Re: [VULN 4/4] Process auth man-in-the-middle
CC list reduced considering I'm going to ask about a slightly different topic. This is fantastic research Sergey, this vuln especially so. On Wed, 3 Nov 2021 at 03:49, Sergey Bugaev wrote: > > To get someone privileged to authenticate to me, I went with the same > exec(/bin/su) trick, which makes the root filesystem reauthenticate all of > the > processes file descriptors. If we place our own port among the file > descriptors, > we'll get a io_reauthenticate () call from the root filesystem on it, which > we'll forward to the proc server, pretending to reauthenticate our process. > > I've been meaning to ask: Why does the hurd attempt to re-authenticate open file descriptors during exec? It seems to eliminate a rather convenient method of delegation; a process opening a descriptor, forking and executing a child, and dropping privileges, while retaining access to that one resource. I realise you can still do this by manipulating ports directly (this only applies specifically to the contents of the descriptor table). Is it required for posix compliance somehow, or was there some other interesting use case? -- William ML Leslie
Re: [VULN 4/4] Process auth man-in-the-middle
On Fri, 5 Nov 2021 at 21:41, Samuel Thibault wrote: > William ML Leslie, le ven. 05 nov. 2021 21:18:50 +1100, a ecrit: > > > which makes the root filesystem reauthenticate all of the > > > processes file descriptors. > > > > It seems to eliminate a rather convenient method of delegation; a > > process opening a descriptor, forking and executing a child, and > > dropping privileges, while retaining access to that one resource. > > reauthenticating doesn't mean closing. File permissions for open are > checked at the open step, not later on. But then there are other things > than just opening a file, such as starting a translator, which we don't > necessarily want to let the unprivileged-with-one-opened-file do. > > Samuel > I see, thank you! -- William ML Leslie
Re: [VULN 4/4] Process auth man-in-the-middle
William ML Leslie, le ven. 05 nov. 2021 21:18:50 +1100, a ecrit: > > which makes the root filesystem reauthenticate all of the > > processes file descriptors. > > It seems to eliminate a rather convenient method of delegation; a > process opening a descriptor, forking and executing a child, and > dropping privileges, while retaining access to that one resource. reauthenticating doesn't mean closing. File permissions for open are checked at the open step, not later on. But then there are other things than just opening a file, such as starting a translator, which we don't necessarily want to let the unprivileged-with-one-opened-file do. Samuel
Re: [VULN 4/4] Process auth man-in-the-middle
William ML Leslie, le ven. 05 nov. 2021 21:18:50 +1100, a ecrit: > I've been meaning to ask: Why does the hurd attempt to re-authenticate open > file descriptors during exec? That's done only when the auth port changes, i.e. uid/gid etc. following a setuid/setgid/etc. trigger. Samuel
Re: [VULN 4/4] Process auth man-in-the-middle
On Fri, Nov 5, 2021 at 1:41 PM Samuel Thibault wrote: > > William ML Leslie, le ven. 05 nov. 2021 21:18:50 +1100, a ecrit: > > > which makes the root filesystem reauthenticate all of the > > > processes file descriptors. > > > > It seems to eliminate a rather convenient method of delegation; a > > process opening a descriptor, forking and executing a child, and > > dropping privileges, while retaining access to that one resource. > > reauthenticating doesn't mean closing. File permissions for open are > checked at the open step, not later on. But then there are other things > than just opening a file, such as starting a translator, which we don't > necessarily want to let the unprivileged-with-one-opened-file do. If I may add to what Samuel has said... The Hurd is a capability system, but not a *pure* capability system: it implements Unix semantics on top of Mach/capabilities. File descriptors, exec, setuid, etc. are all Unix concepts. And in a Unix system, you definitely expect that a change in a process' authority applies to things like which files it is allowed to open through a file descriptor that refers to a directory, which is why all the file descriptors have to be reauthenticated when the auth changes. The file descriptor table is really meant to hold I/O objects, not arbitrary ports; and I/O objects are expected to behave in a predictable way when reauthenticated: you keep access to the object, but the object reconsiders what further things you can do to it / access through it, according to your new authority. As another example, you can open a device while you have root access, then drop privileges; you'll still be able to read or write the file descriptor, but you won't be able to fchown/file_chown it. If you ignore the Unix personality of the system, you can indeed pass any ports to an otherwise unprivileged task, but you better do it outside of the file descriptors table. The Mach mechanisms for that are the bootstrap port and mach_ports_register (). Sergey
Re: [VULN 4/4] Process auth man-in-the-middle
On Fri, 5 Nov 2021 at 22:17, Sergey Bugaev wrote: > On Fri, Nov 5, 2021 at 1:41 PM Samuel Thibault > wrote: > > > > William ML Leslie, le ven. 05 nov. 2021 21:18:50 +1100, a ecrit: > > > > which makes the root filesystem reauthenticate all of the > > > > processes file descriptors. > > > > > > It seems to eliminate a rather convenient method of delegation; a > > > process opening a descriptor, forking and executing a child, and > > > dropping privileges, while retaining access to that one resource. > > > > reauthenticating doesn't mean closing. File permissions for open are > > checked at the open step, not later on. But then there are other things > > than just opening a file, such as starting a translator, which we don't > > necessarily want to let the unprivileged-with-one-opened-file do. > > If I may add to what Samuel has said... > > The Hurd is a capability system, but not a *pure* capability system: > it implements Unix semantics on top of Mach/capabilities. File > descriptors, exec, setuid, etc. are all Unix concepts. Indeed, and the reason I ask is because I'd like to enable more capability patterns, even though Mach and The HURD don't protect ports like they