LoadPin only enforces the read-only origin of kernel file reads. Whether
or not it was a partial read isn't important. Remove the overly
conservative checks so that things like partial firmware reads will
succeed (i.e. reading a firmware header).

Fixes: 2039bda1fa8d ("LSM: Add "contents" flag to kernel_read_file hook")
Cc: Paul Moore <paul@paul-moore.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-security-module@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 security/loadpin/loadpin.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
index de41621f4998..110a5ab2b46b 100644
--- a/security/loadpin/loadpin.c
+++ b/security/loadpin/loadpin.c
@@ -122,21 +122,11 @@ static void loadpin_sb_free_security(struct super_block *mnt_sb)
 	}
 }
 
-static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
-			     bool contents)
+static int loadpin_check(struct file *file, enum kernel_read_file_id id)
 {
 	struct super_block *load_root;
 	const char *origin = kernel_read_file_id_str(id);
 
-	/*
-	 * If we will not know that we'll be seeing the full contents
-	 * then we cannot trust a load will be complete and unchanged
-	 * off disk. Treat all contents=false hooks as if there were
-	 * no associated file struct.
-	 */
-	if (!contents)
-		file = NULL;
-
 	/* If the file id is excluded, ignore the pinning. */
 	if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
 	    ignore_read_file_id[id]) {
@@ -192,9 +182,25 @@ static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
 	return 0;
 }
 
+static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
+			     bool contents)
+{
+	/*
+	 * LoadPin only cares about the _origin_ of a file, not its
+	 * contents, so we can ignore the "are full contents available"
+	 * argument here.
+	 */
+	return loadpin_check(file, id);
+}
+
 static int loadpin_load_data(enum kernel_load_data_id id, bool contents)
 {
-	return loadpin_read_file(NULL, (enum kernel_read_file_id) id, contents);
+	/*
+	 * LoadPin only cares about the _origin_ of a file, not its
+	 * contents, so a NULL file is passed, and we can ignore the
+	 * state of "contents".
+	 */
+	return loadpin_check(NULL, (enum kernel_read_file_id) id);
 }
 
 static struct security_hook_list loadpin_hooks[] __lsm_ro_after_init = {
-- 
2.34.1

On Wed, Dec 14, 2022 at 11:51:15AM +0800, Ping-Ke Shih wrote:
> From: Kees Cook <keescook@chromium.org>
> 
> > LoadPin only enforces the read-only origin of kernel file reads. Whether
> > or not it was a partial read isn't important. Remove the overly
> > conservative checks so that things like partial firmware reads will
> > succeed (i.e. reading a firmware header).
> > 
> > Fixes: 2039bda1fa8d ("LSM: Add "contents" flag to kernel_read_file hook")
> > Cc: Paul Moore <paul@paul-moore.com>
> > Cc: James Morris <jmorris@namei.org>
> > Cc: "Serge E. Hallyn" <serge@hallyn.com>
> > Cc: linux-security-module@vger.kernel.org
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> 
> Tested-by: Ping-Ke Shih <pkshih@realtek.com>

Thanks for testing!

-Kees

> 
> > ---
> >  security/loadpin/loadpin.c | 30 ++++++++++++++++++------------
> >  1 file changed, 18 insertions(+), 12 deletions(-)
> > 
> > diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
> > index de41621f4998..110a5ab2b46b 100644
> > --- a/security/loadpin/loadpin.c
> > +++ b/security/loadpin/loadpin.c
> > @@ -122,21 +122,11 @@ static void loadpin_sb_free_security(struct super_block *mnt_sb)
> >  	}
> >  }
> >  
> > -static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
> > -			     bool contents)
> > +static int loadpin_check(struct file *file, enum kernel_read_file_id id)
> >  {
> >  	struct super_block *load_root;
> >  	const char *origin = kernel_read_file_id_str(id);
> >  
> > -	/*
> > -	 * If we will not know that we'll be seeing the full contents
> > -	 * then we cannot trust a load will be complete and unchanged
> > -	 * off disk. Treat all contents=false hooks as if there were
> > -	 * no associated file struct.
> > -	 */
> > -	if (!contents)
> > -		file = NULL;
> > -
> >  	/* If the file id is excluded, ignore the pinning. */
> >  	if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
> >  	    ignore_read_file_id[id]) {
> > @@ -192,9 +182,25 @@ static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
> >  	return 0;
> >  }
> >  
> > +static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
> > +			     bool contents)
> > +{
> > +	/*
> > +	 * LoadPin only cares about the _origin_ of a file, not its
> > +	 * contents, so we can ignore the "are full contents available"
> > +	 * argument here.
> > +	 */
> > +	return loadpin_check(file, id);
> > +}
> > +
> >  static int loadpin_load_data(enum kernel_load_data_id id, bool contents)
> >  {
> > -	return loadpin_read_file(NULL, (enum kernel_read_file_id) id, contents);
> > +	/*
> > +	 * LoadPin only cares about the _origin_ of a file, not its
> > +	 * contents, so a NULL file is passed, and we can ignore the
> > +	 * state of "contents".
> > +	 */
> > +	return loadpin_check(NULL, (enum kernel_read_file_id) id);
> >  }
> >  
> >  static struct security_hook_list loadpin_hooks[] __lsm_ro_after_init = {
> > -- 
> > 2.34.1
> 
> 

-- 
Kees Cook

On Fri, Dec 09, 2022 at 11:54:57AM -0800, Kees Cook wrote:
> LoadPin only enforces the read-only origin of kernel file reads. Whether
> or not it was a partial read isn't important. Remove the overly
> conservative checks so that things like partial firmware reads will
> succeed (i.e. reading a firmware header).
> 
> Fixes: 2039bda1fa8d ("LSM: Add "contents" flag to kernel_read_file hook")
> Cc: Paul Moore <paul@paul-moore.com>
> Cc: James Morris <jmorris@namei.org>
> Cc: "Serge E. Hallyn" <serge@hallyn.com>

Acked-by: Serge Hallyn <serge@hallyn.com>

Seems reasonable.

