Re: [PATCH] selinux: fix off-by-one in setprocattr

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Feb 7, 2017 at 2:43 PM, Paul Moore <paul@paul-moore.com> wrote:
> On Tue, Jan 31, 2017 at 11:54 AM, Stephen Smalley <sds@tycho.nsa.gov> wrote:
>> SELinux tries to support setting/clearing of /proc/pid/attr attributes
>> from the shell by ignoring terminating newlines and treating an
>> attribute value that begins with a NUL or newline as an attempt to
>> clear the attribute.  However, the test for clearing attributes has
>> always been wrong; it has an off-by-one error, and this could further
>> lead to reading past the end of the allocated buffer since commit
>> bb646cdb12e75d82258c2f2e7746d5952d3e321a ("proc_pid_attr_write():
>> switch to memdup_user()").  Fix the off-by-one error.
>>
>> Even with this fix, setting and clearing /proc/pid/attr attributes
>> from the shell is not straightforward since the interface does not
>> support multiple write() calls (so shells that write the value and
>> newline separately will set and then immediately clear the attribute,
>> requiring use of echo -n to set the attribute), whereas trying to use
>> echo -n "" to clear the attribute causes the shell to skip the
>> write() call altogether since POSIX says that a zero-length write
>> causes no side effects. Thus, one must use echo -n to set and echo
>> without -n to clear, as in the following example:
>> $ echo -n unconfined_u:object_r:user_home_t:s0 > /proc/$$/attr/fscreate
>> $ cat /proc/$$/attr/fscreate
>> unconfined_u:object_r:user_home_t:s0
>> $ echo "" > /proc/$$/attr/fscreate
>> $ cat /proc/$$/attr/fscreate
>>
>> Note the use of /proc/$$ rather than /proc/self, as otherwise
>> the cat command will read its own attribute value, not that of the shell.
>>
>> There are no users of this facility to my knowledge; possibly we
>> should just get rid of it.

I'm not sure which facility you're referring to here, but setpriv(1)
uses /proc/self/attr/current and /proc/self/attr/exec.

--Andy