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Date:   Wed, 11 Aug 2021 12:49:04 -0500
From:   "Christopher M. Riedl" <cmr@...ux.ibm.com>
To:     "Christophe Leroy" <christophe.leroy@...roup.eu>,
        <linuxppc-dev@...ts.ozlabs.org>
Cc:     <keescook@...omium.org>, <peterz@...radead.org>, <x86@...nel.org>,
        <npiggin@...il.com>, <linux-hardening@...r.kernel.org>,
        <tglx@...utronix.de>, <dja@...ens.net>
Subject: Re: [PATCH v5 0/8] Use per-CPU temporary mappings for patching on
 Radix MMU

On Thu Aug 5, 2021 at 4:03 AM CDT, Christophe Leroy wrote:
>
>
> Le 13/07/2021 à 07:31, Christopher M. Riedl a écrit :
> > When compiled with CONFIG_STRICT_KERNEL_RWX, the kernel must create
> > temporary mappings when patching itself. These mappings temporarily
> > override the strict RWX text protections to permit a write. Currently,
> > powerpc allocates a per-CPU VM area for patching. Patching occurs as
> > follows:
> > 
> > 	1. Map page in per-CPU VM area w/ PAGE_KERNEL protection
> > 	2. Patch text
> > 	3. Remove the temporary mapping
> > 
> > While the VM area is per-CPU, the mapping is actually inserted into the
> > kernel page tables. Presumably, this could allow another CPU to access
> > the normally write-protected text - either malicously or accidentally -
> > via this same mapping if the address of the VM area is known. Ideally,
> > the mapping should be kept local to the CPU doing the patching [0].
> > 
> > x86 introduced "temporary mm" structs which allow the creation of mappings
> > local to a particular CPU [1]. This series intends to bring the notion of a
> > temporary mm to powerpc's Book3s64 Radix MMU and harden it by using such a
> > mapping for patching a kernel with strict RWX permissions.
> > 
> > The first four patches implement an LKDTM test "proof-of-concept" which
> > exploits the potential vulnerability (ie. the temporary mapping during patching
> > is exposed in the kernel page tables and accessible by other CPUs) using a
> > simple brute-force approach. This test is implemented for both powerpc and
> > x86_64. The test passes on powerpc Radix with this new series, fails on
> > upstream powerpc, passes on upstream x86_64, and fails on an older (ancient)
> > x86_64 tree without the x86_64 temporary mm patches. The remaining patches add
> > support for and use a temporary mm for code patching on powerpc with the Radix
> > MMU.
>
> I think four first patches (together with last one) are quite
> independent from the heart of the
> series itself which is patches 5, 6, 7. Maybe you should split that
> series it two series ? After all
> those selftests are nice to have but are not absolutely necessary, that
> would help getting forward I
> think.
>

Hmm you're probably right. The selftest at least proves there is a
potential attack which I think is necessary for any hardening related
series/patch. I'll split the series into separate powerpc temp mm and
LKDTM series for the next spin.

> > 
> > Tested boot, ftrace, and repeated LKDTM "hijack":
> > 	- QEMU+KVM (host: POWER9 Blackbird): Radix MMU w/ KUAP
> > 	- QEMU+KVM (host: POWER9 Blackbird): Hash MMU
> > 
> > Tested repeated LKDTM "hijack":
> > 	- QEMU+KVM (host: AMD desktop): x86_64 upstream
> > 	- QEMU+KVM (host: AMD desktop): x86_64 w/o percpu temp mm to
> > 	  verify the LKDTM "hijack" test fails
> > 
> > Tested boot and ftrace:
> > 	- QEMU+TCG: ppc44x (bamboo)
> > 	- QEMU+TCG: g5 (mac99)
> > 
> > I also tested with various extra config options enabled as suggested in
> > section 12) in Documentation/process/submit-checklist.rst.
> > 
> > v5:	* Only support Book3s64 Radix MMU for now. There are some issues with
> > 	  the previous implementation on the Hash MMU as pointed out by Nick
> > 	  Piggin. Fixing these is not trivial so we only support the Radix MMU
> > 	  for now. I tried using realmode (no data translation) to patch with
> > 	  Hash to at least avoid exposing the patch mapping to other CPUs but
> > 	  this doesn't reliably work either since we cannot access vmalloc'd
> > 	  space in realmode.
>
> So you now accept to have two different mode depending on the platform ?

By necessity yes.

> As far as I remember I commented some time ago that non SMP didn't need
> that feature and you were
> reluctant to have two different implementations. What made you change
> your mind ? (just curious).
>

The book3s64 hash mmu support is a pain ;) Supporting both the temp-mm
and vmalloc implementations turned out to be relatively simple - I
initially thought this would be messier. For now we will support both;
however, in the future I'd still like to implement the percpu temp-mm
support for the Hash MMU as well. I suppose we could re-evaluate then if
we want/need both implementations (I know you're in favor of keeping the
vmalloc-based approach for performance reasons on non-SMP).

