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

From: Richard Guy Briggs

Date: Wed Jul 22 2026 - 09:31:47 EST

On 2026-07-22 10:05, Ricardo Robaina wrote:
> On Tue, Jul 21, 2026 at 7:18 PM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> >
> > On 2026-07-13 14:00, Ricardo Robaina 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 ...
> >
> > This looks like it is missing "a0=..."?
> >
> > > type=FSCONFIG : fs_cmd=1 fs_key=source fs_val="tmpfs" fs_aux=0
> > > ----
> > > type=SYSCALL : syscall=fsconfig a1=FSCONFIG_CMD_CREATE ...
> >
> > This looks like it is missing "a0=..."?
> > I'm guessing this isn't actually real test output?
> >
>
> Thanks for reviewing this patch, Richard!
>
> That's real test output; I just trimmed the SYSCALL record output to
> fit in the commit message. Since this patch does not touch this
> record, I don't see a problem. I can
> add a0=... for consistency if you prefer, though.

Consistent use of ... would help to clarify that.

> > > type=FSCONFIG : fs_cmd=6 fs_key=(null) fs_val=(null) fs_aux=0
> >
> > Was there a fs_key output here that didn't make it due to the issue below?
>
> Actually, in this case, the test result happened to be right (that's
> why I missed it).
> FSCONFIG_CMD_CREATE has no key or value, so both are NULL[1].

Yup, fair, had to ask. :-)

> [1] https://man7.org/linux/man-pages/man2/fsconfig.2.html
>
> > > ----
> > > type=SYSCALL : syscall=fsconfig a0=0x4 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
> > > Signed-off-by: Ricardo Robaina <rrobaina@xxxxxxxxxx>
> > > ---
> > > 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>";
> > > 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/include/linux/audit.h b/include/linux/audit.h
> > > index 45abb3722d30..978eb881974c 100644
> > > --- a/include/linux/audit.h
> > > +++ b/include/linux/audit.h
> > > @@ -449,6 +449,8 @@ extern void __audit_tk_injoffset(struct timespec64 offset);
> > > extern void __audit_ntp_log(const struct audit_ntp_data *ad);
> > > extern void __audit_log_nfcfg(const char *name, u8 af, unsigned int nentries,
> > > enum audit_nfcfgop op, gfp_t gfp);
> > > +extern void __audit_log_fsconfig(unsigned int cmd, const char *key,
> > > + const char *value, int aux);
> > >
> > > static inline void audit_ipc_obj(struct kern_ipc_perm *ipcp)
> > > {
> > > @@ -598,6 +600,13 @@ static inline void audit_log_nfcfg(const char *name, u8 af,
> > > __audit_log_nfcfg(name, af, nentries, op, gfp);
> > > }
> > >
> > > +static inline void audit_log_fsconfig(unsigned int cmd, const char *key,
> > > + const char *value, int aux)
> > > +{
> > > + if (!audit_dummy_context())
> > > + __audit_log_fsconfig(cmd, key, value, aux);
> > > +}
> > > +
> > > extern int audit_n_rules;
> > > extern int audit_signals;
> > > #else /* CONFIG_AUDITSYSCALL */
> > > @@ -730,6 +739,10 @@ static inline void audit_log_nfcfg(const char *name, u8 af,
> > > enum audit_nfcfgop op, gfp_t gfp)
> > > { }
> > >
> > > +static inline void audit_log_fsconfig(unsigned int cmd, const char *key,
> > > + const char *value, int aux)
> > > +{ }
> > > +
> > > #define audit_n_rules 0
> > > #define audit_signals 0
> > > #endif /* CONFIG_AUDITSYSCALL */
> > > diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
> > > index e8f5ce677df7..5cd0eb602528 100644
> > > --- a/include/uapi/linux/audit.h
> > > +++ b/include/uapi/linux/audit.h
> > > @@ -122,6 +122,7 @@
> > > #define AUDIT_OPENAT2 1337 /* Record showing openat2 how args */
> > > #define AUDIT_DM_CTRL 1338 /* Device Mapper target control */
> > > #define AUDIT_DM_EVENT 1339 /* Device Mapper events */
> > > +#define AUDIT_FSCONFIG 1341 /* Record showing fsconfig args */
> > >
> > > #define AUDIT_AVC 1400 /* SE Linux avc denial or grant */
> > > #define AUDIT_SELINUX_ERR 1401 /* Internal SE Linux Errors */
> > > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > > index 6610e667c728..222bf79ee9bd 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=");
> > > + if (value)
> >
> > This looks like it should be "if (key)".
>
> Good catch!
>
> > > + 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
> >
> > - RGB
>
> -Ricardo

- RGB

--
Richard Guy Briggs <rgb@xxxxxxxxxx>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
Upstream IRC: SunRaycer
Voice: +1.613.860 2354 SMS: +1.613.518.6570