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The trouble with symbolic links

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Re: The trouble with symbolic links

#31

Post by indymike »

joosters wrote: post #8
mongol wrote: post #7

"X is fundamentally broken" is a tired trope. To me, something is broken if it is no longer working as intended. It used to work, but now it does not - it is broken. If something works as intended, but its utility is limited, and it can be improved, it is not broken.

Symlinks work as intended, but they cause a lot of unintended security vulnerabilities, i.e. they break lots of otherwise-functioning code. You can play with your words and redefine their meanings, but the vulnerabilities remain.

> You can play with your words and redefine their meanings, but the vulnerabilities remain.

I don't think OP was trying to play with words, I do think there's an absolute "this symlink thing is a vuln" vs an absolute "I use symlinks to make X work" argument. Symlinks have always been at the line between the absolutes. They do enable a great deal of functionality but they can be a security risk, and source of bugs when developers don't handle them correctly. That said, they are heavily used feature on unix like oses. My /usr/bin on Ubuntu has 48 of them (most were put there by apt installed packages).

Re: The trouble with symbolic links

#32

Post by ChrisSD »

ziml77 wrote: post #19
the8472 wrote: post #15

Earlier quoted context omitted.

Part of the problem is that handles on directories on which one can then use the the *at family of syscalls are not first-class citizens in many programming languages. Which in turn might be due to portability concerns with windows, e.g. Java's SecureDirectoryStream isn't available there[0]. Apparently windows does have an openat-like API[1], but it's low-level. Programmers aren't using them because the language stan…

Weird that you need to dip down to an Nt* function to open a child given a parent handle. It's not like it's unheard of to be able to do that in the Windows API. The registry is also a hierarchical system and opening any key requires passing in a parent key handle (the root handles are predefined).

It's a quirk of history, imho. If Win32 had been written without concern for what came before, it probably would have more closely followed NT conventions.

But it wasn't. It followed on from Win16 and DOS so, to an extent, it emulated DOS-style path and file handling. After all, that's what developers and users were familiar with. The Windows registry did not have all this baggage so it followed the style of the NT kernel.

Though this doesn't explain why Win32 never added CreateFileRelativeToDirectoryHandleW

Re: The trouble with symbolic links

#33

Post by Beltalowda »

littlestymaar wrote: post #9
mongol wrote: post #7

"X is fundamentally broken" is a tired trope. To me, something is broken if it is no longer working as intended. It used to work, but now it does not - it is broken. If something works as intended, but its utility is limited, and it can be improved, it is not broken.

That “X − designed in the 70s when we had no idea of anything regarding computers − is fundamentally broken” isn't so surprising after all. In fact, computers are probably the only place in the entire technology landscape where we keep using almost unmodified stuff from the 70s and decided we cannot change it because there's too much things relying on it. I don't like breaking everything all the time more than anyone…

> In fact, computers are probably the only place in the entire technology landscape where we keep using almost unmodified stuff from the 70s and decided we cannot change it because there's too much things relying on it.

Bridges and buildings from the 1970s (and much older) are still working fine today.

The thing is, if I do decide to replace my bridge or building because it's old and outdated then I can just replace that one thing without affecting much else. With computers, that is obviously not the case: you need to replace the entire city.

Plus, it's not really the case that we "keep using almost unmodified stuff from the 70s"; while many concepts remained the same and things remained compatible, things have been greatly extended and modified since; it's like those old buildings that were built during the middle ages (or sometimes even earlier) that have been changed and upgraded extensively throughout the centuries to the point you really need to know where to look to see it's actually a centuries-old building.

Re: The trouble with symbolic links

#34

Post by bhawks »

A posix filesystem by itself is not a defensible security perimeter. Symlinks introduce security problems but there are other sources as well. If you have a system where processes with different trust profiles share a common view of a file system you have to assume one can manipulate the filesystem state to subvert the other.

Android has dealt with this via locking down and isolating apps to their own filesystems. Cross app communication and data sharing utilizes IPC primitives that have rich caller/Callie information that can be used to build capabilities and authn/authz checks.

The posix filesystem just doesn't have the abstractions/expressiveness one would need to build a robust security perimeter between untrusted apps.

Re: The trouble with symbolic links

#35

Post by faragon »

ziml77 wrote: post #17
faragon wrote: post #13

Earlier quoted context omitted.

You could create symlinks since Windows Vista without permission elevation, if having disabled the UAC ("User Account Control").

Disabling UAC meant there was nowhere to elevate to because you just had admin rights all the time.

You're right, thank you.

Re: The trouble with symbolic links

#36

Post by ploxiln »

3pt14159 wrote: post #30
mongol wrote: post #7

"X is fundamentally broken" is a tired trope. To me, something is broken if it is no longer working as intended. It used to work, but now it does not - it is broken. If something works as intended, but its utility is limited, and it can be improved, it is not broken.

The word "broken" came from before we had constant arms races in technology. Obviously we don't call clubs broken because we now have rapid artillery, but there was enough time between clubs and swords, and swords and guns to allow transitions away. When I'm exposed to a core OS feature I expect by default that it should not come with expected, critical security vulnerabilities. It is rational to expect a user to use…

In the modern world, the demand seems to be that every tool be perfectly safe in every situation no matter what you do (and it seems practically nothing lives up to this demand, given the ever increasing river of silly CVEs for almost every component, like regex DoS on build tools).

