Date: Tue, 10 Nov 2020 21:43:21 +0300 From: "Vladimir D. Seleznev" <vseleznv@...linux.org> To: oss-security@...ts.openwall.com Cc: "Demi M. Obenour" <demiobenour@...il.com> Subject: Re: The importance of mutual authentication: Local Privilege Escalation in X11 On Tue, Nov 10, 2020 at 12:51:27PM -0500, Demi M. Obenour wrote: > On 11/10/20 11:43 AM, Vladimir D. Seleznev wrote: > > On Mon, Nov 09, 2020 at 11:00:50AM -0500, Demi M. Obenour wrote: > >> [...skip...] > >> ### Placing the X socket in a secure directory > >> > >> X11 is usually used with AF_UNIX sockets. In this case, performing > >> the attack requires that either the directory containing the X socket > >> be writable by an attacker, or that the abstract namespace is in use. > >> If neither condition is met, the attack is thwarted. In this case, the > >> server is implicitly authenticated by being able to write to a location > >> on the file system. On systems other than macOS, placing the X socket > >> in a non-default directory requires changes to X. On Linux, this also > >> requires that abstract sockets be disabled in the X client libraries. > >> > >> A user’s home directory is a safe location on virtually all systems. > >> /run/user/$UID is a good choice when it is secure and available, > >> such as on systemd-based Linux distributions. /tmp/.X11-unix can > >> be made safer by ensuring that it is created before any untrusted > >> code runs and ensuring that untrusted code cannot write to it. > >> For example, it could be owned by root and have 0755 permissions. > >> For this to be effective, untrusted code must not be allowed to start > >> if creating /tmp/.X11-unix fails; this can be enforced by dropping > >> into single-user mode in this case. Furthermore, if the standard > >> location for lock files (/tmp/.X*-lock) is used, there is still a > >> potential denial of service, as anyone can create a lock file and > >> prevent the legitimate server from starting. > > > > This contravenes the ability to run X11 client from another user. The > > idea is that X11 server allows any clients with right credentials > > regardless of theirs processes UID or GID to connect to the server. > > Indeed it does, and I mention cryptographic authentication mechanisms > below. Instead of /tmp, /run/X11 would work just as well. It is > the mutual authentication that matters. Do I understand you correctly: you propose to forbid running X11 clients which processes belong to another users? In that case it is a bad idea: I would like to run untrusted clients with special UIDs. Or if I understand you wrongly, please explain how client of other user can connect to the socket placed in /run/user/$UID with these strict access permissions 0700? -- WBR, Vladimir D. Seleznev
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