linux-kernel - Re: [PATCH v2] lsm,ima: new LSM hook security_kernel_module_read_file to access decompressed kernel module

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Message-ID: <CAHC9VhS6xWvu5TjjS4MRGFEWxdAhg-Xsf6L+=K0k8U+fgiAtTQ@mail.gmail.com>
Date: Wed, 5 Nov 2025 10:42:05 -0500
From: Paul Moore <paul@...l-moore.com>
To: Mimi Zohar <zohar@...ux.ibm.com>
Cc: Coiby Xu <coxu@...hat.com>, linux-integrity@...r.kernel.org, 
	linux-security-module@...r.kernel.org, Karel Srot <ksrot@...hat.com>, 
	James Morris <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>, 
	Luis Chamberlain <mcgrof@...nel.org>, Petr Pavlu <petr.pavlu@...e.com>, 
	Daniel Gomez <da.gomez@...nel.org>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Roberto Sassu <roberto.sassu@...wei.com>, Dmitry Kasatkin <dmitry.kasatkin@...il.com>, 
	Eric Snowberg <eric.snowberg@...cle.com>, open list <linux-kernel@...r.kernel.org>, 
	"open list:MODULE SUPPORT" <linux-modules@...r.kernel.org>
Subject: Re: [PATCH v2] lsm,ima: new LSM hook security_kernel_module_read_file
 to access decompressed kernel module

On Wed, Nov 5, 2025 at 9:07 AM Mimi Zohar <zohar@...ux.ibm.com> wrote:
> On Tue, 2025-11-04 at 21:47 -0500, Paul Moore wrote:
> > Assuming I'm understanding the problem correctly, I think you're
> > making this harder than it needs to be.  I believe something like this
> > should solve the problem without having to add more conditionals
> > around the hooks in kernel_read_file(), and limiting the multiple
> > security_kernel_post_read_file() calls to just the compressed case ...
> > and honestly in each of the _post_read_file() calls in the compressed
> > case, the buffer contents have changed so it somewhat makes sense.
>
> > Given the code below, IMA could simply ignore the
> > READING_MODULE_COMPRESSED case (or whatever it is the IMA needs to do
> > in that case) and focus on the READING_MODULE case as it does today.
> > I expect the associated IMA patch would be both trivial and small.
> >
> > diff --git a/kernel/module/main.c b/kernel/module/main.c
> > index c66b26184936..b435c498ec01 100644
> > --- a/kernel/module/main.c
> > +++ b/kernel/module/main.c
> > @@ -3675,17 +3675,19 @@ static int idempotent_wait_for_completion(struct idempot
> > ent *u)
> >
> > static int init_module_from_file(struct file *f, const char __user * uargs, int
> > flags)
> > {
> > +       bool compressed = !!(flags & MODULE_INIT_COMPRESSED_FILE);
> >        struct load_info info = { };
> >        void *buf = NULL;
> >        int len;
> >
> > -       len = kernel_read_file(f, 0, &buf, INT_MAX, NULL, READING_MODULE);
> > +       len = kernel_read_file(f, 0, &buf, INT_MAX, NULL,
> > +                              compressed ? READING_MODULE_COMPRESSED : READING_
> > MODULE);
> >        if (len < 0) {
> >                mod_stat_inc(&failed_kreads);
> >                return len;
> >        }
> >
> > -       if (flags & MODULE_INIT_COMPRESSED_FILE) {
> > +       if (compressed) {
> >                int err = module_decompress(&info, buf, len);
> >                vfree(buf); /* compressed data is no longer needed */
> >                if (err) {
> > @@ -3693,6 +3695,14 @@ static int init_module_from_file(struct file *f, const ch
> > ar __user * uargs, int
> >                        mod_stat_add_long(len, &invalid_decompress_bytes);
> >                        return err;
> >                }
> > +
> > +               err = security_kernel_post_read_file(f,
> > +                                                    (char *)info.hdr, info.len,
> > +                                                    READING_MODULE);
>
> Without changing the enumeration here, IMA would not be able to differentiate
> the first call to security_kernel_post_read_file() and this one.  The first call
> would result in unnecessary error messages.

Given the patch snippet above, in the case where an uncompressed
module is passed into init_module_from_file() there would be the
following checks, in this order:

 * kernel_read_file()
 -> security_kernel_read_file(READING_MODULE)
 -> security_kernel_post_read_file(READING_MODULE)
 * init_module_from_file()
 -> NONE

... this should be the same as the current behavior.

In the case where a compressed module is passed into
init_module_from_file() there would be the following checks, in this
order:

 * kernel_read_file()
 -> security_kernel_read_file(READING_MODULE_COMPRESSED)
 -> security_kernel_post_read_file(READING_MODULE_COMPRESSED)
 * init_module_from_file()
 -> security_kernel_post_read_file(READING_MODULE)

... the two differences being that the hook calls in
kernel_read_file() use the READING_MODULE_COMPRESSED id, which seems
appropriate as the data passed to the hook is the compressed
representation, and the additional _post_read_file() hook call in
init_module_from_file() using the READING_MODULE id, as the data
passed to the hook is now uncompressed.  Not only should IMA be able
to easily differentiate between the two _post_read_file() calls, but
it should have access to both the compressed and uncompressed data.

-- 
paul-moore.com