[PATCH] man2: new page describing memfd_secret() system call

[Mike Rapoport]

From: Mike Rapoport <rppt@xxxxxxxxxxxxx>

Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
---
 man2/memfd_secret.2 | 176 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 176 insertions(+)
 create mode 100644 man2/memfd_secret.2

diff --git a/man2/memfd_secret.2 b/man2/memfd_secret.2
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+.\" Copyright (c) 2020, IBM Corporation.
+.\" Written by Mike Rapoport <rppt@xxxxxxxxxxxxx>
+.\"
+.\" Based on memfd_create(2) man page
+.\" Copyright (C) 2014 Michael Kerrisk <mtk.manpages@xxxxxxxxx>
+.\" and Copyright (C) 2014 David Herrmann <dh.herrmann@xxxxxxxxx>
+.\"
+.\" %%%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
+.B 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 ENOSYS
+.BR memfd_secret ()
+is not implemented on this architecture.
+.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.
+.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 the privileged code.
+The mappings created using
+.BR memfd_secret ()
+are hidden from the kernel as well.
+.PP
+If an architecture supports
+.B 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.
+.SH SEE ALSO
+.BR fcntl (2),
+.BR ftruncate (2),
+.BR mlock (2),
+.BR mmap (2),
+.BR setrlimit (2),
-- 
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