Message-ID: <9d279fd3d7b3cbb2778183ec777d6b9da8a64b82.camel@linux.ibm.com>
Date: Thu, 16 Oct 2025 22:31:35 -0400
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Coiby Xu <coxu@...hat.com>
Cc: linux-integrity@...r.kernel.org,
        Dmitry Torokhov
 <dmitry.torokhov@...il.com>,
        Karel Srot <ksrot@...hat.com>,
        Roberto Sassu
 <roberto.sassu@...wei.com>,
        Dmitry Kasatkin	 <dmitry.kasatkin@...il.com>,
        Eric Snowberg <eric.snowberg@...cle.com>,
        Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn"	 <serge@...lyn.com>,
        "open list:SECURITY SUBSYSTEM"	 <linux-security-module@...r.kernel.org>,
        open list	 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ima: Fall back to default kernel module signature
 verification

On Thu, 2025-10-16 at 11:46 +0800, Coiby Xu wrote:
> On Tue, Sep 30, 2025 at 04:28:14PM -0400, Mimi Zohar wrote:
> > On Tue, 2025-09-30 at 09:57 -0400, Mimi Zohar wrote:
> > > On Sun, 2025-09-28 at 11:03 +0800, Coiby Xu wrote:
> > > > Currently, for any IMA policy that requires appraisal for kernel modules
> > > > e.g. ima_policy=secure_boot, PowerPC architecture specific policy,
> > > > booting will fail because IMA will reject a kernel module which will
> > > > be decompressed in the kernel space and then have its signature
> > > > verified.
> > > > 
> > > > This happens because when in-kernel module decompression
> > > > (CONFIG_MODULE_DECOMPRESS) is enabled, kmod will use finit_module
> > > > syscall instead of init_module to load a module. And IMA mandates IMA
> > > > xattr verification for finit_module unless appraise_type=imasig|modsig
> > > > is specified in the rule.  However currently initramfs doesn't support
> > > > xattr. And IMA rule "func=MODULE_CHECK appraise_type=imasig|modsig"
> > > > doesn't work either because IMA will treat to-be-decompressed kernel
> > > > module as not having module signature as it can't decompress kernel
> > > > module to check if signature exists.
> > > > 
> > > > So fall back to default kernel module signature verification when we have
> > > > no way to verify IMA xattr.
> > > > 
> > > > Reported-by: Karel Srot <ksrot@...hat.com>
> > > > Signed-off-by: Coiby Xu <coxu@...hat.com>
> > > > ---
> > > > Another approach will be to make IMA decompress the kernel module to
> > > > check the signature. This requires refactoring kernel module code to
> > > > make the in-kernel module decompressing feature modular and seemingly
> > > > more efforts are needed. A second disadvantage is it feels
> > > > counter-intuitive to verify the same kernel module signature twice. And
> > > > we still need to make ima_policy=secure_boot allow verifying appended
> > > > module signature.
> > > > 
> > > > Anyways, I'm open to suggestions and can try the latter approach if
> > > > there are some benefits I'm not aware of or a better approach.
> > > 
> > > Coiby, there are multiple issues being discussed here.  Before deciding on an
> > > appropriate solution, let's frame the issues(s) properly.
> 
> Hi Mimi,
> 
> Thanks for listing and framing the issues! Sorry, it took me a while to
> go through your feedback as I also had a 8-day holiday.
> 
> > > 
> > > 1. The finit_module syscall eventually calls init_module_from_file() to read the
> > > module into memory and then decompress it.  The problem is that the kernel
> > > module signature verification occurs during the kernel_read_file(), before the
> > > kernel module is decompressed.  Thus, the appended kernel module signature
> > > cannot be verified.
> 
> Since IMA only accesses a kernel module as a fd or struct file*, even if
> IMA hook is triggerd after kernel module is decompressed, IMA still
> doesn't know the default verification has passed or can't access the
> decompressed kernel buffer [2] to do the verification by itself.
> 
> [2] https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/module/main.c?h=v6.17#n3689
> 
> > > 
> > > 2. CPIO doesn't have xattr support. There were multiple attempts at including
> > > xattrs in CPIO, but none were upstreamed [1].  If file signatures stored in
> > > security.ima were available in the initramfs, then finit_module() could verify
> > > them, as opposed to the appended kernel module signature.
> 
> Thanks you for pointing me to the work [1]. I'll take a more careful
> look at [1]. I think CPIO supporting xattr can be a long-term solution
> and also close a important security gap.
> 
> > > 
> > > 3. The issues described above are generic, not limited to Power.  When
> > > CONFIG_MODULE_SIG is configured, the arch specific IMA policy rules do not
> > > include an "appraise func=MODULE_CHECK".
> 
> Yes, the issue is not limited to Power. And thanks for correcting me
> that Power arch specific IMA policy rules don't have this problem! Sorry
> I misread the code.
> 
> > > 
> > > 4. Unlike the arch specific IMA policy rules, the built-in secure boot IMA
> > > policy, specified on the boot command line as "ima_policy=secure_boot", always
> > > enforces the IMA signature stored in security.ima.
> > > 
> > > Partial solutions without kernel changes:
> > > - Enable CONFIG_MODULE_SIG  (Doesn't solve 4)
> > > - Disable kernel module compression.
> > > 
> > > Complete solution:
> > > - Pick up and upstream Roberto Sassu's last version of initramfs support [1].
> > > - Somehow prevent kernel_read_file() from failing when the kernel_read_file_id
> > > enumeration is READING_MODULE and the kernel module is compressed.  The change
> > > might be limited to ima_post_read_file().
> > 
> > or perhaps not totally.
> > 
> > init_module_from_file() doesn't pass the flags variable to kernel_read_file().
> > You might want to consider defining a new kernel_read_file_id enumeration named
> > READING_COMPRESSED_MODULE.
> 
> Thanks for suggesting the solutions! I like the solution of CPIO
> supporting xattr but it seems it won't land in upstream soon. So I
> prefer the last approach. I've implemented one [3] by defining a new
> kernel_read_file_id enumeration, would you like me to post the patches
> to the mailing list directly?
> 
> [3] https://github.com/coiby/linux/tree/in_kernel_decompression_ima

A few thoughts, before you post the patch.

1. In the general case, the kernel module could be compressed, but without an
appended signature.  The new code should only defer appended signature
verification, if there isn't an xattr or appended signature.

2. Instead of defining an additional process_measurement() argument to identify
compressed kernel modules, to simplify the code it might be possible to define a
new "func" named COMPRESSED_MODULE_CHECK.

 +       [READING_COMPRESSED_MODULE] = MODULE_CHECK,  -> COMPRESSED_MODULE_CHECK

3.  The patch title "ima: Use default kernel module signature verification for
compressed ones" is a bit off.  It should be something along the lines of "ima:
defer compressed kernel module appended signature verification".

4. Simplify the patch description.

> > 
Mimi

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