On 2/8/2022 09:17, William Roberts wrote:
<snip>
This is getting too long for me.
I don't have a strong opinion either way. If one were to allow this
using a policy rule, it would result in a major policy breakage. The
rule would turn on extended perm checks across the entire system,
which the SELinux Reference Policy isn't written for. I can't speak
to the Android policy, but I would imagine it would be the similar
problem there too.
Excuse me if I am wrong but AFAIK adding a xperm rule does not turn on
xperm checks across the entire system.
It doesn't as you state below its target + class.
If i am not mistaken it will turn on xperm checks only for the
operations that have the same source and target/target class.
That's correct.
Just to clarify (Demi Marie also mentioned this earlier in the thread),
what I originally meant was how to emulate this patch by using policy
rules means you need a rule that allows the two ioctls on all domains
for all objects. That results in xperms checks enabled everywhere.
This is also why i don't (with the exception TIOSCTI for termdev
chr_file) use xperms by default.
1. it is really easy to selectively filter ioctls by adding xperm rules
for end users (and since ioctls are often device/driver specific they
know best what is needed and what not)
and FIONCLEX can be trivially bypassed unless fcntl(F_SETFD)
2. if you filter ioctls in upstream policy for example like i do with
TIOSCTI using for example (allowx foo bar (ioctl chr_file (not
(0xXXXX)))) then you cannot easily exclude additional ioctls later where source is
foo and target/tclass is bar/chr_file because there is already a rule in
place allowing the ioctl (and you cannot add rules)
Currently, fcntl flag F_SETFD is never checked, it's silently allowed, but
the equivalent FIONCLEX and FIOCLEX are checked. So if you wrote policy
to block the FIO*CLEX flags, it would be bypassable through F_SETFD and
FD_CLOEXEC. So the patch proposed makes the FIO flags behave like
F_SETFD. Which means upstream policy users could drop this allow, which
could then remove the target/class rule and allow all icotls. Which is easy
to prevent and fix you could be a rule in to allowx 0 as documented in the
wiki: https://selinuxproject.org/page/XpermRules
The questions I think we have here are:
1. Do we agree that the behavior between SETFD and the FIO flags are equivalent?
I think they are.
2. Do we want the interfaces to behave the same?
I think they should.
If you can bypass FIONCLEX and FIOCLEX checks by F_SETFD and FD_CLOEXEC,
then I agree that the two FIO checks don't have value and can be skipped
as F_SETFD is.
3. Do upstream users of the policy construct care?
The patch is backwards compat, but I don't want their to be cruft
floating around with extra allowxperm rules.
Reference policy does not have any xperm rules at this time. I looked
at the Fedora policy, and that doesn't have any.
--
Chris PeBenito
