Date: Thu, 03 May 2018 17:57:55 -0400 From: Mimi Zohar <zohar@...ux.vnet.ibm.com> To: "Eric W. Biederman" <ebiederm@...ssion.com> Cc: Kees Cook <keescook@...omium.org>, David Howells <dhowells@...hat.com>, Matthew Garrett <mjg59@...gle.com>, linux-integrity@...r.kernel.org, linux-security-module@...r.kernel.org, kexec@...ts.infradead.org, linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com Subject: Re: [PATCH 0/3] kexec: limit kexec_load syscall On Thu, 2018-05-03 at 16:38 -0500, Eric W. Biederman wrote: > Mimi Zohar <zohar@...ux.vnet.ibm.com> writes: > > > [Cc'ing Kees and kernel-hardening] > > > > On Thu, 2018-05-03 at 15:13 -0500, Eric W. Biederman wrote: > >> Mimi Zohar <zohar@...ux.vnet.ibm.com> writes: > >> > >> > In environments that require the kexec kernel image to be signed, prevent > >> > using the kexec_load syscall. In order for LSMs and IMA to differentiate > >> > between kexec_load and kexec_file_load syscalls, this patch set adds a > >> > call to security_kernel_read_file() in kexec_load_check(). > >> > >> Having thought about it some more this justification for these changes > >> does not work. The functionality of kexec_load is already root-only. > >> So in environments that require the kernel image to be signed just don't > >> use kexec_load. Possibly even compile kexec_load out to save space > >> because you will never need it. You don't need a new security hook to > >> do any of that. Userspace is a very fine mechanism for being the > >> instrument of policy. > > > > True, for those building their own kernel, they can disable the old > > syscalls. The concern is not for those building their own kernels, > > but for those using stock kernels. > > > > By adding an LSM hook here in the kexec_load syscall, as opposed to an > > IMA specific hook, other LSMs can piggy back on top of it. Currently, > > both load_pin and SELinux are gating the kernel module syscalls based > > on security_kernel_read_file. > > > > If there was a similar option for the kernel image, I'm pretty sure > > other LSMs would use it. > > > > From an IMA perspective, there needs to be some method for only > > allowing signed code to be loaded, executed, etc. - kernel modules, > > kernel image/initramfs, firmware, policies. > > What is the IMA perspective. Why can't IMA trust appropriately > authorized userspace? Suppose a system owner wants to define a system wide policy that requires all code be signed - kernel modules, firmware, kexec image & initramfs, executables, mmapped files, etc - without having to rebuild the kernel. Without a call in kexec_load that isn't possible. > > >> If you don't trust userspace that needs to be spelled out very clearly. > >> You need to talk about what your threat models are. > >> > >> If the only justification is so that that we can't boot windows if > >> someone hacks into userspace it has my nack because that is another kind > >> of complete non-sense. > > > > The usecase is the ability to gate the kexec_load usage in stock > > kernels. > > But kexec_load is already gated. It requires CAP_SYS_BOOT. It isn't a matter of kexec_load already being gated, but of wanting a single place for defining a system wide policy, as described above. Mimi > > >> Given that you are not trusting userspace this changeset also probably > >> needs to have the kernel-hardening list cc'd. Because the only possible > >> justification I can imagine for something like this is kernel-hardening. > > > > Sure, Cc'ing linux-hardening and Kees. > > > > Mimi >
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