From: Mike Rapoport rppt@linux.ibm.com
Signed-off-by: Mike Rapoport rppt@linux.ibm.com Cowritten-by: Alejandro Colomar alx.manpages@gmail.com Acked-by: Alejandro Colomar alx.manpages@gmail.com Signed-off-by: Alejandro Colomar alx.manpages@gmail.com ---
Hi Mike,
I added that note about not having a wrapper, fixed a few minor formatting and wording issues, and sorted ERRORS alphabetically.
Cheers,
Alex
man2/memfd_secret.2 | 178 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 178 insertions(+) create mode 100644 man2/memfd_secret.2
diff --git a/man2/memfd_secret.2 b/man2/memfd_secret.2 new file mode 100644 index 000000000..4e617aa0e --- /dev/null +++ b/man2/memfd_secret.2 @@ -0,0 +1,178 @@ +." Copyright (c) 2020, IBM Corporation. +." Written by Mike Rapoport rppt@linux.ibm.com +." +." Based on memfd_create(2) man page +." Copyright (C) 2014 Michael Kerrisk mtk.manpages@gmail.com +." and Copyright (C) 2014 David Herrmann dh.herrmann@gmail.com +." +." %%%LICENSE_START(GPLv2+) +." +." This program is free software; you can redistribute it and/or modify +." it under the terms of the GNU General Public License as published by +." the Free Software Foundation; either version 2 of the License, or +." (at your option) any later version. +." +." This program is distributed in the hope that it will be useful, +." but WITHOUT ANY WARRANTY; without even the implied warranty of +." MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +." GNU General Public License for more details. +." +." You should have received a copy of the GNU General Public +." License along with this manual; if not, see +." http://www.gnu.org/licenses/. +." %%%LICENSE_END +." +.TH MEMFD_SECRET 2 2020-08-02 Linux "Linux Programmer's Manual" +.SH NAME +memfd_secret - create an anonymous file to map secret memory regions +.SH SYNOPSIS +.nf +.B #include <linux/secretmem.h> +.PP +.BI "int memfd_secret(unsigned long " flags ");" +.fi +.PP +.IR Note : +There is no glibc wrapper for this system call; see NOTES. +.SH DESCRIPTION +.BR memfd_secret () +creates an anonymous file and returns a file descriptor that refers to it. +The file can only be memory-mapped; +the memory in such mapping +will have stronger protection than usual memory mapped files, +and so it can be used to store application secrets. +Unlike a regular file, a file created with +.BR memfd_secret () +lives in RAM and has a volatile backing storage. +Once all references to the file are dropped, it is automatically released. +The initial size of the file is set to 0. +Following the call, the file size should be set using +.BR ftruncate (2). +.PP +The memory areas obtained with +.BR mmap (2) +from the file descriptor are exclusive to the owning context. +These areas are removed from the kernel page tables +and only the page table of the process holding the file descriptor +maps the corresponding physical memory. +.PP +The following values may be bitwise ORed in +.IR flags +to control the behavior of +.BR memfd_secret (2): +.TP +.BR FD_CLOEXEC +Set the close-on-exec flag on the new file descriptor. +See the description of the +.B O_CLOEXEC +flag in +.BR open (2) +for reasons why this may be useful. +.PP +.TP +.BR SECRETMEM_UNCACHED +In addition to excluding memory areas from the kernel page tables, +mark the memory mappings uncached in the page table of the owning process. +Such mappings can be used to prevent speculative loads +and cache-based side channels. +This mode of +.BR memfd_secret () +is not supported on all architectures. +.PP +See also NOTES below. +.PP +As its return value, +.BR memfd_secret () +returns a new file descriptor that can be used to refer to an anonymous file. +This file descriptor is opened for both reading and writing +.RB ( O_RDWR ) +and +.B O_LARGEFILE +is set for the file descriptor. +.PP +With respect to +.BR fork (2) +and +.BR execve (2), +the usual semantics apply for the file descriptor created by +.BR memfd_secret (). +A copy of the file descriptor is inherited by the child produced by +.BR fork (2) +and refers to the same file. +The file descriptor is preserved across +.BR execve (2), +unless the close-on-exec flag has been set. +.PP +The memory regions backed with +.BR memfd_secret () +are locked in the same way as +.BR mlock (2), +however the implementation will not try to +populate the whole range during the +.BR mmap () +call. +The amount of memory allowed for memory mappings +of the file descriptor obeys the same rules as +.BR mlock (2) +and cannot exceed +.BR RLIMIT_MEMLOCK . +.SH RETURN VALUE +On success, +.BR memfd_secret () +returns a new file descriptor. +On error, -1 is returned and +.I errno +is set to indicate the error. +.SH ERRORS +.TP +.B EINVAL +.I flags +included unknown bits. +.TP +.B EMFILE +The per-process limit on the number of open file descriptors has been reached. +.TP +.B EMFILE +The system-wide limit on the total number of open files has been reached. +.TP +.B ENOMEM +There was insufficient memory to create a new anonymous file. +.TP +.B ENOSYS +.BR memfd_secret () +is not implemented on this architecture. +.SH VERSIONS +The +.BR memfd_secret (2) +system call first appeared in Linux 5.X; +.SH CONFORMING TO +The +.BR memfd_secret (2) +system call is Linux-specific. +.SH NOTES +The +.BR memfd_secret (2) +system call provides an ability to hide information +from the operating system. +Normally Linux userspace mappings are protected from other users, +but they are visible to privileged code. +The mappings created using +.BR memfd_secret () +are hidden from the kernel as well. +.PP +If an architecture supports +.BR SECRETMEM_UNCACHED , +the mappings also have protection from speculative execution vulnerabilties, +at the expense of increased memory access latency. +Care should be taken when using +.B SECRETMEM_UNCACHED +to avoid degrading application performance. +.PP +Glibc does not provide a wrapper for this system call; call it using +.BR syscall (2). +.SH SEE ALSO +.BR fcntl (2), +.BR ftruncate (2), +.BR mlock (2), +.BR mmap (2), +.BR setrlimit (2)