Messages in this thread | | | From | Paul Moore <> | Date | Thu, 22 Sep 2022 21:18:34 -0400 | Subject | Re: [PATCH 1/2] powerpc/pseries: block untrusted device tree changes when locked down |
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On Thu, Sep 22, 2022 at 3:38 PM Nathan Lynch <nathanl@linux.ibm.com> wrote: > > The /proc/powerpc/ofdt interface allows the root user to freely alter > the in-kernel device tree, enabling arbitrary physical address writes > via drivers that could bind to malicious device nodes, thus making it > possible to disable lockdown. > > Historically this interface has been used on the pseries platform to > facilitate the runtime addition and removal of processor, memory, and > device resources (aka Dynamic Logical Partitioning or DLPAR). Years > ago, the processor and memory use cases were migrated to designs that > happen to be lockdown-friendly: device tree updates are communicated > directly to the kernel from firmware without passing through untrusted > user space. I/O device DLPAR via the "drmgr" command in powerpc-utils > remains the sole legitimate user of /proc/powerpc/ofdt, but it is > already broken in lockdown since it uses /dev/mem to allocate argument > buffers for the rtas syscall. So only illegitimate uses of the > interface should see a behavior change when running on a locked down > kernel. > > Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com> > --- > arch/powerpc/platforms/pseries/reconfig.c | 5 +++++ > include/linux/security.h | 1 + > security/security.c | 1 + > 3 files changed, 7 insertions(+)
A couple of small nits below, but in general this seems reasonable. However, as we are currently at -rc6 I would like us to wait to merge this until after the upcoming merge window closes (I don't like merging new functionality into -next at -rc6).
https://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/lsm.git/tree/README.md
> diff --git a/include/linux/security.h b/include/linux/security.h > index 7bd0c490703d..1ca8dbacd3cc 100644 > --- a/include/linux/security.h > +++ b/include/linux/security.h > @@ -122,6 +122,7 @@ enum lockdown_reason { > LOCKDOWN_XMON_WR, > LOCKDOWN_BPF_WRITE_USER, > LOCKDOWN_DBG_WRITE_KERNEL, > + LOCKDOWN_DEVICE_TREE,
I would suggest moving LOCKDOWN_DEVICE_TREE to be next to LOCKDOWN_ACPI_TABLES. It's not a hard requirement, but it seems like a nice idea to group similar things when we can.
> LOCKDOWN_INTEGRITY_MAX, > LOCKDOWN_KCORE, > LOCKDOWN_KPROBES, > diff --git a/security/security.c b/security/security.c > index 4b95de24bc8d..2863fc31eec6 100644 > --- a/security/security.c > +++ b/security/security.c > @@ -60,6 +60,7 @@ const char *const lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = { > [LOCKDOWN_XMON_WR] = "xmon write access", > [LOCKDOWN_BPF_WRITE_USER] = "use of bpf to write user RAM", > [LOCKDOWN_DBG_WRITE_KERNEL] = "use of kgdb/kdb to write kernel RAM", > + [LOCKDOWN_DEVICE_TREE] = "modifying device tree contents",
Might as well move this one too.
> [LOCKDOWN_INTEGRITY_MAX] = "integrity", > [LOCKDOWN_KCORE] = "/proc/kcore access", > [LOCKDOWN_KPROBES] = "use of kprobes", > -- > 2.37.3
-- paul-moore.com
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