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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id p62sm334352pfb.93.2020.05.19.14.17.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 May 2020 14:17:23 -0700 (PDT) Date: Tue, 19 May 2020 14:17:22 -0700 From: Kees Cook To: "Eric W. Biederman" Cc: Al Viro , Andrew Morton , Tetsuo Handa , Eric Biggers , Dmitry Vyukov , linux-fsdevel@vger.kernel.org, linux-security-module@vger.kernel.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, John Johansen Subject: Re: [PATCH 0/4] Relocate execve() sanity checks Message-ID: <202005191342.97EE972E3@keescook> References: <20200518055457.12302-1-keescook@chromium.org> <87a724t153.fsf@x220.int.ebiederm.org> <202005190918.D2BD83F7C@keescook> <87o8qjstyw.fsf@x220.int.ebiederm.org> <202005191052.0A6B1D5843@keescook> <87sgfvrckr.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87sgfvrckr.fsf@x220.int.ebiederm.org> Sender: linux-api-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Tue, May 19, 2020 at 01:42:28PM -0500, Eric W. Biederman wrote: > Kees Cook writes: > > > On Tue, May 19, 2020 at 12:41:27PM -0500, Eric W. Biederman wrote: > >> Kees Cook writes: > >> > and given the LSM hooks, I think the noexec check is too late as well. > >> > (This is especially true for the coming O_MAYEXEC series, which will > >> > absolutely need those tests earlier as well[1] -- the permission checking > >> > is then in the correct place: during open, not exec.) I think the only > >> > question is about leaving the redundant checks in fs/exec.c, which I > >> > think are a cheap way to retain a sense of robustness. > >> > >> The trouble is when someone passes through changes one of the permission > >> checks for whatever reason (misses that they are duplicated in another > >> location) and things then fail in some very unexpected way. > > > > Do you think this series should drop the "late" checks in fs/exec.c? > > Honestly, the largest motivation for me to move the checks earlier as > > I've done is so that other things besides execve() can use FMODE_EXEC > > during open() and receive the same sanity-checking as execve() (i.e the > > O_MAYEXEC series -- the details are still under discussion but this > > cleanup will be needed regardless). > > I think this series should drop the "late" checks in fs/exec.c It feels > less error prone, and it feels like that would transform this into > something Linus would be eager to merge because series becomes a cleanup > that reduces line count. Yeah, that was my initial sense too. I just started to get nervous about removing the long-standing exec sanity checks. ;) > I haven't been inside of open recently enough to remember if the > location you are putting the check fundamentally makes sense. But the > O_MAYEXEC bits make a pretty strong case that something of the sort > needs to happen. Right. I *think* it's correct place for now, based on my understanding of the call graph (which is why I included it in the commit logs). > I took a quick look but I can not see clearly where path_noexec > and the regular file tests should go. > > I do see that you have code duplication with faccessat which suggests > that you haven't put the checks in the right place. Yeah, I have notes on the similar call sites (which I concluded, perhaps wrongly) to ignore: do_faccessat() user_path_at(dfd, filename, lookup_flags, &path); if (acc_mode & MAY_EXEC .... path_noexec() inode_permission(inode, mode | MAY_ACCESS); This appears to be strictly advisory, and the path_noexec() test is there to, perhaps, avoid surprises when doing access() then fexecve()? I would note, however, that that path-based LSMs appear to have no hook in this call graph at all. I was expecting a call like: security_file_permission(..., mode | MAY_ACCESS) but I couldn't find one (or anything like it), so only inode_permission() is being tested (which means also the existing execve() late tests are missed, and the newly added S_ISREG() test from do_dentry_open() is missed). prctl_set_mm_exe_file() err = -EACCESS; if (!S_ISREG(inode->i_mode) || path_noexec(&exe.file->f_path)) goto exit; err = inode_permission(inode, MAY_EXEC); This is similar (no path-based LSM hooks present, only inode_permission() used for permission checking), but it is at least gated by CAP_SYS_ADMIN. And this bring me to a related question from my review: does dentry_open() intentionally bypass security_inode_permission()? I.e. it calls vfs_open() not do_open(): openat2(dfd, char * filename, open_how) build_open_flags(open_how, open_flags) do_filp_open(dfd, filename, open_flags) path_openat(nameidata, open_flags, flags) file = alloc_empty_file(open_flags, current_cred()); do_open(nameidata, file, open_flags) may_open(path, acc_mode, open_flag) inode_permission(inode, MAY_OPEN | acc_mode) security_inode_permission(inode, acc_mode) vfs_open(path, file) do_dentry_open(file, path->dentry->d_inode, open) if (unlikely(f->f_flags & FMODE_EXEC && !S_ISREG(inode->i_mode))) ... security_file_open(f) /* path-based LSMs check for open here * and use FMODE_* flags to determine how a file * is being opened. */ open() vs dentry_open(path, flags, cred) f = alloc_empty_file(flags, cred); vfs_open(path, f); I would expect dentry_open() to mostly duplicate a bunch of path_openat(), but it lacks the may_open() call, etc. I really got the feeling that there was some new conceptual split needed inside do_open() where the nameidata details have been finished, after we've gained the "file" information, but before we've lost the "path" information. For example, may_open(path, ...) has no sense of "file", though it does do the inode_permission() call. Note also that may_open() is used in do_tmpfile() too, and has a comment implying it needs to be checking only a subset of the path details. So I'm not sure how to split things up. So, that's why I put the new checks just before the may_open() call in do_open(): it's the most central, positions itself correctly for dealing with O_MAYEXEC, and doesn't appear to make any existing paths worse. > I am wondering if we need something distinct to request the type of the > file being opened versus execute permissions. Well, this is why I wanted to centralize it -- the knowledge of how a file is going to be used needs to be tested both by the core VFS (S_ISREG, path_noexec) and the LSMs. Things were inconsistent before. > All I know is being careful and putting the tests in a good logical > place makes the code more maintainable, whereas not being careful > results in all kinds of sharp corners that might be exploitable. > So I think it is worth digging in and figuring out where those checks > should live. Especially so that code like faccessat does not need > to duplicate them. I think this is the right place with respect to execve(), though I think there are other cases that could use to be improved (or at least made more consistent). -Kees -- Kees Cook