Message-ID: <1f4b4ff2ba92560f8b3cdd441e0844758eb080a4.camel@kernel.org>
Date: Thu, 15 Jan 2026 14:45:22 -0500
From: Jeff Layton <jlayton@...nel.org>
To: Frederick Lawler <fred@...udflare.com>, Roberto Sassu
	 <roberto.sassu@...weicloud.com>
Cc: Mimi Zohar <zohar@...ux.ibm.com>, Roberto Sassu
 <roberto.sassu@...wei.com>,  Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
 Eric Snowberg <eric.snowberg@...cle.com>, Paul Moore	
 <paul@...l-moore.com>, James Morris <jmorris@...ei.org>, "Serge E. Hallyn"	
 <serge@...lyn.com>, "Darrick J. Wong" <djwong@...nel.org>, Christian
 Brauner	 <brauner@...nel.org>, Josef Bacik <josef@...icpanda.com>, 
	linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org, 
	linux-security-module@...r.kernel.org, kernel-team@...udflare.com
Subject: Re: [PATCH] ima: Detect changes to files via kstat changes rather
 than i_version

On Thu, 2026-01-15 at 12:10 -0600, Frederick Lawler wrote:
> On Thu, Jan 15, 2026 at 12:46:37PM +0100, Roberto Sassu wrote:
> > On Mon, 2026-01-12 at 16:32 -0600, Frederick Lawler wrote:
> > > Commit 1cf7e834a6fb ("xfs: switch to multigrain timestamps")
> > > introduced a means to track change detection for an inode
> > > via ctime updates, opposed to setting kstat.change_cookie when
> > > calling into xfs_vn_getattr().
> > > 
> > > This introduced a regression because IMA caches kstat.change_cookie
> > > to compare against an inode's i_version directly in
> > > integrity_inode_attrs_changed(), and thus could be out of date
> > > depending on how file systems increment i_version.
> > > 
> > > To address this, require integrity_inode_attrs_changed() to query
> > > vfs_getattr_nosec() to compare the cached version against
> > > kstat.change_cookie directly. This ensures that when updates occur,
> > > we're accessing the same changed inode version on changes, and fallback
> > > to compare against an artificial version generated from kstat.ctime
> > > via integrity_ctime_guard() when there's no detected change
> > > to the kstat.change_cookie.
> > > 
> > > This ensures that in the absence of i_version support for file systems,
> > > and in the absence of a kstat.change_cookie update, we ultimately have a
> > > unique-enough version to compare against.
> > > 
> > > The exact implementation for integrity_ctime_guard() is to ensure that
> > > if tv_sec or tv_nsec are zero, there's some value to store back into
> > > struct integrity_inode_attributes.version. This also avoids the need to
> > > add additional storage and comparisons.
> > > 
> > > Lastly, because EVM still relies on querying and caching a backing inode's
> > > i_version, the integrity_inode_attrs_changed() falls back to the
> > > original inode.i_version != cached comparison. This maintains the
> > > invariant that a re-evaluation in unknown change detection circumstances
> > > is required.
> > > 
> > > Link: https://lore.kernel.org/all/aTspr4_h9IU4EyrR@CMGLRV3
> > > Suggested-by: Jeff Layton <jlayton@...nel.org>
> > > Signed-off-by: Frederick Lawler <fred@...udflare.com>
> > > ---
> > > We uncovered a case in kernels >= 6.13 where XFS is no longer updating
