From: Andy Lutomirski Sent: November 13, 2018 at 5:47:16 PM GMT > To: Nadav Amit <nadav.amit@xxxxxxxxx> > Cc: Igor Stoppa <igor.stoppa@xxxxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, 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 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. Oh.. Of course. It is sort of a measure to prevent a single malicious/faulty write from corrupting the sensitive memory. Doing so limits the code that can compromise the security by a write to the protected data-structures (rephrasing for myself). I think I should add it as a comment in your patch.
