From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f69.google.com (mail-pg0-f69.google.com [74.125.83.69]) by kanga.kvack.org (Postfix) with ESMTP id F35866B0386 for ; Thu, 17 Nov 2016 19:38:37 -0500 (EST) Received: by mail-pg0-f69.google.com with SMTP id g186so227479530pgc.2 for ; Thu, 17 Nov 2016 16:38:37 -0800 (PST) Received: from out03.mta.xmission.com (out03.mta.xmission.com. [166.70.13.233]) by mx.google.com with ESMTPS id 127si5286474pgg.233.2016.11.17.16.38.36 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 17 Nov 2016 16:38:37 -0800 (PST) From: ebiederm@xmission.com (Eric W. Biederman) References: <87twcbq696.fsf@x220.int.ebiederm.org> <20161018135031.GB13117@dhcp22.suse.cz> <8737jt903u.fsf@xmission.com> <20161018150507.GP14666@pc.thejh.net> <87twc9656s.fsf@xmission.com> <20161018191206.GA1210@laptop.thejh.net> <87r37dnz74.fsf@xmission.com> <87k2d5nytz.fsf_-_@xmission.com> <87y41kjn6l.fsf@xmission.com> <20161019172917.GE1210@laptop.thejh.net> <87pomwi5p2.fsf@xmission.com> <87pomwghda.fsf@xmission.com> <87twb6avk8.fsf_-_@xmission.com> <87inrmavax.fsf_-_@xmission.com> <87mvgxwtjv.fsf@xmission.com> Date: Thu, 17 Nov 2016 18:35:57 -0600 In-Reply-To: (Andy Lutomirski's message of "Thu, 17 Nov 2016 16:10:47 -0800") Message-ID: <87shqpvd3m.fsf@xmission.com> MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [REVIEW][PATCH 2/3] exec: Don't allow ptracing an exec of an unreadable file Sender: owner-linux-mm@kvack.org List-ID: To: Andy Lutomirski Cc: Linux Containers , Oleg Nesterov , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , Linux FS Devel , Michal Hocko , Jann Horn , Willy Tarreau , Kees Cook Andy Lutomirski writes: > On Thu, Nov 17, 2016 at 3:55 PM, Eric W. Biederman > wrote: >> Andy Lutomirski writes: >> >>> On Thu, Nov 17, 2016 at 9:08 AM, Eric W. Biederman >>> wrote: >>>> >>>> It is the reasonable expectation that if an executable file is not >>>> readable there will be no way for a user without special privileges to >>>> read the file. This is enforced in ptrace_attach but if we are >>>> already attached there is no enforcement if a readonly executable >>>> is exec'd. >>>> >>>> Therefore do the simple thing and if there is a non-dumpable >>>> executable that we are tracing without privilege fail to exec it. >>>> >>>> Fixes: v1.0 >>>> Cc: stable@vger.kernel.org >>>> Reported-by: Andy Lutomirski >>>> Signed-off-by: "Eric W. Biederman" >>>> --- >>>> fs/exec.c | 8 +++++++- >>>> 1 file changed, 7 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/fs/exec.c b/fs/exec.c >>>> index fdec760bfac3..de107f74e055 100644 >>>> --- a/fs/exec.c >>>> +++ b/fs/exec.c >>>> @@ -1230,6 +1230,11 @@ int flush_old_exec(struct linux_binprm * bprm) >>>> { >>>> int retval; >>>> >>>> + /* Fail if the tracer can't read the executable */ >>>> + if ((bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP) && >>>> + !ptracer_capable(current, bprm->mm->user_ns)) >>>> + return -EPERM; >>>> + >>> >>> At the very least, I think that BINPRM_FLAGS_ENFORCE_NONDUMP needs to >>> check capable_wrt_inode_uidgid too. Otherwise we risk breaking: >>> >>> $ gcc whatever.c >>> $ chmod 400 a.out >>> $ strace a.out >> >> It is an invariant that if you have caps in mm->user_ns you will >> also be capable_write_inode_uidgid of all files that a process exec's. > > I meant to check whether you *are* the owner, too. I don't follow. BINPRM_FLAGS_ENFORCE_NONDUMP is only set if the caller of exec does not have inode_permission(inode, MAY_READ). Which in your example would have guaranteed that BINPRM_FLAGS_ENFORCE_NONDUMP would have be unset. The ptracer_capable thing is only asking in this instance if we can ignore the nondumpable status because we have CAP_SYS_PTRACE over a user namespace that includes all of the files that would_dump was called on (mm->user_ns). ptrace_access_vm in the replacement patch has essentially the same permission check. It is just at PTRACE_PEEKTEXT, PTRACE_PEEKDATA, PTRACE_POKETEXT, or PTRACE_POKEDATA time. So I am curious if you are seeing something that is worth fixing. >> My third patch winds up changing mm->user_ns to maintain this invariant. >> >> It is also true that Willy convinced me while this check is trivial it >> will break historic uses so I have replaced this patch with: >> "ptrace: Don't allow accessing an undumpable mm. > > I think that's better. Eric -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org