codeblog code is freedom — patching my itch

September 28, 2016

security things in Linux v4.5

Filed under: Chrome OS,Debian,Kernel,Security,Ubuntu,Ubuntu-Server — kees @ 1:58 pm

Previously: v4.4. Some things I found interesting in the Linux kernel v4.5:

CONFIG_IO_STRICT_DEVMEM

The CONFIG_STRICT_DEVMEM setting that has existed for a long time already protects system RAM from being accessible through the /dev/mem device node to root in user-space. Dan Williams added CONFIG_IO_STRICT_DEVMEM to extend this so that if a kernel driver has reserved a device memory region for use, it will become unavailable to /dev/mem also. The reservation in the kernel was to keep other kernel things from using the memory, so this is just common sense to make sure user-space can’t stomp on it either. Everyone should have this enabled. (And if you have a system where you discover you need IO memory access from userspace, you can boot with “iomem=relaxed” to disable this at runtime.)

If you’re looking to create a very bright line between user-space having access to device memory, it’s worth noting that if a device driver is a module, a malicious root user can just unload the module (freeing the kernel memory reservation), fiddle with the device memory, and then reload the driver module. So either just leave out /dev/mem entirely (not currently possible with upstream), build a monolithic kernel (no modules), or otherwise block (un)loading of modules (/proc/sys/kernel/modules_disabled).

ptrace fsuid checking

Jann Horn fixed some corner-cases in how ptrace access checks were handled on special files in /proc. For example, prior to this fix, if a setuid process temporarily dropped privileges to perform actions as a regular user, the ptrace checks would not notice the reduced privilege, possibly allowing a regular user to trick a privileged process into disclosing things out of /proc (ASLR offsets, restricted directories, etc) that they normally would be restricted from seeing.

ASLR entropy sysctl

Daniel Cashman standardized the way architectures declare their maximum user-space ASLR entropy (CONFIG_ARCH_MMAP_RND_BITS_MAX) and then created a sysctl (/proc/sys/vm/mmap_rnd_bits) so that system owners could crank up entropy. For example, the default entropy on 32-bit ARM was 8 bits, but the maximum could be as much as 16. If your 64-bit kernel is built with CONFIG_COMPAT, there’s a compat version of the sysctl as well, for controlling the ASLR entropy of 32-bit processes: /proc/sys/vm/mmap_rnd_compat_bits.

Here’s how to crank your entropy to the max, without regard to what architecture you’re on:

for i in "" "compat_"; do f=/proc/sys/vm/mmap_rnd_${i}bits; n=$(cat $f); while echo $n > $f ; do n=$(( n + 1 )); done; done

strict sysctl writes

Two years ago I added a sysctl for treating sysctl writes more like regular files (i.e. what’s written first is what appears at the start), rather than like a ring-buffer (what’s written last is what appears first). At the time it wasn’t clear what might break if this was enabled, so a WARN was added to the kernel. Since only one such string showed up in searches over the last two years, the strict writing mode was made the default. The setting remains available as /proc/sys/kernel/sysctl_writes_strict.

seccomp UM support

Mickaël Salaün added seccomp support (and selftests) for user-mode Linux. Moar architectures!

seccomp NNP vs TSYNC fix

Jann Horn noticed and fixed a problem where if a seccomp filter was already in place on a process (after being installed by a privileged process like systemd, a container launcher, etc) then the setting of the “no new privs” flag could be bypassed when adding filters with the SECCOMP_FILTER_FLAG_TSYNC flag set. Bypassing NNP meant it might be possible to trick a buggy setuid program into doing things as root after a seccomp filter forced a privilege drop to fail (generally referred to as the “sendmail setuid flaw”). With NNP set, a setuid program can’t be run in the first place.

That’s it! Next I’ll cover v4.6

Edit: Added notes about “iomem=…”

© 2016, Kees Cook. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 License.
CC BY-SA 4.0

September 27, 2016

security things in Linux v4.4

Filed under: Chrome OS,Debian,Kernel,Security,Ubuntu,Ubuntu-Server — kees @ 2:47 pm

Previously: v4.3. Continuing with interesting security things in the Linux kernel, here’s v4.4. As before, if you think there’s stuff I missed that should get some attention, please let me know.

