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.