It's important to understand the scope of the issue. If you create and operate on your own symlinks in your own folders, there is no problem. The problem is when a more privileged user operates on folders that can be written to by less privileged users, for example system daemons (like a /tmp cleanup, or a web server serving /home/*/www), or suid binaries. These things need to be written very carefully, it is now clear.

But if I'm working with my own files, media, source code, build tools, web pages, etc, in my own folders, then symlinks are still fine.

And there is an existing setting to mitigate a couple common forms of the issue that does exist when accessing folders other users can write to: https://www.kernel.org/doc/html/latest/admin-guide/sysctl/fs...

Re: The trouble with symbolic links

#37

Post by kllrnohj »

littlestymaar wrote: post #9
mongol wrote: post #7

"X is fundamentally broken" is a tired trope. To me, something is broken if it is no longer working as intended. It used to work, but now it does not - it is broken. If something works as intended, but its utility is limited, and it can be improved, it is not broken.

That “X − designed in the 70s when we had no idea of anything regarding computers − is fundamentally broken” isn't so surprising after all. In fact, computers are probably the only place in the entire technology landscape where we keep using almost unmodified stuff from the 70s and decided we cannot change it because there's too much things relying on it. I don't like breaking everything all the time more than anyone…

I don't think that's entirely true. There's plenty of major systems that have made fundamentally incompatible breaking changes in order to move things forward. Windows did that with Vista, Android did that with Linux (eg, app sandboxing per UID, heavily restricted filesystem access to shared directories), etc..

It's kinda mainly desktop/server Linux where there's this inability to move forward.

Re: The trouble with symbolic links

#38

Post by js2 »

I'm sorry symlinks are a thorn in Jeremy's side, but they are useful from a user's perspective. Hard links don't fill the same need. You can't normally hard link directories. If a file has multiple links, finding them all normally requires scanning the entire file system, so deleting a file now becomes harder. A file with multiple links doesn't have an obvious canonical path.

As an example of all these issues, I manage a bunch of Mac build hosts with multiple Xcode versions installed. We only retain the most recent patch release of each major.minor version, but drop compatibility symlinks in place for the other versions. On macOS, an application is just a directory. So for example we'll have:

  Xcode-13.0.app -> Xcode-13.0.1.app
  Xcode-13.0.1.app
From a simple "ls" it's obvious which versions are installed and which are just compatibility shims. Symlinks are just so damn convenient for use cases like this. Hard links don't cut the mustard here.

So there are more reasons for symlinks than just "hard links are restricted to linking within the same filesystem", but yes, that too.

Probably I'm just lacking imagination and there's a solution that offers the advantages of symlinks with none of the downsides, but in my experience, we see this sort of indirection all over the computing landscape, so it seems like there's a fundamental need for it.

Re: The trouble with symbolic links

#39

Post by rob_c »

the8472 wrote: post #15
rob_c wrote: post #12

Earlier quoted context omitted.

> but they cause a lot of unintended security vulnerabilities No, bad coders on UNIX platforms do this. The code may be valid code, but if it's intending to support running on UNIX it should do it properly not assuming it's on a FAT32 filesystem in 2022. Or, even better, run the code you don't trust on fat32 filesystems, see how far that gets.

Part of the problem is that handles on directories on which one can then use the the *at family of syscalls are not first-class citizens in many programming languages. Which in turn might be due to portability concerns with windows, e.g. Java's SecureDirectoryStream isn't available there[0]. Apparently windows does have an openat-like API[1], but it's low-level. Programmers aren't using them because the language stan…

This a serious issue in support of the language for running on a UNIX system.

Frankly blaming the kernel/filesystem for this is like saying I want my 720p monitor to display 8k correctly, it must be the display-drivers fault...

Re: The trouble with symbolic links

#40

Post by rob_c »

joosters wrote: post #14
rob_c wrote: post #12

Earlier quoted context omitted.

> but they cause a lot of unintended security vulnerabilities No, bad coders on UNIX platforms do this. The code may be valid code, but if it's intending to support running on UNIX it should do it properly not assuming it's on a FAT32 filesystem in 2022. Or, even better, run the code you don't trust on fat32 filesystems, see how far that gets.

Don't victim blame. Do you really think that using open(), stat(), lstat() (!), realpath(), mkdir(), rename() etc etc etc is a sign of a bad coder? The problem is that the APIs set you up for unexpected failure, and even some of the provided workarounds to 'safely' handle symlinks don't do it well enough. In the case of symlinks, I think it's fair to blame the tools rather than the workman.

This is not victim blame. RTFM READ IT!!!

Most sane languages and low level tools describe what you want and how to work correctly.

If you don't want this feature in the filesystem, move to one that doesn't support it, or better yet submit a patch to run the filesytem you want with this feature deactivted for "security concerns".

Demanding a whole OS change the way it works for bad/lazy/inept coders is akin to 2 people getting blind drunk and blaming the other person or the drink for the stupid things they did. Take some responsibility.

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