seccomp Checkpoint/Restore-In-Userspace

Tycho Andersen added a way to extract and restore seccomp filters from running processes via PTRACE_SECCOMP_GET_FILTER under CONFIG_CHECKPOINT_RESTORE. This is a continuation of his work (that I failed to mention in my prior post) from v4.3, which introduced a way to suspend and resume seccomp filters. As I mentioned at the time (and for which he continues to quote me) “this feature gives me the creeps.” :)

x86 W^X detection

Stephen Smalley noticed that there was still a range of kernel memory (just past the end of the kernel code itself) that was incorrectly marked writable and executable, defeating the point of CONFIG_DEBUG_RODATA which seeks to eliminate these kinds of memory ranges. He corrected this in v4.3 and added CONFIG_DEBUG_WX in v4.4 which performs a scan of memory at boot time and yells loudly if unexpected memory protection are found. To nobody’s delight, it was shortly discovered the UEFI leaves chunks of memory in this state too, which posed an ugly-to-solve problem (which Matt Fleming addressed in v4.6).

x86_64 vsyscall CONFIG

I introduced a way to control the mode of the x86_64 vsyscall with a build-time CONFIG selection, though the choice I really care about is CONFIG_LEGACY_VSYSCALL_NONE, to force the vsyscall memory region off by default. The vsyscall memory region was always mapped into process memory at a fixed location, and it originally posed a security risk as a ROP gadget execution target. The vsyscall emulation mode was added to mitigate the problem, but it still left fixed-position static memory content in all processes, which could still pose a security risk. The good news is that glibc since version 2.15 doesn’t need vsyscall at all, so it can just be removed entirely. Any kernel built this way that discovered they needed to support a pre-2.15 glibc could still re-enable it at the kernel command line with “vsyscall=emulate”.

That’s it for v4.4. Tune in tomorrow for v4.5!

© 2016, Kees Cook. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 License.
CC BY-SA 4.0

September 26, 2016

security things in Linux v4.3

Filed under: Chrome OS,Debian,Kernel,Security,Ubuntu,Ubuntu-Server — kees @ 2:54 pm

When I gave my State of the Kernel Self-Protection Project presentation at the 2016 Linux Security Summit, I included some slides covering some quick bullet points on things I found of interest in recent Linux kernel releases. Since there wasn’t a lot of time to talk about them all, I figured I’d make some short blog posts here about the stuff I was paying attention to, along with links to more information. This certainly isn’t everything security-related or generally of interest, but they’re the things I thought needed to be pointed out. If there’s something security-related you think I should cover from v4.3, please mention it in the comments. I’m sure I haven’t caught everything. :)

A note on timing and context: the momentum for starting the Kernel Self Protection Project got rolling well before it was officially announced on November 5th last year. To that end, I included stuff from v4.3 (which was developed in the months leading up to November) under the umbrella of the project, since the goals of KSPP aren’t unique to the project nor must the goals be met by people that are explicitly participating in it. Additionally, not everything I think worth mentioning here technically falls under the “kernel self-protection” ideal anyway — some things are just really interesting userspace-facing features.

So, to that end, here are things I found interesting in v4.3:

CONFIG_CPU_SW_DOMAIN_PAN

Russell King implemented this feature for ARM which provides emulated segregation of user-space memory when running in kernel mode, by using the ARM Domain access control feature. This is similar to a combination of Privileged eXecute Never (PXN, in later ARMv7 CPUs) and Privileged Access Never (PAN, coming in future ARMv8.1 CPUs): the kernel cannot execute user-space memory, and cannot read/write user-space memory unless it was explicitly prepared to do so. This stops a huge set of common kernel exploitation methods, where either a malicious executable payload has been built in user-space memory and the kernel was redirected to run it, or where malicious data structures have been built in user-space memory and the kernel was tricked into dereferencing the memory, ultimately leading to a redirection of execution flow.

This raises the bar for attackers since they can no longer trivially build code or structures in user-space where they control the memory layout, locations, etc. Instead, an attacker must find areas in kernel memory that are writable (and in the case of code, executable), where they can discover the location as well. For an attacker, there are vastly fewer places where this is possible in kernel memory as opposed to user-space memory. And as we continue to reduce the attack surface of the kernel, these opportunities will continue to shrink.

While hardware support for this kind of segregation exists in s390 (natively separate memory spaces), ARM (PXN and PAN as mentioned above), and very recent x86 (SMEP since Ivy-Bridge, SMAP since Skylake), ARM is the first upstream architecture to provide this emulation for existing hardware. Everyone running ARMv7 CPUs with this kernel feature enabled suddenly gains the protection. Similar emulation protections (PAX_MEMORY_UDEREF) have been available in PaX/Grsecurity for a while, and I’m delighted to see a form of this land in upstream finally.

To test this kernel protection, the ACCESS_USERSPACE and EXEC_USERSPACE triggers for lkdtm have existed since Linux v3.13, when they were introduced in anticipation of the x86 SMEP and SMAP features.

