oss-sec mailing list archives
Re: Security risk of server side text editing in general and vim.tiny specifically
From: Fiedler Roman <Roman.Fiedler () ait ac at>
Date: Fri, 3 Nov 2017 15:53:02 +0000
From: Solar Designer [mailto:solar () openwall com] ...The bug may be in the documentation/specification: in my opinion, documentation of good, security aware software should a) implement things considering security bordercases (vim.tiny reporting, that a file wasreplacedor symlink encountered, proceed?)Those special cases you list are just a tip of the iceberg.or b) state, they are not made for that purpose. Even when such statements are redundant for many differenttools,they give users at least the chance to learn, that an operation is dangerous and may link to additional information, e.g. the link you provided below on secure root file access. Why has each plastic bag of a new consumer device printed "There is a risk that children pull them over their head and suffocate." for safety reasons,My guess is mostly for legal reasons, although safety was also involved at some point.
It's safety also for the producer, so that he cannot be sued. But in an IT-perspective: safety and security are often conflicting, but sometimes - like in that case with vim, they are two sides of the same coin: If a tool is not concurrency/thread-safe, any concurrent interaction might be a security risk - exactly as shown here with the arbitrary file overwrite POC.
but in software development, we assume, everybody knows and do notincludesuch warnings at least in the footer of man pages?I don't assume everybody knows. On the contrary, I know that most people don't know, nor do they want to know. When I tell, or ask my fellow sysadmins to follow safer practices, they just get annoyed, in part because the safer practices are too complicated, too brittle, and sometimes also not perfectly safe. But do we really need to include this in every man page? I wish there were a better place.
I'm not sure on that: safety and security has a lot to do with awareness and awareness is raised, when admins are reminded frequently, e.g. when reading manuals. Perhaps they will not do it in all their daily work (who really washes his hands ALWAYS before touching food?), but the might think about it in the right moment (when eating in a very dirty place or preparing food for the whole company on an important server machine).
Editing of non-root files by root should be safe (or be made safe by making changes to the editors where necessary) only in the rare special case when those files are located in a trusted directory. For example, editing as root /var/run/foo owned by user foo should be safe as long as /, /var, and /var/run are owned by root, but editing as root /home/foo/foo or /tmp/foo is unsafe and is likely to stay so.I would need to check that on vim.tiny. As stat-ing, getxattr, renaming, chmod, ... are not atomic, I am not sure if vim.tiny as example would fulfil your expectations. But before that: why do you expect the software to behave like that, when it is not stated anywhere?What I said in the paragraph quoted above is that I expect very little ("only in the rare special case"), and even that might not be true yet (but we should make it true for specific tools if so). I focus on this special case because it's tenable. What you say about non-atomicity of those syscalls is not a security issue when the directory and all parent directories are trusted.
They still can be: check those two syscalls (seems the vim tests the "fchown" call on unrelated file in same directory beforehand): open("4913", O_WRONLY|O_CREAT|O_EXCL|O_NOFOLLOW, 0107777) = 3 fchown(3, 1000, 100) = 0 While the file edited was a 07777 (4 times seven!) user owned file, vim creates an intermediate world writabe root-owned suid-binary. This is not nice, but would be hard to exploit on modern systems: a resource depletion augmented race against a running root-binary from lower-priv user is hard, we would need to OOM-kill vim (perhaps easier to trigger than thought if the edited, user controlled file is very large, thus vim has very bad score compared to fork bomb launching thousands of ping/mount commands) - otherwise while file is open, I think the exec syscall would not exec the file. And of course we would need to write to it without dropping the SUID-bit doing so, requiring some other SUID binary to do that for us (see exploitDB for examples). Summarizing, we got an ugly intermediate state in a trusted directory due to non-atomic operation where exploitation might only be limited by the creativity of the attacker, but for us very hard to rule out. Also put it mathematically: there exist file system states, that a user can create with n operations, but another user (root) needs m operations with always m>n to create the same state in a secure manner. With all the race complexity, it might be fun to analyse, if there exist user controlled states that cannot be reached in secure manner by root, no matter how large m is.
It can still be a reliability and a safety issue e.g. if two sysadmins try to edit a file, but I thought that was beyond scope of our discussion.
Yes, this is "process failure" on human side, should be left out.
..It is tricky to access files in an untrusted directory safely. Programs that knowingly do it end up using O_EXCL or O_NOFOLLOW|O_NOCTTYandsuch, and doing various *stat() calls, and even that is sometimes not enough. It'd be naive to expect the same from every other program accepting an arbitrary pathname.From my point of view, this mandates something like a "libSecureOpen"(tryingto get that into libc as first step might be in vain), which has a solid implementation also considering different UNIX-system peculiarities andshouldbe used by open source software doing that kind of risky operations.IIRC, something like this was proposed in 1990s, albeit not for that extensive a use.
