On Sun, 2017-06-11 at 13:44 +0200, Mickaël Salaün wrote: > On 10/06/2017 07:27, Tetsuo Handa wrote: > > Kees Cook wrote: > >> On Fri, Jun 9, 2017 at 10:23 AM, Matt Brown <[email protected]> wrote: > >>> what does everyone thing about a envp_blacklist option that is a list of > >>> environmental variables that will be stripped from exec calls. This can > >>> be done in the LSM hook bprm_check_security. > >>> > >>> Is there any reason on a hardened system why you would need the > >>> PYTHONINSPECT environmental variable? > >> > >> As part of shebang, it likely makes sense to whitelist (rather than > >> blacklist) the env of the restricted interpreters. Though this is > >> starting to get complex. :P > > > > Blacklisting environment variables is dangerous. I think that > > administrators can afford whitelisting environment variable names. > > I think that implementing whitelist of environment variable names > > as an independent LSM module would be fine. > > > > While it is true that things starts getting complex if we check environment > > variables, shebang will already become complex if it starts worrying about > > updating inode number list in order to close the race window between doing > > creat()+write()+close()+chmod()+rename() by the package manager and teaching > > the kernel the new inode number determined by creat(). We will need an > > interface for allowing the package manager to teach the kernel the new inode > > number and modification of the package manager, for the kernel side is doing > > inode number based blacklisting while user side can execute it before > > rename().
I don't think we're trying to protect against executing the interpreter prior to the rename. Rename, itself, would trigger associating the interpreter name with an inode number. > > -- > > To unsubscribe from this list: send the line "unsubscribe > > linux-security-module" in > > the body of a message to [email protected] > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > > > Using filesystem xattr seems like a good idea for this kind of > exceptions and instead of a hardcoded interpreter path. Something like > "security.tpe.interpreter=1|2" (bitmask for interpreter-only and/or CLI) > and "security.tpe.environment=HOME,LOGNAME" would be quite flexible to > configure a security policy for some binaries. This could also be > protected by IMA/EVM, if needed. Checking for the existence of an xattr without caching is relatively slow. I'm not sure that we would want to go this route. > This kind of xattr should be writable by the owner of the file. The TPE > LSM [1] could then take these xattr into account according to the TPE > policy. Security xattrs are only writable by root. Mimi > The "security.tpe.environment" could also be set on a script file to be > part of the union with the interpreter's environment whitelist. This may > be needed to be able to use environment variables as configuration in a > script. > > In the future, a "security.tpe.memory" could contain a set of flags as > PaX uses for mprotect-like exceptions (user.pax.flags). > > Userland daemons such as paxctld or paxrat could be used (with some > tweaks) to keep a consistent TPE policy over time. > > Mickaël > > > [1] https://lkml.kernel.org/r/[email protected] >