Ambient Capabilities

Andy Lutomirski (with Christoph Lameter and Serge Hallyn) implemented a way for processes to pass capabilities across exec() in a sensible manner. Until Ambient Capabilities, any capabilities available to a process would only be passed to a child process if the new executable was correctly marked with filesystem capability bits. This turns out to be a real headache for anyone trying to build an even marginally complex “least privilege” execution environment. The case that Chrome OS ran into was having a network service daemon responsible for calling out to helper tools that would perform various networking operations. Keeping the daemon not running as root and retaining the needed capabilities in children required conflicting or crazy filesystem capabilities organized across all the binaries in the expected tree of privileged processes. (For example you may need to set filesystem capabilities on bash!) By being able to explicitly pass capabilities at runtime (instead of based on filesystem markings), this becomes much easier.

For more details, the commit message is well-written, almost twice as long as than the code changes, and contains a test case. If that isn’t enough, there is a self-test available in tools/testing/selftests/capabilities/ too.

PowerPC and Tile support for seccomp filter

Michael Ellerman added support for seccomp to PowerPC, and Chris Metcalf added support to Tile. As the seccomp maintainer, I get excited when an architecture adds support, so here we are with two. Also included were updates to the seccomp self-tests (in tools/testing/selftests/seccomp), to help make sure everything continues working correctly.

That’s it for v4.3. If I missed stuff you found interesting, please let me know! I’m going to try to get more per-version posts out in time to catch up to v4.8, which appears to be tentatively scheduled for release this coming weekend. Next: v4.4.

© 2016, Kees Cook. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 License.
CC BY-SA 4.0

November 11, 2015

evolution of seccomp

Filed under: Debian,Kernel,Security,Ubuntu,Ubuntu-Server — kees @ 10:01 am

I’m excited to see other people thinking about userspace-to-kernel attack surface reduction ideas. Theo de Raadt recently published slides describing Pledge. This uses the same ideas that seccomp implements, but with less granularity. While seccomp works at the individual syscall level and in addition to killing processes, it allows for signaling, tracing, and errno spoofing. As de Raadt mentions, Pledge could be implemented with seccomp very easily: libseccomp would just categorize syscalls.

I don’t really understand the presentation’s mention of “Optional Security”, though. Pledge, like seccomp, is an opt-in feature. Nothing in the kernel refuses to run “unpledged” programs. I assume his point was that when it gets ubiquitously built into programs (like stack protector), it’s effectively not optional (which is alluded to later as “comprehensive applicability ~= mandatory mitigation”). Regardless, this sensible (though optional) design gets me back to his slide on seccomp, which seems to have a number of misunderstandings:

  • A Turing complete eBPF program watches your program Strictly speaking, seccomp is implemented using a subset of BPF, not eBPF. And since BPF (and eBPF) programs are guaranteed to halt, it makes seccomp filters not Turing complete.
  • Who watches the watcher? I don’t even understand this. It’s in the kernel. The kernel watches your program. Just like always. If this is a question of BPF program verification, there is literally a program verifier that checks various properties of the BPF program.
  • seccomp program is stored elsewhere This, with the next statement, is just totally misunderstood. Programs using seccomp define their program in their own code. It’s used the same way as the Pledge examples are shown doing.
  • Easy to get desyncronized either program is updated As above, this just isn’t the case. The only place where this might be true is when using seccomp on programs that were not written natively with seccomp. In that case, yes, desync is possible. But that’s one of the advantages of seccomp’s design: a program launcher (like minijail or systemd) can declare a seccomp filter for a program that hasn’t yet been ported to use one natively.
  • eBPF watcher has no real idea what the program under observation is doing… I don’t understand this statement. I don’t see how Pledge would “have a real idea” either: they’re both doing filtering. If we get AI out of our syscall filters, we’re in serious trouble. :)

OpenBSD has some interesting advantages in the syscall filtering department, especially around sockets. Right now, it’s hard for Linux syscall filtering to understand why a given socket is being used. Something like SOCK_DNS seems like it could be quite handy.

Another nice feature of Pledge is the path whitelist feature. As it’s still under development, I hope they expand this to include more things than just paths. Argument inspection is a weak point for seccomp, but under Linux, most of the arguments are ultimately exposed to the LSM layer. Last year I experimented with creating a “seccomp LSM” for path matching where programs could declare whitelists, similar to standard LSMs.

So, yes, Linux “could match this API on seccomp”. It’d just take some extensions to libseccomp to implement pledge(), as I described at the top. With OpenBSD doing a bunch of analysis work on common programs, it’d be excellent to see this usable on Linux too. So far on Linux, only a few programs (e.g. Chrome, vsftpd) have bothered to do this using seccomp, and it could be argued that this is ultimately due to how fine grained it is.

© 2015, Kees Cook. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 License.
CC BY-SA 4.0

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