Date: Thu, 03 May 2018 16:38:57 -0500 From: ebiederm@...ssion.com (Eric W. Biederman) To: Mimi Zohar <zohar@...ux.vnet.ibm.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 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? >> 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. >> 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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