On Mon, Nov 16, 2020 at 10:01:37PM +0100, Alejandro Colomar wrote: > From: Mike Rapoport <rppt@xxxxxxxxxxxxx> > > Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx> > Cowritten-by: Alejandro Colomar <alx.manpages@xxxxxxxxx> > Acked-by: Alejandro Colomar <alx.manpages@xxxxxxxxx> > Signed-off-by: Alejandro Colomar <alx.manpages@xxxxxxxxx> > --- > > Hi Mike, > > I added that note about not having a wrapper, > fixed a few minor formatting and wording issues, > and sorted ERRORS alphabetically. Thanks, Alejandro! > 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@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 > +.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) > -- > 2.29.2 > -- Sincerely yours, Mike.