> > > struct kstat.change_cookie on i_op getattr() access calls. Instead, XFS is
> > > using multigrain ctime (as well as other file systems) for
> > > change detection in commit 1cf7e834a6fb ("xfs: switch to
> > > multigrain timestamps").
> > > 
> > > Because file systems may implement i_version as they see fit, IMA
> > > caching may be behind as well as file systems that don't support/export
> > > i_version. Thus we're proposing to compare against the kstat.change_cookie
> > > directly to the cached version, and fall back to a ctime guard when
> > > that's not updated.
> > > 
> > > EVM is largely left alone since there's no trivial way to query a file
> > > directly in the LSM call paths to obtain kstat.change_cookie &
> > > kstat.ctime to cache. Thus retains accessing i_version directly.
> > > 
> > > Regression tests will be added to the Linux Test Project instead of
> > > selftest to help catch future file system changes that may impact
> > > future evaluation of IMA.
> > > 
> > > I'd like this to be backported to at least 6.18 if possible.
> > > 
> > > Below is a simplified test that demonstrates the issue:
> > > 
> > > _fragment.config_
> > > CONFIG_XFS_FS=y
> > > CONFIG_OVERLAY_FS=y
> > > CONFIG_IMA=y
> > > CONFIG_IMA_WRITE_POLICY=y
> > > CONFIG_IMA_READ_POLICY=y
> > > 
> > > _./test.sh_
> > > 
> > > IMA_POLICY="/sys/kernel/security/ima/policy"
> > > TEST_BIN="/bin/date"
> > > MNT_BASE="/tmp/ima_test_root"
> > > 
> > > mkdir -p "$MNT_BASE"
> > > mount -t tmpfs tmpfs "$MNT_BASE"
> > > mkdir -p "$MNT_BASE"/{xfs_disk,upper,work,ovl}
> > > 
> > > dd if=/dev/zero of="$MNT_BASE/xfs.img" bs=1M count=300
> > > mkfs.xfs -q "$MNT_BASE/xfs.img"
> > > mount "$MNT_BASE/xfs.img" "$MNT_BASE/xfs_disk"
> > > cp "$TEST_BIN" "$MNT_BASE/xfs_disk/test_prog"
> > > 
> > > mount -t overlay overlay -o \
> > > "lowerdir=$MNT_BASE/xfs_disk,upperdir=$MNT_BASE/upper,workdir=$MNT_BASE/work" \
> > > "$MNT_BASE/ovl"
> > > 
> > > echo "audit func=BPRM_CHECK uid=$(id -u nobody)" > "$IMA_POLICY"
> > > 
> > > target_prog="$MNT_BASE/ovl/test_prog"
> > > setpriv --reuid nobody "$target_prog"
> > > setpriv --reuid nobody "$target_prog"
> > > setpriv --reuid nobody "$target_prog"
> > > 
> > > audit_count=$(dmesg | grep -c "file=\"$target_prog\"")
> > > 
> > > if [[ "$audit_count" -eq 1 ]]; then
> > >         echo "PASS: Found exactly 1 audit event."
> > > else
> > >         echo "FAIL: Expected 1 audit event, but found $audit_count."
> > >         exit 1
> > > fi
> > > ---
> > > Changes since RFC:
> > > - Remove calls to I_IS_VERSION()
> > > - Function documentation/comments
> > > - Abide IMA/EVM change detection fallback invariants
> > > - Combined ctime guard into version for attributes struct
> > > - Link to RFC: https://lore.kernel.org/r/20251229-xfs-ima-fixup-v1-1-6a717c939f7c@cloudflare.com
> > > ---
> > >  include/linux/integrity.h         | 42 +++++++++++++++++++++++++++++++++++----
> > >  security/integrity/evm/evm_main.c |  5 ++---
> > >  security/integrity/ima/ima_api.c  | 11 +++++++---
> > >  security/integrity/ima/ima_main.c | 15 +++++---------
> > >  4 files changed, 53 insertions(+), 20 deletions(-)
> > > 
