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

[Mimi Zohar <zohar@linux.ibm.com>]

On Mon, 2025-09-15 at 13:55 +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 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@redhat.com>

Thanks, Coiby.  The patch is now queued in next-integrity.
> ---
>  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