linux-kernel - Re: [PATCH] ima: don't clear IMA_DIGSIG flag when setting non-IMA xattr

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Message-ID: <d252b04934908e7e65a3299bfeffc282c7b0b12f.camel@linux.ibm.com>
Date: Thu, 04 Sep 2025 22:41:42 -0400
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Coiby Xu <coxu@...hat.com>, linux-integrity@...r.kernel.org
Cc: 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: don't clear IMA_DIGSIG flag when setting non-IMA
 xattr

On Tue, 2025-09-02 at 12:25 +0800, Coiby Xu wrote:
> Currently when both IMA and EVM are in fix mode, the IMA signature will
> be reset to IMA hash if a program first stores IMA signature in
> security.ima and then sets security.selinux for a file. For example, on
> Fedora, after booting the kernel with "ima_appraise=fix evm=fix
> ima_policy=appraise_tcb" and installing rpm-plugin-ima, reinstalling a
> package will not make good reference IMA signature generated. Instead
> IMA hash is generated,
>     # getfattr -m - -d -e hex /usr/bin/bash
>     # file: usr/bin/bash
>     security.ima=0x0404...
> 
> This happens because when setting selinux.selinux, the IMA_DIGSIG flag
> that had been set early was cleared. As a result, IMA hash is generated
> when the file is closed.
> 
> Here's a minimal C reproducer,
> 
>     #include <stdio.h>
>     #include <sys/xattr.h>
>     #include <fcntl.h>
>     #include <unistd.h>
>     #include <string.h>
>     #include <stdlib.h>
> 
>     int main() {
>         const char* file_path = "/usr/sbin/test_binary";
>         const char* hex_string = "030204d33204490066306402304";
>         int length = strlen(hex_string);
>         char* ima_attr_value;
>         int fd;
> 
>         fd = open(file_path, O_WRONLY|O_CREAT|O_EXCL, 0644);
>         if (fd == -1) {
>             perror("Error opening file");
>             return 1;
>         }
> 
>         ima_attr_value = (char*)malloc(length / 2 );
>         for (int i = 0, j = 0; i < length; i += 2, j++) {
>             sscanf(hex_string + i, "%2hhx", &ima_attr_value[j]);
>         }
> 
>         if (fsetxattr(fd, "security.ima", ima_attr_value, length/2, 0) == -1) {
>             perror("Error setting extended attribute");
>             close(fd);
>             return 1;
>         }
> 
>         const char* selinux_value= "system_u:object_r:bin_t:s0";
>         if (fsetxattr(fd, "security.selinux", selinux_value, strlen(selinux_value), 0) == -1) {
>             perror("Error setting extended attribute");
>             close(fd);
>             return 1;
>         }
> 
>         close(fd);
> 
>         return 0;
>     }
> 
> Signed-off-by: Coiby Xu <coxu@...hat.com>

Thanks, Coiby.  Agreed, the ability to clear the IMA_DIGSIG flag should be
limited to security.ima xattr and probably security.evm xattr.  Writing other
security xattrs should not affect the IMA_DIGSIG flag.

Even without an IMA appraise policy, the security xattrs are written out to the
filesystem, but the IMA_DIGSIG flag is not cached.

Please document the tristate values:
0: clear IMA_DIGSIG
1: set IMA_DIGSIG
-1: don't change IMA_DIGSIG

> ---
>  security/integrity/ima/ima_appraise.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index f435eff4667f..fc82161f8b30 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -708,7 +708,7 @@ static void ima_reset_appraise_flags(struct inode *inode, int digsig)
>  	set_bit(IMA_CHANGE_XATTR, &iint->atomic_flags);
>  	if (digsig)
>  		set_bit(IMA_DIGSIG, &iint->atomic_flags);

This matches both -1 and 1.  Test "digsig == 1" here.

> -	else
> +	else if (digsig != -1)

and test "digsig == 0" here.

>  		clear_bit(IMA_DIGSIG, &iint->atomic_flags);
>  }
>  
> @@ -794,6 +794,8 @@ static int ima_inode_setxattr(struct mnt_idmap *idmap, struct dentry *dentry,
>  		digsig = (xvalue->type == EVM_IMA_XATTR_DIGSIG);
>  	} else if (!strcmp(xattr_name, XATTR_NAME_EVM) && xattr_value_len > 0) {
>  		digsig = (xvalue->type == EVM_XATTR_PORTABLE_DIGSIG);
> +	} else if (result != 1) {

The "if (result != 1)" test is redundant.

thanks,

Mimi

> +		digsig = -1;
>  	}
>  	if (result == 1 || evm_revalidate_status(xattr_name)) {
>  		ima_reset_appraise_flags(d_backing_inode(dentry), digsig);