A missed chance for software development/awareness ...
You say "risky operations", but under the threat model you imply (root using almost any tool on pathnames with components writable by a user) almost all filesystem accesses are risky.
With "that kind of risky operations" I aimed to refer to file system operations on untrusted input. Still such a library could make the inherent service contracts more clear by selecting the appropriate method, e.g. open a file, do not care if it is in same directory or symlinked but make sure, that the user starting the link chain would also have permissions to read the final target, wherever it is. Or another procedure "reliable, crash-safe replacement of file within a directory" (exactly the vim usecase).
To partially achieve what you seem to want to achieve, almost all uses of open(2) and fopen(3), etc. would need to be replaced with "secure" alternatives, ...
or better with a more high-level API, using kernel features ..at() and O_BENEATH where available on a given platform ...
.. and that would be bad in many ways, including breaking of customary behavior of traditional Unix command-line programs, which existing scripts rely on.
Why would scripts break? Insecure command invocation is not caught by "secure alternatives" for file system operation anyway. And for program execution: as long as there is no adversary (no races), both worlds (secure and insecure file operations) would come to the same result.
We could proceed with introduction of isatty(3) and env var checks, but this would get messy.
isatty yes, env vars seem different topic to me.
I say "partially" because there's no way for a program to know that the file it's looking at is still the file the user had looked at when they decided to run the program against that pathname. Not only the file itself could have been replaced, but an upper directory could have been. I included some steps to deal with this in the example referenced in my previous message, and one of the steps is a double-check by the user themselves after having created a hard link in a trusted directory.
Yes, so true. But as soon as tools would have the capability to even detect such manipulations, they could start providing simple command line options for users aware of the problem. Think of "vim.tiny -s [path]" where "-s" is for "--secure": in that mode vim.tiny would warn, if any path component up to the file to be edited is not owned by the user invoking vim or root. So "vim.tiny -s /run/xxx" and "vim.tiny -s /var/run/xxx" would be OK on Ubuntu (all symlinks/dirs root owned) while "vim.tiny -s /var/www/uploads/somedir/x.html" would fail for root if upload dir is not owned by root. The caring user then could still do a "cd /var/www/uploads/somedir", "pwd" and then decide if he wants to edit.
I suppose some alternate OS could introduce a paradigm where a user's view of the filesystem would be frozen when they stat() a file or list a directory and unfrozen after they've accessed a file in there. This is another can of worms. I guess it's more realistically (or less unrealistically) done for one thread in a program (with each thread having its own filesystem view freeze) rather than for a user's shell running multiple programs one after another.
Very nice idea, but unless the big 5 come up with it, quite unrealistic, I assume.
Other software should explicitely declare: "is not safe for operating on file of different users/NFS in untrusted environments".This is true for 99%+ of Unix software. Exceptions are few (like some uses of "ln", and even then there's the issue of parent directories).
True - but why not declare it? "ln is not safe creating links within directories or to files not owned by yourself or the root user". To stay with the example of the plastic bags: as long as no one declared it, nobody seemed to care about avoiding it or replacing it with less risky alternatives. Changes start in mind first, not in RAM or disk bits.
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Current thread:
- Re: nvi crash recovery (was Re: [oss-security] Re: Security risk of server side text editing in general and vim.tiny specifically), (continued)
- Re: Re: Security risk of server side text editing in general and vim.tiny specifically Christos Zoulas (Nov 03)
- AW: Re: Security risk of server side text editing in general and vim.tiny specifically Fiedler Roman (Nov 06)
- Re: Security risk of server side text editing in general and vim.tiny specifically Solar Designer (Nov 13)
- AW: Security risk of server side text editing in general and vim.tiny specifically Fiedler Roman (Nov 13)
- Re: Security risk of server side text editing in general and vim.tiny specifically Fiedler Roman (Nov 03)
- Re: Security risk of server side text editing in general and vim.tiny specifically Fiedler Roman (Nov 03)
- Re: Security risk of server side text editing in general and vim.tiny specifically Solar Designer (Nov 03)
- Re: Security risk of server side text editing in general and vim.tiny specifically Solar Designer (Nov 03)
- Re: Security risk of server side text editing in general and vim.tiny specifically Leonid Isaev (Nov 05)
- Re: Security risk of server side text editing in general and vim.tiny specifically Solar Designer (Nov 03)
- Re: Security risk of server side text editing in general and vim.tiny specifically Fiedler Roman (Nov 03)