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

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Message-ID: <53wb5tzech2k4k25xy2heq7ohmp2elw2a7l4x3nfk6fajfydur@5thsinydau5x>
Date: Mon, 8 Sep 2025 19:12:07 +0800
From: Coiby Xu <coxu@...hat.com>
To: Mimi Zohar <zohar@...ux.ibm.com>
Cc: linux-integrity@...r.kernel.org, 
	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 Thu, Sep 04, 2025 at 10:41:42PM -0400, Mimi Zohar wrote:
>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.

Thanks for confirming it!

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

It seems I miss some context for the above sentence. If no IMA policy is
configured, no ima_iint_cache will be created. If you mean non-appraisal
policy, will not caching IMA_DIGSIG flag cause any problem?

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

Addressed in v2. Thanks for the suggestion!

>
>> ---
>>  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.

I've fixed them in v2. Thanks for reviewing the patch and correcting my
careless mistakes! I'll check if my mind is in a clear thinking state
next time.


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

-- 
Best regards,
Coiby