncate(2).
The memory areas backing the file created with memfd_secret(2)
are visible 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 corresponding physical memory. (Thus, the
pages in the region can't be accessed by the kernel itself, so
that, for example, pointers to the region can't be passed to
system calls.)
The following values may be bitwise ORed in flags to control the
behavior 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 descriptor.
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 descriptor is inherited by the child produced by fork(2) and
refers to the same file. The file descriptor is preserved across
execve(2), unless 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 allocated 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.
On success, memfd_secret() returns a new file descriptor. On
error, -1 is returned and errno is set to indicate the error.
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_enable=1.
Linux.
Linux 5.14.
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 circumstances, 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 attack 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 difficulty of the
attack, especially when other protections like the kernel
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 accidentally 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
overall system performance, therefore the system call is disabled
by default 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_secret() from a hybernation image, hybernation is prevented
when there are active memfd_secret() users.
fcntl(2), ftruncate(2), mlock(2), memfd_create(2), mmap(2),
setrlimit(2)
Linux man-pages (unreleased) (date) memfd_secret(2)
Pages that refer to this page: madvise(2), memfd_create(2), memfd_secret(2), syscalls(2)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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