From: Aleksa Sarai <cyphar@cyphar.com> To: Andy Lutomirski <luto@amacapital.net> Cc: "Steve Grubb" <sgrubb@redhat.com>, "Florian Weimer" <fweimer@redhat.com>, "Mickaël Salaün" <mic@digikod.net>, linux-kernel@vger.kernel.org, "Alexei Starovoitov" <ast@kernel.org>, "Al Viro" <viro@zeniv.linux.org.uk>, "Andy Lutomirski" <luto@kernel.org>, "Christian Heimes" <christian@python.org>, "Daniel Borkmann" <daniel@iogearbox.net>, "Eric Chiang" <ericchiang@google.com>, "James Morris" <jmorris@namei.org>, "Jan Kara" <jack@suse.cz>, "Jann Horn" <jannh@google.com>, "Jonathan Corbet" <corbet@lwn.net>, "Kees Cook" <keescook@chromium.org>, "Matthew Garrett" <mjg59@google.com>, "Matthew Wilcox" <willy@infradead.org>, "Michael Kerrisk" <mtk.manpages@gmail.com>, "Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>, "Mimi Zohar" <zohar@linux.ibm.com>, "Philippe Trébuchet" <philippe.trebuchet@ssi.gouv.fr>, "Scott Shell" <scottsh@microsoft.com>, "Sean Christopherson" <sean.j.christopherson@intel.com>, "Shuah Khan" <shuah@kernel.org>, "Song Liu" <songliubraving@fb.com>, "Steve Dower" <steve.dower@python.org>, "Thibaut S autereau" <thibaut.sautereau@ssi.gouv.fr>, "Vincent Strubel" <vincent.strubel@ssi.gouv.fr>, "Yves-Alexis Perez" <yves-alexis.perez@ssi.gouv.fr>, kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-security-module@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v2 0/5] Add support for O_MAYEXEC Date: Sat, 7 Sep 2019 08:44:10 +1000 [thread overview] Message-ID: <20190906224410.lffd6l5lnm4z3hht@yavin.dot.cyphar.com> (raw) In-Reply-To: <C95B704C-F84F-4341-BDE7-CD70C5DDBEEF@amacapital.net> [-- Attachment #1: Type: text/plain, Size: 2905 bytes --] On 2019-09-06, Andy Lutomirski <luto@amacapital.net> wrote: > > On Sep 6, 2019, at 12:07 PM, Steve Grubb <sgrubb@redhat.com> wrote: > > > >> On Friday, September 6, 2019 2:57:00 PM EDT Florian Weimer wrote: > >> * Steve Grubb: > >>> Now with LD_AUDIT > >>> $ LD_AUDIT=/home/sgrubb/test/openflags/strip-flags.so.0 strace ./test > >>> 2>&1 | grep passwd openat(3, "passwd", O_RDONLY) = 4 > >>> > >>> No O_CLOEXEC flag. > >> > >> I think you need to explain in detail why you consider this a problem. > > > > Because you can strip the O_MAYEXEC flag from being passed into the kernel. > > Once you do that, you defeat the security mechanism because it never gets > > invoked. The issue is that the only thing that knows _why_ something is being > > opened is user space. With this mechanism, you can attempt to pass this > > reason to the kernel so that it may see if policy permits this. But you can > > just remove the flag. > > I’m with Florian here. Once you are executing code in a process, you > could just emulate some other unapproved code. This series is not > intended to provide the kind of absolute protection you’re imagining. I also agree, though I think that there is a separate argument to be made that there are two possible problems with O_MAYEXEC (which might not be really big concerns): * It's very footgun-prone if you didn't call O_MAYEXEC yourself and you pass the descriptor elsewhere. You need to check f_flags to see if it contains O_MAYEXEC. Maybe there is an argument to be made that passing O_MAYEXECs around isn't a valid use-case, but in that case there should be some warnings about that. * There's effectively a TOCTOU flaw (even if you are sure O_MAYEXEC is in f_flags) -- if the filesystem becomes re-mounted noexec (or the file has a-x permissions) after you've done the check you won't get hit with an error when you go to use the file descriptor later. To fix both you'd need to do what you mention later: > What the kernel *could* do is prevent mmapping a non-FMODE_EXEC file > with PROT_EXEC, which would indeed have a real effect (in an iOS-like > world, for example) but would break many, many things. And I think this would be useful (with the two possible ways of executing .text split into FMODE_EXEC and FMODE_MAP_EXEC, as mentioned in a sister subthread), but would have to be opt-in for the obvious reason you outlined. However, we could make it the default for openat2(2) -- assuming we can agree on what the semantics of a theoretical FMODE_EXEC should be. And of course we'd need to do FMODE_UPGRADE_EXEC (which would need to also permit fexecve(2) though probably not PROT_EXEC -- I don't think you can mmap() an O_PATH descriptor). -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/> [-- Attachment #2: signature.asc --] [-- Type: application/pgp-signature, Size: 228 bytes --]
