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Message-ID: <asadtbf7itw2343rjwvqfdvhmofhu7k6n4vsos2lojt6gqtudv@l7cgdsfbuuku>
Date: Mon, 15 Sep 2025 14:03:32 +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 v2] ima: don't clear IMA_DIGSIG flag when setting non-IMA
xattr
On Wed, Sep 10, 2025 at 08:21:03AM -0400, Mimi Zohar wrote:
>On Mon, 2025-09-08 at 18:58 +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.
>
>The problem description should be generic.
>
>-> and then writes some other security xattr for the file.
Good advice! I've applied it to v3 with a slight change,
"... then writes/removes some other security xattr" in v3,
>
>Start a new paragraph here for the example.
>> 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.
>
>Start a new paragraph here, adding a sentence describing the solution to the
>problem. For example,
>
>Prevent replacing the IMA file signature with a file hash, by preventing the
>IMA_DIGSIG flag from being reset.
Thanks for the suggestion, applied to v3.
>
>>
>> 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. The updated patch looks good. Have you looked at the other
>calls to ima_reset_appraise_flags() to make sure they don't need to be adjusted?
Great question! I've updated the other two places in v3 which can
address two additional cases,
- remove xattr like security.evm
- set/remove ACL
Note I tried to write a C program to set/remove ACL but somehow the C
version of "setfacl -m u:test:r" always lead to "acl_set_fd: Invalid
argument". I bypass this issue by setting system.posix_acl_access xattr
directly. We can get the value by "getfattr -m system.posix_acl_access
-d -e hex" after "setfacl -m u:test:r /usr/sbin/test_binary".
>
>thanks,
>
>Mimi
>
>> ---
>> security/integrity/ima/ima_appraise.c | 15 +++++++++++++--
>> 1 file changed, 13 insertions(+), 2 deletions(-)
>>
>> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
>> index f435eff4667f..4e4750ea41ad 100644
>> --- a/security/integrity/ima/ima_appraise.c
>> +++ b/security/integrity/ima/ima_appraise.c
>> @@ -694,6 +694,15 @@ static int ima_protect_xattr(struct dentry *dentry, const char *xattr_name,
>> return 0;
>> }
>>
>> +/*
>> + * ima_reset_appraise_flags - reset ima_iint_cache flags
>> + *
>> + * @digsig: whether to clear/set IMA_DIGSIG flag, tristate values
>> + * 0: clear IMA_DIGSIG
>> + * 1: set IMA_DIGSIG
>> + * -1: don't change IMA_DIGSIG
>> + *
>> + */
>> static void ima_reset_appraise_flags(struct inode *inode, int digsig)
>> {
>> struct ima_iint_cache *iint;
>> @@ -706,9 +715,9 @@ static void ima_reset_appraise_flags(struct inode *inode, int digsig)
>> return;
>> iint->measured_pcrs = 0;
>> set_bit(IMA_CHANGE_XATTR, &iint->atomic_flags);
>> - if (digsig)
>> + if (digsig == 1)
>> set_bit(IMA_DIGSIG, &iint->atomic_flags);
>> - else
>> + else if (digsig == 0)
>> clear_bit(IMA_DIGSIG, &iint->atomic_flags);
>> }
>>
>> @@ -794,6 +803,8 @@ 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 {
>> + digsig = -1;
>> }
>> if (result == 1 || evm_revalidate_status(xattr_name)) {
>> ima_reset_appraise_flags(d_backing_inode(dentry), digsig);
>
--
Best regards,
Coiby
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