Re: [PATCH v2] audit: add FSCONFIG auxiliary record to log filesystem configuration

[Paul Moore <paul@paul-moore.com>]

On Jul 27, 2026 Ricardo Robaina <rrobaina@redhat.com> wrote:
> 
> Modern mount tools (util-linux >= 2.39.1) use the new mount API
> (fsopen, fsconfig, fsmount, move_mount) instead of the legacy mount(2)
> syscall. The generic SYSCALL audit record logs the fsconfig syscall but
> does not capture the configuration parameters, creating an audit gap for
> critical mount information such as the device being mounted.
> 
> Add an FSCONFIG auxiliary record that logs the command type, parameter
> name (key), parameter value, and aux parameter passed to fsconfig(2).
> 
>  ----
>  type=SYSCALL : syscall=fsconfig ... a1=FSCONFIG_SET_STRING ...
>  type=FSCONFIG : fs_cmd=1 fs_key=source fs_val="tmpfs" fs_aux=0
>  ----
>  type=SYSCALL : syscall=fsconfig ... a1=FSCONFIG_CMD_CREATE ...
>  type=FSCONFIG : fs_cmd=6 fs_key=(null) fs_val=(null) fs_aux=0
>  ----
>  type=SYSCALL : syscall=fsconfig ... a1=FSCONFIG_SET_BINARY ...
>  type=FSCONFIG : fs_cmd=2 fs_key=hidepid fs_val="<binary>" fs_aux=4
> 
> Link: https://github.com/linux-audit/audit-kernel/issues/153
> Acked-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Ricardo Robaina <rrobaina@redhat.com>
> ---
> Changes in v2:
> - Fixed null-check for fs_key field.
> - Clarified trimmed SYSCALL output in commit message examples. 
> 
>  fs/fsopen.c                |  7 +++++++
>  include/linux/audit.h      | 13 +++++++++++++
>  include/uapi/linux/audit.h |  1 +
>  kernel/auditsc.c           | 27 +++++++++++++++++++++++++++
>  4 files changed, 48 insertions(+)
> 
> diff --git a/fs/fsopen.c b/fs/fsopen.c
> index ae19e5136598..9b3c02f59df4 100644
> --- a/fs/fsopen.c
> +++ b/fs/fsopen.c
> @@ -15,6 +15,7 @@
>  #include <linux/namei.h>
>  #include <linux/file.h>
>  #include <uapi/linux/mount.h>
> +#include <linux/audit.h>
>  #include "internal.h"
>  #include "mount.h"
>  
> @@ -357,6 +358,7 @@ SYSCALL_DEFINE5(fsconfig,
>  	struct fs_context *fc;
>  	int ret;
>  	int lookup_flags = 0;
> +	const char *value_str = NULL;
>  
>  	struct fs_parameter param = {
>  		.type	= fs_value_is_undefined,
> @@ -423,6 +425,7 @@ SYSCALL_DEFINE5(fsconfig,
>  			goto out_key;
>  		}
>  		param.size = strlen(param.string);
> +		value_str = param.string;
>  		break;
>  	case FSCONFIG_SET_BINARY:
>  		param.type = fs_value_is_blob;
> @@ -432,6 +435,7 @@ SYSCALL_DEFINE5(fsconfig,
>  			ret = PTR_ERR(param.blob);
>  			goto out_key;
>  		}
> +		value_str = "<binary>";

Is there a reason why we're not logging the binary data?  We might want
to impose a size limit to truncate the logged data (although we have
provisions to log rather large binary chunks), but I don't see a reason
why we couldn't log the binary data as a hex string.

>  		break;
>  	case FSCONFIG_SET_PATH_EMPTY:
>  		lookup_flags = LOOKUP_EMPTY;
> @@ -445,6 +449,7 @@ SYSCALL_DEFINE5(fsconfig,
>  		}
>  		param.dirfd = aux;
>  		param.size = strlen(param.name->name);
> +		value_str = param.name->name;
>  		break;
>  	case FSCONFIG_SET_FD:
>  		param.type = fs_value_is_file;
> @@ -458,6 +463,8 @@ SYSCALL_DEFINE5(fsconfig,
>  		break;
>  	}
>  
> +	audit_log_fsconfig(cmd, param.key, value_str, aux);
> +
>  	ret = mutex_lock_interruptible(&fc->uapi_mutex);
>  	if (ret == 0) {
>  		ret = vfs_fsconfig_locked(fc, cmd, &param);

...

> diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> index 6610e667c728..fefc5c5dd4aa 100644
> --- a/kernel/auditsc.c
> +++ b/kernel/auditsc.c
> @@ -2882,6 +2882,33 @@ void __audit_log_nfcfg(const char *name, u8 af, unsigned int nentries,
>  }
>  EXPORT_SYMBOL_GPL(__audit_log_nfcfg);
>  
> +void __audit_log_fsconfig(unsigned int cmd, const char *key,
> +			  const char *value, int aux)
> +{
> +	struct audit_buffer *ab;
> +
> +	ab = audit_log_start(audit_context(), GFP_KERNEL, AUDIT_FSCONFIG);
> +	if (!ab)
> +		return;
> +
> +	audit_log_format(ab, "fs_cmd=%u", cmd);
> +	audit_log_format(ab, " fs_key=");

Calling into audit_log_format() is expensive due to the string
processing, we should combine this into a single audit_log_format()
call:

 audit_log_format(ab, "fs_cmd=%u fs_key=", cmd);

> +	if (key)
> +		audit_log_untrustedstring(ab, key);
> +	else
> +		audit_log_format(ab, "(null)");
> +
> +	audit_log_format(ab, " fs_val=");
> +	if (value)
> +		audit_log_untrustedstring(ab, value);
> +	else
> +		audit_log_format(ab, "(null)");
> +
> +	audit_log_format(ab, " fs_aux=%d", aux);
> +
> +	audit_log_end(ab);
> +}
> +
>  static void audit_log_task(struct audit_buffer *ab)
>  {
>  	kuid_t auid, uid;
> -- 
> 2.53.0

--
paul-moore.com