WARNING: multiple messages have this Message-ID (diff)
From: Aleksa Sarai <cyphar@cyphar.com> To: Andy Lutomirski <luto@amacapital.net> Cc: "Steve Grubb" <sgrubb@redhat.com>, "Florian Weimer" <fweimer@redhat.com>, "Mickaël Salaün" <mic@digikod.net>, linux-kernel@vger.kernel.org, "Alexei Starovoitov" <ast@kernel.org>, "Al Viro" <viro@zeniv.linux.org.uk>, "Andy Lutomirski" <luto@kernel.org>, "Christian Heimes" <christian@python.org>, "Daniel Borkmann" <daniel@iogearbox.net>, "Eric Chiang" <ericchiang@google.com>, "James Morris" <jmorris@namei.org>, "Jan Kara" <jack@suse.cz>, "Jann Horn" <jannh@google.com>, "Jonathan Corbet" <corbet@lwn.net>, "Kees Cook" <keescook@chromium.org>, "Matthew Garrett" <mjg59@google.com>, "Matthew Wilcox" <willy@infradead.org>, "Michael Kerrisk" <mtk.manpages@gmail.com>, "Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>, "Mimi Zohar" <zohar@linux.ibm.com> Subject: Re: [PATCH v2 0/5] Add support for O_MAYEXEC Date: Sat, 7 Sep 2019 08:44:10 +1000 [thread overview] Message-ID: <20190906224410.lffd6l5lnm4z3hht@yavin.dot.cyphar.com> (raw) In-Reply-To: <C95B704C-F84F-4341-BDE7-CD70C5DDBEEF@amacapital.net> [-- Attachment #1: Type: text/plain, Size: 2905 bytes --] On 2019-09-06, Andy Lutomirski <luto@amacapital.net> wrote: > > On Sep 6, 2019, at 12:07 PM, Steve Grubb <sgrubb@redhat.com> wrote: > > > >> On Friday, September 6, 2019 2:57:00 PM EDT Florian Weimer wrote: > >> * Steve Grubb: > >>> Now with LD_AUDIT > >>> $ LD_AUDIT=/home/sgrubb/test/openflags/strip-flags.so.0 strace ./test > >>> 2>&1 | grep passwd openat(3, "passwd", O_RDONLY) = 4 > >>> > >>> No O_CLOEXEC flag. > >> > >> I think you need to explain in detail why you consider this a problem. > > > > Because you can strip the O_MAYEXEC flag from being passed into the kernel. > > Once you do that, you defeat the security mechanism because it never gets > > invoked. The issue is that the only thing that knows _why_ something is being > > opened is user space. With this mechanism, you can attempt to pass this > > reason to the kernel so that it may see if policy permits this. But you can > > just remove the flag. > > I’m with Florian here. Once you are executing code in a process, you > could just emulate some other unapproved code. This series is not > intended to provide the kind of absolute protection you’re imagining. I also agree, though I think that there is a separate argument to be made that there are two possible problems with O_MAYEXEC (which might not be really big concerns): * It's very footgun-prone if you didn't call O_MAYEXEC yourself and you pass the descriptor elsewhere. You need to check f_flags to see if it contains O_MAYEXEC. Maybe there is an argument to be made that passing O_MAYEXECs around isn't a valid use-case, but in that case there should be some warnings about that. * There's effectively a TOCTOU flaw (even if you are sure O_MAYEXEC is in f_flags) -- if the filesystem becomes re-mounted noexec (or the file has a-x permissions) after you've done the check you won't get hit with an error when you go to use the file descriptor later. To fix both you'd need to do what you mention later: > What the kernel *could* do is prevent mmapping a non-FMODE_EXEC file > with PROT_EXEC, which would indeed have a real effect (in an iOS-like > world, for example) but would break many, many things. And I think this would be useful (with the two possible ways of executing .text split into FMODE_EXEC and FMODE_MAP_EXEC, as mentioned in a sister subthread), but would have to be opt-in for the obvious reason you outlined. However, we could make it the default for openat2(2) -- assuming we can agree on what the semantics of a theoretical FMODE_EXEC should be. And of course we'd need to do FMODE_UPGRADE_EXEC (which would need to also permit fexecve(2) though probably not PROT_EXEC -- I don't think you can mmap() an O_PATH descriptor). -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/> [-- Attachment #2: signature.asc --] [-- Type: application/pgp-signature, Size: 228 bytes --]
