Hello On Thu, Jun 29, 2023 at 3:30 AM Dominique Martinet <asmadeus@xxxxxxxxxxxxx> wrote: > > Jeff Xu wrote on Wed, Jun 28, 2023 at 09:33:27PM -0700: > > > > BTW I find the current behaviour rather hard to use: setting this to 2 > > > > should still set NOEXEC by default in my opinion, just refuse anything > > > > that explicitly requested EXEC. > > > > > > And I just noticed it's not possible to lower the value despite having > > > CAP_SYS_ADMIN: what the heck?! I have never seen such a sysctl and it > > > just forced me to reboot because I willy-nilly tested in the init pid > > > namespace, and quite a few applications that don't require exec broke > > > exactly as I described below. > > > > > > If the user has CAP_SYS_ADMIN there are more container escape methods > > > than I can count, this is basically free pass to root on main namespace > > > anyway, you're not protecting anything. Please let people set the sysctl > > > to what they want. > > > > Yama has a similar setting, for example, 3 (YAMA_SCOPE_NO_ATTACH) > > will not allow downgrading at runtime. > > > > Since this is a security feature, not allowing downgrading at run time > > is part of the security consideration. I hope you understand. > > I didn't remember yama had this stuck bit; that still strikes me as > unusual, and if you require a custom LSM rule for memfd anyway I don't > see why it couldn't enforce that the sysctl is unchanged, but sure. > > Please, though: > - I have a hard time thinking of 1 as a security flag in general (even > if I do agree a sloppy LSM rule could require it); I would only lock 2 > - please make it clear, I don't see any entry in the sysctl > documentation[1] about memfd_noexec, there should be one and you can > copy the wording from yama's doc[2]: "Once set, this sysctl value cannot > be changed" > [1] Documentation/admin-guide/sysctl/vm.rst > [2] Documentation/admin-guide/LSM/Yama.rst > Thanks for the suggestion. Yes, it would be good to have some documentation. I will send patch to update Documentation/admin-guide/sysctl/vm.rst > > Either way as it stands I still don't think one can expect most > userspace applications to be converted until some libc wrapper takes > care of the retry logic and a couple of years, so I'll go look for > another way of filtering this (and eventually setting this to 1) as you > suggested. > I'll leave the follow-up up to you and won't bother you more. > Not bothered at all! and thanks for reporting the bug to improve the quality of memfd_noexec ! Much appreciated. -Jeff > Thanks, > -- > Dominique Martinet | Asmadeus
