On Tue, Nov 13, 2018 at 9:43 AM Nadav Amit <nadav.amit@xxxxxxxxx> wrote: > > From: Andy Lutomirski > Sent: November 13, 2018 at 5:16:09 PM GMT > > To: Igor Stoppa <igor.stoppa@xxxxxxxxx> > > Cc: Kees Cook <keescook@xxxxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Nadav Amit <nadav.amit@xxxxxxxxx>, Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>, Matthew Wilcox <willy@xxxxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, James Morris <jmorris@xxxxxxxxx>, Michal Hocko <mhocko@xxxxxxxxxx>, Kernel Hardening <kernel-hardening@xxxxxxxxxxxxxxxxxx>, linux-integrity <linux-integrity@xxxxxxxxxxxxxxx>, LSM List <linux-security-module@xxxxxxxxxxxxxxx>, Igor Stoppa <igor.stoppa@xxxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, Jonathan Corbet <corbet@xxxxxxx>, Laura Abbott <labbott@xxxxxxxxxx>, Randy Dunlap <rdunlap@xxxxxxxxxxxxx>, Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>, open list:DOCUMENTATION <linux-doc@xxxxxxxxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > Subject: Re: [PATCH 10/17] prmem: documentation > > > > > > On Tue, Nov 13, 2018 at 6:25 AM Igor Stoppa <igor.stoppa@xxxxxxxxx> wrote: > >> Hello, > >> I've been studying v4 of the patch-set [1] that Nadav has been working on. > >> Incidentally, I think it would be useful to cc also the > >> security/hardening ml. > >> The patch-set seems to be close to final, so I am resuming this discussion. > >> > >> On 30/10/2018 19:06, Andy Lutomirski wrote: > >> > >>> I support the addition of a rare-write mechanism to the upstream kernel. And I think that there is only one sane way to implement it: using an mm_struct. That mm_struct, just like any sane mm_struct, should only differ from init_mm in that it has extra mappings in the *user* region. > >> > >> After reading the code, I see what you meant. > >> I think I can work with it. > >> > >> But I have a couple of questions wrt the use of this mechanism, in the > >> context of write rare. > >> > >> > >> 1) mm_struct. > >> > >> Iiuc, the purpose of the patchset is mostly (only?) to patch kernel code > >> (live patch?), which seems to happen sequentially and in a relatively > >> standardized way, like replacing the NOPs specifically placed in the > >> functions that need patching. > >> > >> This is a bit different from the more generic write-rare case, applied > >> to data. > >> > >> As example, I have in mind a system where both IMA and SELinux are in use. > >> > >> In this system, a file is accessed for the first time. > >> > >> That would trigger 2 things: > >> - evaluation of the SELinux rules and probably update of the AVC cache > >> - IMA measurement and update of the measurements > >> > >> Both of them could be write protected, meaning that they would both have > >> to be modified through the write rare mechanism. > >> > >> While the events, for 1 specific file, would be sequential, it's not > >> difficult to imagine that multiple files could be accessed at the same time. > >> > >> If the update of the data structures in both IMA and SELinux must use > >> the same mm_struct, that would have to be somehow regulated and it would > >> introduce an unnecessary (imho) dependency. > >> > >> How about having one mm_struct for each writer (core or thread)? > > > > I don't think that helps anything. I think the mm_struct used for > > prmem (or rare_write or whatever you want to call it) should be > > entirely abstracted away by an appropriate API, so neither SELinux nor > > IMA need to be aware that there's an mm_struct involved. It's also > > entirely possible that some architectures won't even use an mm_struct > > behind the scenes -- x86, for example, could have avoided it if there > > were a kernel equivalent of PKRU. Sadly, there isn't. > > > >> 2) Iiuc, the purpose of the 2 pages being remapped is that the target of > >> the patch might spill across the page boundary, however if I deal with > >> the modification of generic data, I shouldn't (shouldn't I?) assume that > >> the data will not span across multiple pages. > > > > The reason for the particular architecture of text_poke() is to avoid > > memory allocation to get it working. i think that prmem/rare_write > > should have each rare-writable kernel address map to a unique user > > address, possibly just by offsetting everything by a constant. For > > rare_write, you don't actually need it to work as such until fairly > > late in boot, since the rare_writable data will just be writable early > > on. > > > >> If the data spans across multiple pages, in unknown amount, I suppose > >> that I should not keep interrupts disabled for an unknown time, as it > >> would hurt preemption. > >> > >> What I thought, in my initial patch-set, was to iterate over each page > >> that must be written to, in a loop, re-enabling interrupts in-between > >> iterations, to give pending interrupts a chance to be served. > >> > >> This would mean that the data being written to would not be consistent, > >> but it's a problem that would have to be addressed anyways, since it can > >> be still read by other cores, while the write is ongoing. > > > > This probably makes sense, except that enabling and disabling > > interrupts means you also need to restore the original mm_struct (most > > likely), which is slow. I don't think there's a generic way to check > > whether in interrupt is pending without turning interrupts on. > > I guess that enabling IRQs might break some hidden assumptions in the code, > but is there a fundamental reason that IRQs need to be disabled? use_mm() > got them enabled, although it is only suitable for kernel threads. > For general rare-writish stuff, I don't think we want IRQs running with them mapped anywhere for write. For AVC and IMA, I'm less sure. --Andy