> > > diff --git a/include/linux/integrity.h b/include/linux/integrity.h
> > > index f5842372359be5341b6870a43b92e695e8fc78af..5eca8aa2769f9238c68bb40885ecc46910524f11 100644
> > > --- a/include/linux/integrity.h
> > > +++ b/include/linux/integrity.h
> > > @@ -9,6 +9,7 @@
> > >  
> > >  #include <linux/fs.h>
> > >  #include <linux/iversion.h>
> > > +#include <linux/kernel.h>
> > >  
> > >  enum integrity_status {
> > >  	INTEGRITY_PASS = 0,
> > > @@ -36,6 +37,14 @@ struct integrity_inode_attributes {
> > >  	dev_t dev;
> > >  };
> > >  
> > > +/*
> > > + * Wrapper to generate an artificial version for a file.
> > > + */
> > > +static inline u64 integrity_ctime_guard(struct kstat stat)
> > > +{
> > > +	return stat.ctime.tv_sec ^ stat.ctime.tv_nsec;
> > 
> > Unfortunately, we cannot take the risk of a collision. Better use all
> > or a packed version.
> 
> Sounds good.
> 
> > 
> > > +}
> > > +
> > >  /*
> > >   * On stacked filesystems the i_version alone is not enough to detect file data
> > >   * or metadata change. Additional metadata is required.
> > > @@ -51,14 +60,39 @@ integrity_inode_attrs_store(struct integrity_inode_attributes *attrs,
> > >  
> > >  /*
> > >   * On stacked filesystems detect whether the inode or its content has changed.
> > > + *
> > > + * Must be called in process context.
> > >   */
> > >  static inline bool
> > >  integrity_inode_attrs_changed(const struct integrity_inode_attributes *attrs,
> > > -			      const struct inode *inode)
> > > +			      struct file *file, struct inode *inode)
> > >  {
> > > -	return (inode->i_sb->s_dev != attrs->dev ||
> > > -		inode->i_ino != attrs->ino ||
> > > -		!inode_eq_iversion(inode, attrs->version));
> > > +	struct kstat stat;
> > > +
> > > +	might_sleep();
> > > +
> > > +	if (inode->i_sb->s_dev != attrs->dev || inode->i_ino != attrs->ino)
> > > +		return true;
> > > +
> > > +	/*
> > > +	 * EVM currently relies on backing inode i_version. While IS_I_VERSION
> > > +	 * is not a good indicator of i_version support, this still retains
> > > +	 * the logic such that a re-evaluation should still occur for EVM, and
> > > +	 * only for IMA if vfs_getattr_nosec() fails.
> > > +	 */
> > > +	if (!file || vfs_getattr_nosec(&file->f_path, &stat,
> > > +				       STATX_CHANGE_COOKIE | STATX_CTIME,
> > > +				       AT_STATX_SYNC_AS_STAT))
> > > +		return !IS_I_VERSION(inode) ||
> > > +			!inode_eq_iversion(inode, attrs->version);
> > > +
> > > +	if (stat.result_mask & STATX_CHANGE_COOKIE)
> > > +		return stat.change_cookie != attrs->version;
> > > +
> > > +	if (stat.result_mask & STATX_CTIME)
> > > +		return integrity_ctime_guard(stat) != attrs->version;
> > 
> > Yes, switching to the new field I guess it works, but I'm wondering if
> > we could have more uniformity across the filesystems, otherwise one has
> > to use one source for filesystem X, another source for filesystem Y.
> 
> Agreed. But I'm under the impression from casual searching, that most
> file systems are likely to support ctime, than setting the change cookie
> with an i_version or even having/updating i_version consistently.
> 
> Is there someone we could CC in here to get another opinion?
> 


