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Date: Mon, 13 May 2019 18:09:11 -0400 From: Mimi Zohar <zohar@...ux.ibm.com> To: Arvind Sankar <nivedita@...m.mit.edu> Cc: Arvind Sankar <niveditas98@...il.com>, Roberto Sassu <roberto.sassu@...wei.com>, Rob Landley <rob@...dley.net>, linux-kernel@...r.kernel.org, linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-integrity@...r.kernel.org, initramfs@...r.kernel.org Subject: Re: [PATCH v2 0/3] initramfs: add support for xattrs in the initial ram disk On Mon, 2019-05-13 at 14:47 -0400, Arvind Sankar wrote: > On Mon, May 13, 2019 at 02:36:24PM -0400, Mimi Zohar wrote: > > > > > > How does this work today then? Is it actually the case that initramfs > > > > just cannot be used on an IMA-enabled system, or it can but it leaves > > > > the initramfs unverified and we're trying to fix that? I had assumed the > > > > latter. > > > Oooh, it's done not by starting IMA appraisal later, but by loading a > > > default policy to ignore initramfs? > > > > Right, when rootfs is a tmpfs filesystem, it supports xattrs, allowing > > for finer grained policies to be defined. This patch set would allow > > a builtin IMA appraise policy to be defined which includes tmpfs. Clarification: finer grain IMA policy rules are normally defined in terms of LSM labels. The LSMs need to enabled, before writing IMA policy rules in terms of the LSM labels. > > > Ok, but wouldn't my idea still work? Leave the default compiled-in > policy set to not appraise initramfs. The embedded /init sets all the > xattrs, changes the policy to appraise tmpfs, and then exec's the real > init? Then everything except the embedded /init and the file with the > xattrs will be appraised, and the embedded /init was verified as part of > the kernel image signature. The only additional kernel change needed > then is to add a config option to the kernel to disallow overwriting the > embedded initramfs (or at least the embedded /init). Yes and no. The current IMA design allows a builtin policy to be specified on the boot command line ("ima_policy="), so that it exists from boot, and allows it to be replaced once with a custom policy. After that, assuming that CONFIG_IMA_WRITE_POLICY is configured, additional rules may be appended. As your embedded /init solution already replaces the builtin policy, the IMA policy couldn't currently be replaced a second time with a custom policy based on LSM labels. Mimi
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