How to access users' files as root safely

The following is an excerpt taken from

Speaking of issues where a more or differently privileged process
accesses files in a directory writable by another process, these are
surprisingly difficult to "fully" deal with, and the majority of
programs are "affected".  Of the common Unix commands, only a handful
are safe to use in untrusted directories (with possible impact of
attacks being a mere DoS against the command itself) - such as "ln".
Yes, I actually use "ln" like this:

# su - user1
$ cp some/dir/file . # hopefully the ~/file name was not taken
$ exit
# ln ~user1/file .
# ls -ld file # sanity check that we were not raced
# chown user2: file
# ln file ~user2/
# rm file
# su - user2
$ mv file target/dir/
$ exit
# su - user1
$ rm file

when I need to copy a file from one untrusted user to another.  Yes,
this is complicated and it has limitations (same fs).  Maybe we need a
special tool for this task (I had some ideas on this - "give" and "take"
commands).  But that's the current situation.

Almost all other commands and programs are unsafe on untrusted
directories.  In my opinion, that's the only correct assumption for a
sysadmin to make, and any other assumption is naive.  Unfortunately,
most sysadmins don't fully realize this (in my experience), but that
does not make those programs any safer, nor does it prompt us to assign
CVE ids against almost all Unix programs.

Unfortunately, su is not perfect. An excerpt from

The admin could "su" to the service pseudo-user, but that allows the
compromised service to attack the admin's terminal, accessing the fd via
/proc or ptrace of a process such as "less" running under the "su"
session.  Only "su" itself is immune from such attacks (since it has its
"dumpable" flag cleared); its child processes are not.  The attacker
would be able to print control characters directly to the terminal fd,
and to issue ioctl's on it, changing the terminal mode.  This might have
a security impact worse than DoS.
internal/accessing-users-files-as-root-safely.txt · Last modified: 2023/05/10 13:46 by solar
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