Most filesystems properly support ctime. The problem is that only some
of them (so far) have multigrain ctime support, so on many filesystems
the ctime has quite coarse granularity (~1 jiffy or so).

Multigrain timestamps fix that. They guarantee that if you do
stat()+write()+stat() that the two stats will get different c/mtime
values. This is why we've disabled reporting the i_version via -
>getattr() in XFS. The ctime now provides better semantics for the
change attribute on XFS.

Most filesystems that support i_version now support multigrain
timestamps, so I sort of expect that in the future, we may end up
removing the i_version from some of these filesystems and just
manufacture it from the ctime.

We do need to convert more in-kernel filesystems to use multigrain
timestamps first though.

> > 
> > Thanks
> > 
> > Roberto
> > 
> > > +
> > > +	return true;
> > >  }
> > >  
> > >  
> > > diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> > > index 73d500a375cb37a54f295b0e1e93fd6e5d9ecddc..6a4e0e246005246d5700b1db590c1759242b9cb6 100644
> > > --- a/security/integrity/evm/evm_main.c
> > > +++ b/security/integrity/evm/evm_main.c
> > > @@ -752,9 +752,8 @@ bool evm_metadata_changed(struct inode *inode, struct inode *metadata_inode)
> > >  	bool ret = false;
> > >  
> > >  	if (iint) {
> > > -		ret = (!IS_I_VERSION(metadata_inode) ||
> > > -		       integrity_inode_attrs_changed(&iint->metadata_inode,
> > > -						     metadata_inode));
> > > +		ret = integrity_inode_attrs_changed(&iint->metadata_inode,
> > > +						    NULL, metadata_inode);
> > >  		if (ret)
> > >  			iint->evm_status = INTEGRITY_UNKNOWN;
> > >  	}
> > > diff --git a/security/integrity/ima/ima_api.c b/security/integrity/ima/ima_api.c
> > > index c35ea613c9f8d404ba4886e3b736c3bab29d1668..8096986f3689781d3cdf6595f330033782f9cc45 100644
> > > --- a/security/integrity/ima/ima_api.c
> > > +++ b/security/integrity/ima/ima_api.c
> > > @@ -272,10 +272,15 @@ int ima_collect_measurement(struct ima_iint_cache *iint, struct file *file,
> > >  	 * to an initial measurement/appraisal/audit, but was modified to
> > >  	 * assume the file changed.
> > >  	 */
> > > -	result = vfs_getattr_nosec(&file->f_path, &stat, STATX_CHANGE_COOKIE,
> > > +	result = vfs_getattr_nosec(&file->f_path, &stat,
> > > +				   STATX_CHANGE_COOKIE | STATX_CTIME,
> > >  				   AT_STATX_SYNC_AS_STAT);
> > > -	if (!result && (stat.result_mask & STATX_CHANGE_COOKIE))
> > > -		i_version = stat.change_cookie;
> > > +	if (!result) {
> > > +		if (stat.result_mask & STATX_CHANGE_COOKIE)
> > > +			i_version = stat.change_cookie;
> > > +		else if (stat.result_mask & STATX_CTIME)
> > > +			i_version = integrity_ctime_guard(stat);
> > > +	}
> > >  	hash.hdr.algo = algo;
> > >  	hash.hdr.length = hash_digest_size[algo];
> > >  
> > > diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> > > index 5770cf691912aa912fc65280c59f5baac35dd725..3a4c32e254f925bba85cb91b63744ac142b3b049 100644
> > > --- a/security/integrity/ima/ima_main.c
> > > +++ b/security/integrity/ima/ima_main.c
> > > @@ -22,6 +22,7 @@
> > >  #include <linux/mount.h>
> > >  #include <linux/mman.h>
> > >  #include <linux/slab.h>
> > > +#include <linux/stat.h>
> > >  #include <linux/xattr.h>
> > >  #include <linux/ima.h>
> > >  #include <linux/fs.h>
> > > @@ -191,18 +192,13 @@ static void ima_check_last_writer(struct ima_iint_cache *iint,
> > >  
> > >  	mutex_lock(&iint->mutex);
> > >  	if (atomic_read(&inode->i_writecount) == 1) {
> > > -		struct kstat stat;
> > > -
> > >  		clear_bit(IMA_EMITTED_OPENWRITERS, &iint->atomic_flags);
> > >  
> > >  		update = test_and_clear_bit(IMA_UPDATE_XATTR,
> > >  					    &iint->atomic_flags);
> > >  		if ((iint->flags & IMA_NEW_FILE) ||
> > > -		    vfs_getattr_nosec(&file->f_path, &stat,
> > > -				      STATX_CHANGE_COOKIE,
> > > -				      AT_STATX_SYNC_AS_STAT) ||
> > > -		    !(stat.result_mask & STATX_CHANGE_COOKIE) ||
> > > -		    stat.change_cookie != iint->real_inode.version) {
> > > +		    integrity_inode_attrs_changed(&iint->real_inode, file,
> > > +						  inode)) {
> > >  			iint->flags &= ~(IMA_DONE_MASK | IMA_NEW_FILE);
> > >  			iint->measured_pcrs = 0;
> > >  			if (update)
> > > @@ -328,9 +324,8 @@ static int process_measurement(struct file *file, const struct cred *cred,
> > >  	real_inode = d_real_inode(file_dentry(file));
> > >  	if (real_inode != inode &&
> > >  	    (action & IMA_DO_MASK) && (iint->flags & IMA_DONE_MASK)) {
> > > -		if (!IS_I_VERSION(real_inode) ||
> > > -		    integrity_inode_attrs_changed(&iint->real_inode,
> > > -						  real_inode)) {
> > > +		if (integrity_inode_attrs_changed(&iint->real_inode,
> > > +						  file, real_inode)) {
> > >  			iint->flags &= ~IMA_DONE_MASK;
> > >  			iint->measured_pcrs = 0;
> > >  		}
> > > 
> > > ---
> > > base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
> > > change-id: 20251212-xfs-ima-fixup-931780a62c2c
> > > 
> > > Best regards,
> > 

-- 
Jeff Layton <jlayton@...nel.org>

Powered by blists - more mailing lists