[PATCH v3] ima: don't clear IMA_DIGSIG flag when setting or removing non-IMA xattr

[Coiby Xu]

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 writes/removes some other security xattr for the
file.

For example, on Fedora, after booting the kernel with "ima_appraise=fix
evm=fix ima_policy=appraise_tcb" and installing rpm-plugin-ima,
installing/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 security.selinux, the IMA_DIGSIG flag
that had been set early was cleared. As a result, IMA hash is generated
when the file is closed.

Similarly, IMA signature can be cleared on file close after removing
security xattr like security.evm or setting/removing ACL.

Prevent replacing the IMA file signature with a file hash, by preventing
the IMA_DIGSIG flag from being reset.

Here's a minimal C reproducer which sets security.selinux as the last
step which can also replaced by removing security.evm or setting ACL,

    #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 at redhat.com>
---
 security/integrity/ima/ima_appraise.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index f435eff4667f..5149ff4fd50d 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);
@@ -807,7 +818,7 @@ static int ima_inode_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
 			     const char *acl_name, struct posix_acl *kacl)
 {
 	if (evm_revalidate_status(acl_name))
-		ima_reset_appraise_flags(d_backing_inode(dentry), 0);
+		ima_reset_appraise_flags(d_backing_inode(dentry), -1);
 
 	return 0;
 }
@@ -815,11 +826,13 @@ static int ima_inode_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
 static int ima_inode_removexattr(struct mnt_idmap *idmap, struct dentry *dentry,
 				 const char *xattr_name)
 {
-	int result;
+	int result, digsig = -1;
 
 	result = ima_protect_xattr(dentry, xattr_name, NULL, 0);
 	if (result == 1 || evm_revalidate_status(xattr_name)) {
-		ima_reset_appraise_flags(d_backing_inode(dentry), 0);
+		if (!strcmp(xattr_name, XATTR_NAME_IMA))
+			digsig = 0;
+		ima_reset_appraise_flags(d_backing_inode(dentry), digsig);
 		if (result == 1)
 			result = 0;
 	}

base-commit: 7aac71907bdea16e2754a782b9d9155449a9d49d
-- 
2.51.0