are capabilities (c.f. hurd/utils/msgport.c for some really cool and impressive "look at how much power you have!" moments). > And in a Unix > system, you definitely expect that a change in a process' authority > applies to things like which files it is allowed to open through a > file descriptor that refers to a directory, which is why all the file > descriptors have to be reauthenticated when the auth changes. The file > descriptor table is really meant to hold I/O objects, not arbitrary > ports; and I/O objects are expected to behave in a predictable way > when reauthenticated: you keep access to the object, but the object > reconsiders what further things you can do to it / access through it, > according to your new authority. I'm still not confident this is the right approach, but if that's limited to changing the behaviour of translator nodes that is much less scary. One wonderful property of capabilities is that delegating them works. You should be able to weaken them deliberately, but having them weakened for you is a bit weird. The stranger thing is having them become more powerful without an explicit action. Implicitly gaining more power on what is supposed to be a capability sounds like a new way to introduce a confused deputy vulnerability. I have read access to this, I exec you, and you have write access. If you were fishing this file out of the file system you'd expect it to have your authority, but if you were passed it from another program, you expect to use the authority that the caller provided and no higher. > As another example, you can open a > device while you have root access, then drop privileges; you'll still > be able to read or write the file descriptor, but you won't be able to > fchown/file_chown it. > > Righto yes, a port to an open file is a port to an io object, and io objects can implement messages like io_mod_owner and so. I'd missed that these were not distinct responsibilities. > If you ignore the Unix personality of the system, you can indeed pass > any ports to an otherwise unprivileged task, but you better do it > outside of the file descriptors table. The Mach mechanisms for that > are the bootstrap port and mach_ports_register (). > Or alternatively, ignore glibc exec() and talk to the exec and proc servers directly. -- William ML Leslie
Re: [VULN 4/4] Process auth man-in-the-middle
Disclaimer: while I'm a fan of capabilities / object capabilities / capability-based security, and also a "Unix person", I (obviously) wasn't among those who have designed Mach, or Hurd, or Unix. So I cannot speak authoritatively, I can only attempt to share what my understanding is. On Fri, Nov 5, 2021 at 3:52 PM William ML Leslie wrote: > > On Fri, 5 Nov 2021 at 22:17, Sergey Bugaev wrote: >> The Hurd is a capability system, but not a *pure* capability system: >> it implements Unix semantics on top of Mach/capabilities. File >> descriptors, exec, setuid, etc. are all Unix concepts. > > > Indeed, and the reason I ask is because I'd like to enable more capability > patterns, even though Mach and The HURD don't protect ports like they are > capabilities (c.f. hurd/utils/msgport.c for some really cool and impressive > "look at how much power you have!" moments). I don't think I understand what you mean by "don't protect ports like they are capabilities". Surely Mach ports are capabilities! You can get the ports that a task has if you have its task port, but in that case you already have full control over the task, so you could make it send the ports to you explicitly. msg_get_init_port () alternatively lets you specify the auth port as a credential, but if you have that you could trivially get the task port too. Capabilities can grant different access to the actual object they reference. In _object_ capability environments (think E or Java), this is not true, you either have a capability to the object or not. But then there are objects/capabilities whose only purpose is to provide limited access to another object/capability, so it's effectively the same thing. In Apple's Mach, there's a "task info port", task_info_t, that only grants you basic inspect-only access to a task. But actual Mach task ports (and the only kind that GNU Mach has) grant you complete, full access to the task. And if you have full access to the task, there's nothing insecure about getting the ports that the task has. > One wonderful property of capabilities is that delegating them works. You > should be able to weaken them deliberately, but having them weakened for you > is a bit weird. The stranger thing is having them become more powerful > without an explicit action. Implicitly gaining more power on what is > supposed to be a capability sounds like a new way to introduce a confused > deputy vulnerability. Well, yes, sure, setuid is the textbook example of how to get confused deputies: a setuid executable serves two masters (the invoker and the file owner), most of its context is set up by the invoker, so it needs to ignore things like LD_LIBRARY_PATH, but you can never remember all the things you have to ignore, and so on. But setuid is also the only way in Unix to raise privileges! Plan 9, for instance, has /dev/capuse, which lets you raise your privileges at run time (after authenticating). The Hurd does the same — you can authenticate to the password server and get a new auth port; and using the msg.defs you can even change auth of other processes this way, without them having to do anything explicitly. But the Hurd still has to keep support for the (arguably inferior) Unix setuid mechanism too. If strict Unix compatibility was not a concern, could addauth & friends replace setuid completely? I believe this would require converting all sorts of setuid utilities into privileged servers. The password server is essentially 'su' turned into a server; all the other setuid utilities would need similar treatment too. So for instance there'd be a ping server that does the privileged operation (using raw sockets) at the request of unprivileged clients. > Or alternatively, ignore glibc exec() and talk to the exec and proc servers > directly. Sure, you could do that, but then you wouldn't get the actual setuid behavior. /bin/su would still run as your user, and would not be able to give you a root shell. Sergey
