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

[Ricardo Robaina]

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.

> >  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].

[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
>
> --
> 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
>

-Ricardo