next prev parent reply other threads:[~2019-09-06 22:44 UTC|newest] Thread overview: 90+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-09-06 15:24 [PATCH v2 0/5] Add support for O_MAYEXEC Mickaël Salaün 2019-09-06 15:24 ` Mickaël Salaün 2019-09-06 15:24 ` [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() Mickaël Salaün 2019-09-06 15:24 ` Mickaël Salaün 2019-09-06 15:56 ` Florian Weimer 2019-09-06 15:56 ` Florian Weimer 2019-09-06 16:06 ` Mickaël Salaün 2019-09-06 16:06 ` Mickaël Salaün 2019-09-06 16:48 ` Jeff Layton 2019-09-06 16:48 ` Jeff Layton 2019-09-06 17:13 ` Aleksa Sarai 2019-09-06 17:13 ` Aleksa Sarai 2019-09-06 19:43 ` Jeff Layton 2019-09-06 19:43 ` Jeff Layton 2019-09-06 20:06 ` Andy Lutomirski 2019-09-06 20:06 ` Andy Lutomirski 2019-09-06 20:51 ` Jeff Layton 2019-09-06 20:51 ` Jeff Layton 2019-09-06 21:27 ` Andy Lutomirski 2019-09-06 21:27 ` Andy Lutomirski 2019-09-06 22:12 ` Aleksa Sarai 2019-09-06 22:12 ` Aleksa Sarai 2019-09-09 9:33 ` Mickaël Salaün 2019-09-09 9:33 ` Mickaël Salaün 2019-09-06 22:05 ` Aleksa Sarai 2019-09-06 22:05 ` Aleksa Sarai 2019-09-06 22:18 ` Aleksa Sarai 2019-09-06 22:18 ` Aleksa Sarai 2019-09-06 17:14 ` Mickaël Salaün 2019-09-06 17:14 ` Mickaël Salaün 2019-09-06 18:38 ` Jeff Layton 2019-09-06 18:38 ` Jeff Layton 2019-09-06 18:41 ` Andy Lutomirski 2019-09-06 18:41 ` Andy Lutomirski 2019-09-09 9:18 ` Mickaël Salaün 2019-09-09 9:18 ` Mickaël Salaün 2019-09-09 15:49 ` Andy Lutomirski 2019-09-09 15:49 ` Andy Lutomirski 2019-09-06 18:44 ` Florian Weimer 2019-09-06 18:44 ` Florian Weimer 2019-09-06 19:03 ` James Morris 2019-09-06 19:03 ` James Morris 2019-09-09 9:25 ` Mickaël Salaün 2019-09-09 9:25 ` Mickaël Salaün 2019-09-09 10:12 ` James Morris 2019-09-09 10:12 ` James Morris 2019-09-09 10:54 ` Mickaël Salaün 2019-09-09 10:54 ` Mickaël Salaün 2019-09-09 12:28 ` Aleksa Sarai 2019-09-09 12:28 ` Aleksa Sarai 2019-09-09 12:33 ` Mickaël Salaün 2019-09-09 12:33 ` Mickaël Salaün 2019-09-09 11:54 ` Aleksa Sarai 2019-09-09 11:54 ` Aleksa Sarai 2019-09-09 12:28 ` Mickaël Salaün 2019-09-09 12:28 ` Mickaël Salaün 2019-09-06 17:07 ` Aleksa Sarai 2019-09-06 17:07 ` Aleksa Sarai 2019-09-06 17:20 ` Christian Brauner 2019-09-06 17:20 ` Christian Brauner 2019-09-06 17:24 ` Mickaël Salaün 2019-09-06 17:24 ` Mickaël Salaün 2019-09-06 17:40 ` Tycho Andersen 2019-09-06 17:40 ` Tycho Andersen 2019-09-06 18:27 ` Florian Weimer 2019-09-06 18:27 ` Florian Weimer 2019-09-06 18:46 ` Tycho Andersen 2019-09-06 18:46 ` Tycho Andersen 2019-09-06 15:24 ` [PATCH v2 2/5] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount propertie Mickaël Salaün 2019-09-06 15:24 ` Mickaël Salaün 2019-09-06 15:24 ` [PATCH v2 3/5] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün 2019-09-06 15:24 ` Mickaël Salaün 2019-09-06 15:24 ` [PATCH v2 4/5] selftest/exec: Add tests for O_MAYEXEC enforcing Mickaël Salaün 2019-09-06 15:24 ` Mickaël Salaün 2019-09-06 15:24 ` [PATCH v2 5/5] doc: Add documentation for the fs.open_mayexec_enforce sysctl Mickaël Salaün 2019-09-06 15:24 ` Mickaël Salaün 2019-09-06 18:50 ` [PATCH v2 0/5] Add support for O_MAYEXEC Steve Grubb 2019-09-06 18:50 ` Steve Grubb 2019-09-06 18:57 ` Florian Weimer 2019-09-06 18:57 ` Florian Weimer 2019-09-06 19:07 ` Steve Grubb 2019-09-06 19:07 ` Steve Grubb 2019-09-06 19:26 ` Andy Lutomirski 2019-09-06 19:26 ` Andy Lutomirski 2019-09-06 22:44 ` Aleksa Sarai [this message] 2019-09-06 22:44 ` Aleksa Sarai 2019-09-09 9:09 ` Mickaël Salaün 2019-09-09 9:09 ` Mickaël Salaün 2019-09-09 0:16 ` James Morris 2019-09-09 0:16 ` James Morris
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