>
> > 	* Use percpu variables for the patching_mm and patching_addr. This
> > 	  avoids the need for synchronization mechanisms entirely. Thanks to
> > 	  Peter Zijlstra for pointing out text_mutex which unfortunately didn't
> > 	  work out without larger code changes in powerpc. Also thanks to Daniel
> > 	  Axtens for comments about using percpu variables for the *percpu* temp
> > 	  mm things off list.
> > 
> > v4:	* It's time to revisit this series again since @jpn and @mpe fixed
> > 	  our known STRICT_*_RWX bugs on powerpc/64s.
> > 	* Rebase on linuxppc/next:
> >            commit ee1bc694fbaec ("powerpc/kvm: Fix build error when PPC_MEM_KEYS/PPC_PSERIES=n")
> > 	* Completely rework how map_patch() works on book3s64 Hash MMU
> > 	* Split the LKDTM x86_64 and powerpc bits into separate patches
> > 	* Annotate commit messages with changes from v3 instead of
> > 	  listing them here completely out-of context...
> > 
> > v3:	* Rebase on linuxppc/next: commit 9123e3a74ec7 ("Linux 5.9-rc1")
> > 	* Move temporary mm implementation into code-patching.c where it
> > 	  belongs
> > 	* Implement LKDTM hijacker test on x86_64 (on IBM time oof) Do
> > 	* not use address zero for the patching address in the
> > 	  temporary mm (thanks @dja for pointing this out!)
> > 	* Wrap the LKDTM test w/ CONFIG_SMP as suggested by Christophe
> > 	  Leroy
> > 	* Comments to clarify PTE pre-allocation and patching addr
> > 	  selection
> > 
> > v2:	* Rebase on linuxppc/next:
> > 	  commit 105fb38124a4 ("powerpc/8xx: Modify ptep_get()")
> > 	* Always dirty pte when mapping patch
> > 	* Use `ppc_inst_len` instead of `sizeof` on instructions
> > 	* Declare LKDTM patching addr accessor in header where it belongs	
> > 
> > v1:	* Rebase on linuxppc/next (4336b9337824)
> > 	* Save and restore second hw watchpoint
> > 	* Use new ppc_inst_* functions for patching check and in LKDTM test
> > 
> > rfc-v2:	* Many fixes and improvements mostly based on extensive feedback
> >            and testing by Christophe Leroy (thanks!).
> > 	* Make patching_mm and patching_addr static and move
> > 	  '__ro_after_init' to after the variable name (more common in
> > 	  other parts of the kernel)
> > 	* Use 'asm/debug.h' header instead of 'asm/hw_breakpoint.h' to
> > 	  fix PPC64e compile
> > 	* Add comment explaining why we use BUG_ON() during the init
> > 	  call to setup for patching later
> > 	* Move ptep into patch_mapping to avoid walking page tables a
> > 	  second time when unmapping the temporary mapping
> > 	* Use KUAP under non-radix, also manually dirty the PTE for patch
> > 	  mapping on non-BOOK3S_64 platforms
> > 	* Properly return any error from __patch_instruction
> >          * Do not use 'memcmp' where a simple comparison is appropriate
> > 	* Simplify expression for patch address by removing pointer maths
> > 	* Add LKDTM test
> > 
> > [0]: https://github.com/linuxppc/issues/issues/224
> > [1]: https://lore.kernel.org/kernel-hardening/20190426232303.28381-1-nadav.amit@gmail.com/
> > 
> > Christopher M. Riedl (8):
> >    powerpc: Add LKDTM accessor for patching addr
> >    lkdtm/powerpc: Add test to hijack a patch mapping
> >    x86_64: Add LKDTM accessor for patching addr
> >    lkdtm/x86_64: Add test to hijack a patch mapping
> >    powerpc/64s: Introduce temporary mm for Radix MMU
> >    powerpc: Rework and improve STRICT_KERNEL_RWX patching
> >    powerpc/64s: Initialize and use a temporary mm for patching on Radix
> >    lkdtm/powerpc: Fix code patching hijack test
> > 
> >   arch/powerpc/include/asm/code-patching.h |   4 +
> >   arch/powerpc/include/asm/debug.h         |   1 +
> >   arch/powerpc/kernel/process.c            |   5 +
> >   arch/powerpc/lib/code-patching.c         | 240 ++++++++++++++++++++---
> >   arch/x86/include/asm/text-patching.h     |   4 +
> >   arch/x86/kernel/alternative.c            |   7 +
> >   drivers/misc/lkdtm/core.c                |   1 +
> >   drivers/misc/lkdtm/lkdtm.h               |   1 +
> >   drivers/misc/lkdtm/perms.c               | 143 ++++++++++++++
> >   9 files changed, 378 insertions(+), 28 deletions(-)
> > 

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