linux-kernel - Re: [PATCH v1 0/7] Remove in-tree usage of MAP

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Thu, 26 Aug 2021 10:48:55 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        "David Laight" <David.Laight@...lab.com>,
        "David Hildenbrand" <david@...hat.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Al Viro" <viro@...iv.linux.org.uk>,
        "Alexey Dobriyan" <adobriyan@...il.com>,
        "Steven Rostedt" <rostedt@...dmis.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Arnaldo Carvalho de Melo" <acme@...nel.org>,
        "Mark Rutland" <mark.rutland@....com>,
        "Alexander Shishkin" <alexander.shishkin@...ux.intel.com>,
        "Jiri Olsa" <jolsa@...hat.com>,
        "Namhyung Kim" <namhyung@...nel.org>,
        "Petr Mladek" <pmladek@...e.com>,
        "Sergey Senozhatsky" <sergey.senozhatsky@...il.com>,
        "Andy Shevchenko" <andriy.shevchenko@...ux.intel.com>,
        "Rasmus Villemoes" <linux@...musvillemoes.dk>,
        "Kees Cook" <keescook@...omium.org>,
        "Greg Ungerer" <gerg@...ux-m68k.org>,
        "Geert Uytterhoeven" <geert@...ux-m68k.org>,
        "Mike Rapoport" <rppt@...nel.org>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Vincenzo Frascino" <vincenzo.frascino@....com>,
        "Chinwen Chang" <chinwen.chang@...iatek.com>,
        "Michel Lespinasse" <walken@...gle.com>,
        "Catalin Marinas" <catalin.marinas@....com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Huang Ying" <ying.huang@...el.com>,
        "Jann Horn" <jannh@...gle.com>, "Feng Tang" <feng.tang@...el.com>,
        "Kevin Brodsky" <Kevin.Brodsky@....com>,
        "Michael Ellerman" <mpe@...erman.id.au>,
        "Shawn Anastasio" <shawn@...stas.io>,
        "Steven Price" <steven.price@....com>,
        "Nicholas Piggin" <npiggin@...il.com>,
        "Christian Brauner" <christian.brauner@...ntu.com>,
        "Jens Axboe" <axboe@...nel.dk>,
        "Gabriel Krisman Bertazi" <krisman@...labora.com>,
        "Peter Xu" <peterx@...hat.com>,
        "Suren Baghdasaryan" <surenb@...gle.com>,
        "Shakeel Butt" <shakeelb@...gle.com>,
        "Marco Elver" <elver@...gle.com>,
        "Daniel Jordan" <daniel.m.jordan@...cle.com>,
        "Nicolas Viennot" <Nicolas.Viennot@...sigma.com>,
        "Thomas Cedeno" <thomascedeno@...gle.com>,
        "Collin Fijalkovich" <cfijalkovich@...gle.com>,
        "Michal Hocko" <mhocko@...e.com>,
        "Miklos Szeredi" <miklos@...redi.hu>,
        "Chengguang Xu" <cgxu519@...ernel.net>,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        "linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>,
        "Linux API" <linux-api@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "<linux-fsdevel@...r.kernel.org>" <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "Florian Weimer" <fweimer@...hat.com>,
        "Michael Kerrisk" <mtk.manpages@...il.com>
Subject: Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

On Fri, Aug 13, 2021, at 5:54 PM, Linus Torvalds wrote:
> On Fri, Aug 13, 2021 at 2:49 PM Andy Lutomirski <luto@...nel.org> wrote:
> >
> > I’ll bite.  How about we attack this in the opposite direction: remove the deny write mechanism entirely.
> 
> I think that would be ok, except I can see somebody relying on it.
> 
> It's broken, it's stupid, but we've done that ETXTBUSY for a _loong_ time.

Someone off-list just pointed something out to me, and I think we should push harder to remove ETXTBSY.  Specifically, we've all been focused on open() failing with ETXTBSY, and it's easy to make fun of anyone opening a running program for write when they should be unlinking and replacing it.

Alas, Linux's implementation of deny_write_access() is correct^Wabsurd, and deny_write_access() *also* returns ETXTBSY if the file is open for write.  So, in a multithreaded program, one thread does:

fd = open("some exefile", O_RDWR | O_CREAT | O_CLOEXEC);
write(fd, some stuff);

<--- problem is here

close(fd);
execve("some exefile");

Another thread does:

fork();
execve("something else");

In between fork and execve, there's another copy of the open file description, and i_writecount is held, and the execve() fails.  Whoops.  See, for example:

https://github.com/golang/go/issues/22315

I propose we get rid of deny_write_access() completely to solve this.

Getting rid of i_writecount itself seems a bit harder, since a handful of filesystems use it for clever reasons.

(OFD locks seem like they might have the same problem.  Maybe we should have a clone() flag to unshare the file table and close close-on-exec things?)

> 
> But you are right that we have removed parts of it over time (no more
> MAP_DENYWRITE, no more uselib()) so that what we have today is a
> fairly weak form of what we used to do.
> 
> And nobody really complained when we weakened it, so maybe removing it
> entirely might be acceptable.
> 
>               Linus
>