Hi Michael, On Mon, Aug 09, 2021 at 04:00:46AM +0200, Michael Kerrisk (man-pages) wrote: > Hi Mike and Alex, > > I think some more work is needed for this page. Mike, would > you be willing to do some work on the points below please? I'm really stretched at the moment, so it'll take a while. What do you say about starting without the elaborate NOTES section and only updating the page according to your comments below? I will add the NOTES section a bit later then. > On 8/8/21 10:41 AM, Alejandro Colomar wrote: > > From: Mike Rapoport <rppt@xxxxxxxxxxxxx> > > > > Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx> > > Signed-off-by: Alejandro Colomar <alx.manpages@xxxxxxxxx> > > --- > > man2/memfd_secret.2 | 146 ++++++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 146 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..466aa4236 > > --- /dev/null > > +++ b/man2/memfd_secret.2 > > @@ -0,0 +1,146 @@ > > +.\" Copyright (c) 2021, 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 access secret memory regions > > +.SH SYNOPSIS > > +.nf > > +.PP > > +.BR "#include <sys/syscall.h>" " /* Definition of " SYS_* " constants */" > > +.B #include <unistd.h> > > +.PP > > +.BI "int syscall(SYS_memfd_secret, unsigned int " flags ); > > +.fi > > +.PP > > +.IR Note : > > +glibc provides no wrapper for > > +.BR memfd_secret (), > > +necessitating the use of > > +.BR syscall (2). > > +.SH DESCRIPTION > > +.BR memfd_secret () > > +creates an anonymous file and returns a file descriptor that refers to it. > > s/file/RAM-based file/ > > > +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 references to the file are dropped, it is automatically released. > > "dropped" is not clear. Should it be something like: > > Once all open references to the file are closed, > > > +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 backing the file created with > > +.BR memfd_create(2) > > +are visible only to the contexts that have access to the file descriptor. > > "contexts" is not clear here. Can you reword to explain what you mean? > (processes, threads, something else?) > > > +These areas are removed from the kernel page tables > > s/These areas are/The memory region is/ > > > +and only the page tables of the processes holding the file descriptor > > +map the corresponding physical memory. > > Perhaps a sentence here such as: > > "(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.)" > > > +.PP > > +The following values may be bitwise ORed in > > +.I flags > > +to control the behavior of > > +.BR memfd_secret (2): > > +.TP > > +.B FD_CLOEXEC > > +Set the close-on-exec flag on the new file descriptor. > > s/.$/, which causes the region to be removed from the process on execve(2)./ > > > +See the description of the > > +.B O_CLOEXEC > > +flag in > > +.BR open (2) > > +for reasons why this may be useful. > > Maybe the previous sentence is not necessary? > > > +.PP > > +As its return value, > > +.BR memfd_secret () > > +returns a new file descriptor that can be used to refer to an anonymous file. > > s/that can be used to refer/that refers/ > > > +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), > > I find the wording here just a little unclear > > How about: > > The memory region is locked into memory in the same way as > with mlock(2), so that it will never be written into swap > > > +however the implementation will not try to> +populate the whole range during the > > +.BR mmap (2) > > +call. > > s/call./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 > > +.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 5.14. > > +.SH CONFORMING TO > > +The > > +.BR memfd_secret (2) > > +system call is Linux-specific. > > +.SH SEE ALSO > > +.BR fcntl (2), > > +.BR ftruncate (2), > > +.BR mlock (2), > > +.BR mmap (2), > > +.BR setrlimit (2) > > I feel like this page could benefit from a NOTES section > that explains the rationale for the system call. This could > note that the fact that the region is not accessible from the > kernel removes a whole class of security attacks. > > Also, the NOTES section could mention the "secretmem_enable" > boot option, what its purpose is, what values it can have, > and what is default behavior if this option is not specified. > > Also, is ti still the case that if this system call is used, > then users can no longer hibernate their systems? If so, > this really should be mentioned in NOTES! > > Also, in NOTES perhaps it is worth mentioning that the > pages in the region can enter the cache (right?). > > Perhaps Jon's articles at https://lwn.net/Articles/865256/ > https://lwn.net/Articits/835342/ and https://lwn.net/Articles/812325/, > as well as your own commit message > (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1507f51255c9) > may inspire some other ideas on details that should be included > in NOTES. > > Thanks, > > Michael > > -- > Michael Kerrisk > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ > Linux/UNIX System Programming Training: http://man7.org/training/ -- Sincerely yours, Mike.
