Re: [PATCH v2] ima: don't clear IMA_DIGSIG flag when setting non-IMA xattr

[Coiby Xu]

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 <c...@redhat.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