So the patch which introduced this was
2039bda1f: LSM: Add "contents" flag to kernel_read_file hook
It sounds like the usage of @contents which it added to ima still
makes sense.  But what about the selinux_kernel_read_file() one?

> Cc: linux-security-module@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  security/loadpin/loadpin.c | 30 ++++++++++++++++++------------
>  1 file changed, 18 insertions(+), 12 deletions(-)
> 
> diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
> index de41621f4998..110a5ab2b46b 100644
> --- a/security/loadpin/loadpin.c
> +++ b/security/loadpin/loadpin.c
> @@ -122,21 +122,11 @@ static void loadpin_sb_free_security(struct super_block *mnt_sb)
>  	}
>  }
>  
> -static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
> -			     bool contents)
> +static int loadpin_check(struct file *file, enum kernel_read_file_id id)
>  {
>  	struct super_block *load_root;
>  	const char *origin = kernel_read_file_id_str(id);
>  
> -	/*
> -	 * If we will not know that we'll be seeing the full contents
> -	 * then we cannot trust a load will be complete and unchanged
> -	 * off disk. Treat all contents=false hooks as if there were
> -	 * no associated file struct.
> -	 */
> -	if (!contents)
> -		file = NULL;
> -
>  	/* If the file id is excluded, ignore the pinning. */
>  	if ((unsigned int)id < ARRAY_SIZE(ignore_read_file_id) &&
>  	    ignore_read_file_id[id]) {
> @@ -192,9 +182,25 @@ static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
>  	return 0;
>  }
>  
> +static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
> +			     bool contents)
> +{
> +	/*
> +	 * LoadPin only cares about the _origin_ of a file, not its
> +	 * contents, so we can ignore the "are full contents available"
> +	 * argument here.
> +	 */
> +	return loadpin_check(file, id);
> +}
> +
>  static int loadpin_load_data(enum kernel_load_data_id id, bool contents)
>  {
> -	return loadpin_read_file(NULL, (enum kernel_read_file_id) id, contents);
> +	/*
> +	 * LoadPin only cares about the _origin_ of a file, not its
> +	 * contents, so a NULL file is passed, and we can ignore the
> +	 * state of "contents".
> +	 */
> +	return loadpin_check(NULL, (enum kernel_read_file_id) id);
>  }
>  
>  static struct security_hook_list loadpin_hooks[] __lsm_ro_after_init = {
> -- 
> 2.34.1

On Mon, Dec 12, 2022 at 03:13:19PM -0600, Serge E. Hallyn wrote:
> On Fri, Dec 09, 2022 at 11:54:57AM -0800, Kees Cook wrote:
> > LoadPin only enforces the read-only origin of kernel file reads. Whether
> > or not it was a partial read isn't important. Remove the overly
> > conservative checks so that things like partial firmware reads will
> > succeed (i.e. reading a firmware header).
> > 
> > Fixes: 2039bda1fa8d ("LSM: Add "contents" flag to kernel_read_file hook")
> > Cc: Paul Moore <paul@paul-moore.com>
> > Cc: James Morris <jmorris@namei.org>
> > Cc: "Serge E. Hallyn" <serge@hallyn.com>
> 
> Acked-by: Serge Hallyn <serge@hallyn.com>
> 
> Seems reasonable.

Thanks!

> So the patch which introduced this was
> 2039bda1f: LSM: Add "contents" flag to kernel_read_file hook
> It sounds like the usage of @contents which it added to ima still
> makes sense.  But what about the selinux_kernel_read_file() one?

I think those continue to make sense since those LSM may be sensitive to
the _content_ (rather than the _origin_) of the file.

-Kees

-- 
Kees Cook

On Tue, Dec 13, 2022 at 11:06 PM Kees Cook <keescook@chromium.org> wrote:
> On Mon, Dec 12, 2022 at 03:13:19PM -0600, Serge E. Hallyn wrote:
> > On Fri, Dec 09, 2022 at 11:54:57AM -0800, Kees Cook wrote:
> > > LoadPin only enforces the read-only origin of kernel file reads. Whether
> > > or not it was a partial read isn't important. Remove the overly
> > > conservative checks so that things like partial firmware reads will
> > > succeed (i.e. reading a firmware header).
> > >
> > > Fixes: 2039bda1fa8d ("LSM: Add "contents" flag to kernel_read_file hook")
> > > Cc: Paul Moore <paul@paul-moore.com>
> > > Cc: James Morris <jmorris@namei.org>
> > > Cc: "Serge E. Hallyn" <serge@hallyn.com>
> >
> > Acked-by: Serge Hallyn <serge@hallyn.com>
> >
> > Seems reasonable.
>
> Thanks!
>
> > So the patch which introduced this was
> > 2039bda1f: LSM: Add "contents" flag to kernel_read_file hook
> > It sounds like the usage of @contents which it added to ima still
> > makes sense.  But what about the selinux_kernel_read_file() one?
>
> I think those continue to make sense since those LSM may be sensitive to
> the _content_ (rather than the _origin_) of the file.

Agreed.  When @contents is false SELinux does a permission check
between the calling process and itself, but when @contents is true it
performs a check between the calling process and the file being read.

-- 
paul-moore.com