Message-ID: <0de47fe9d7c03398ce26c97739b69e89ecb425f2.camel@kernel.org>
Date: Tue, 20 Jan 2026 17:24:48 -0500
From: Jeff Layton <jlayton@...nel.org>
To: Frederick Lawler <fred@...udflare.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 v2] ima: Fallback to ctime check for FS without
 kstat.change_cookie

On Tue, 2026-01-20 at 15:49 -0600, Frederick Lawler wrote:
> On Tue, Jan 20, 2026 at 04:22:55PM -0500, Jeff Layton wrote:
> > On Tue, 2026-01-20 at 14:20 -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 to
> > > an i_version when calling into xfs_vn_getattr().
> > > 
> > > This introduced a regression for IMA such that an action
> > > performed on a LOWER inode on a stacked file systems always
> > > requires a re-evaluation if the LOWER file system does not
> > > leverage kstat.change_cookie to track inode i_version or lacks
> > > i_version support all together.
> > > 
> > > In the case of stacking XFS on XFS, an action on either the LOWER or UPPER
> > > will require re-evaluation. Stacking TPMFS on XFS for instance, once the
> > > inode is UPPER is mutated, IMA resumes normal behavior because TMPFS
> > > leverages generic_fillattr() to update the change cookie.
> > > 
> > > This is 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 could be out of date depending on how file systems set
> > > kstat.change_cookie.
> > > 
> > > 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 kstat.ctime when STATX_CHANGE_COOKIE is missing from
> > > result mask.
> > > 
> > > Lastly, because EVM still relies on querying and caching a inode's
> > > i_version directly, the integrity_inode_attrs_changed() falls back to the
> > > original inode.i_version != cached comparison.
> > > 
> > > 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
> > > ---
> > > Changes in v2:
> > > - Updated commit description + message to clarify the problem.
> > > - compare struct timespec64 to avoid collision possibility [Roberto].
> > > - Don't check inode_attr_changed() in ima_check_last_writer()
> > > - Link to v1: https://lore.kernel.org/r/20260112-xfs-ima-fixup-v1-1-8d13b6001312@cloudflare.com
> > > ---
> > >  include/linux/integrity.h           | 40 ++++++++++++++++++++++++++++++++-----
> > >  security/integrity/evm/evm_crypto.c |  4 +++-
> > >  security/integrity/evm/evm_main.c   |  5 ++---
> > >  security/integrity/ima/ima_api.c    | 20 +++++++++++++------
> > >  security/integrity/ima/ima_main.c   | 18 ++++++++++-------
> > >  5 files changed, 65 insertions(+), 22 deletions(-)
> > > 
> > > diff --git a/include/linux/integrity.h b/include/linux/integrity.h
> > > index f5842372359be5341b6870a43b92e695e8fc78af..46f57402b790c9c275b85f0b30819fa196fc20c7 100644
> > > --- a/include/linux/integrity.h
> > > +++ b/include/linux/integrity.h
> > > @@ -9,6 +9,8 @@
> > >  
> > >  #include <linux/fs.h>
> > >  #include <linux/iversion.h>
> > > +#include <linux/kernel.h>
> > > +#include <linux/time64.h>
> > >  
> > >  enum integrity_status {
> > >  	INTEGRITY_PASS = 0,
> > > @@ -31,6 +33,7 @@ static inline void integrity_load_keys(void)
> > >  
> > >  /* An inode's attributes for detection of changes */
> > >  struct integrity_inode_attributes {
> > > +	struct timespec64 ctime;
> > >  	u64 version;		/* track inode changes */
> > >  	unsigned long ino;
> > >  	dev_t dev;
> > > @@ -42,8 +45,10 @@ struct integrity_inode_attributes {
> > >   */
> > >  static inline void
> > >  integrity_inode_attrs_store(struct integrity_inode_attributes *attrs,
> > > -			    u64 i_version, const struct inode *inode)
> > > +			    u64 i_version, struct timespec64 ctime,
> > > +			    const struct inode *inode)
> > >  {
> > > +	attrs->ctime = ctime;
> > >  	attrs->version = i_version;
> > >  	attrs->dev = inode->i_sb->s_dev;
> > >  	attrs->ino = inode->i_ino;
> > > @@ -51,14 +56,39 @@ 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);
> > 
> > Seems like a reasonable way to handle getattr failure, given the
> > limitations on EVM.
> > 
> > > +
> > > +	if (stat.result_mask & STATX_CHANGE_COOKIE)
> > > +		return stat.change_cookie != attrs->version;
> > > +
> > > +	if (stat.result_mask & STATX_CTIME)
> > > +		return !timespec64_equal(&stat.ctime, &attrs->ctime);
> > > +
> > > +	return true;
> > >  }
> > >  
> > >  
> > > diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> > > index a5e730ffda57fbc0a91124adaa77b946a12d08b4..361ee7b216247a0d6d2f518e82fb6e92dc355afe 100644
> > > --- a/security/integrity/evm/evm_crypto.c
> > > +++ b/security/integrity/evm/evm_crypto.c
> > > @@ -297,10 +297,12 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry,
> > >  	hmac_add_misc(desc, inode, type, data->digest);
> > >  
> > >  	if (inode != d_backing_inode(dentry) && iint) {
> > > +		struct timespec64 ctime = {0};
> > > +
> > >  		if (IS_I_VERSION(inode))
> > >  			i_version = inode_query_iversion(inode); 
> > 
> > It would be nice if you could change this codepath as well, but that
> > looks like a much bigger project. FWIW, I took a quick look:
> > 
> > The problematic codepath is get/setxattr, which can call into this but
> > currently only receives a dentry and not a struct path. That would
> > probably have to be changed. I think it's doable but it's a fair bit of
> > yak shaving.
> 
> Agreed. I was looking at that a couple weeks ago to include a patch 2
> for EVM, and came to the same conclusion. I did find struct iattr on the
> setxattr which has struct file *, but that seemed dubious to at best.
> 

Yeah, it's not set in all cases. setxattr is a pathname-based
operation, so an open file sometimes just doesn't exist.
 
> Alternatively, create security_get/setxattr() equivalent hooks, or
> have an internal mapping of sorts.
> 

> 
Those are possibilities too.

A dentry without a mount is somewhat ambiguous though. IMO, it would be
best to just change all of the xattr_handler functions to take struct
path instead of a dentry.

Maybe a good topic for LSF/MM...

> > >  		integrity_inode_attrs_store(&iint->metadata_inode, i_version,
> > > -					    inode);
> > > +					    ctime, inode);
> > >  	}
> > >  
> > >  	/* Portable EVM signatures must include an IMA hash */
> > > 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..0d8e0a3ebd34b70bb1b4cc995aae5d4adc90a585 100644
> > > --- a/security/integrity/ima/ima_api.c
> > > +++ b/security/integrity/ima/ima_api.c
> > > @@ -254,6 +254,7 @@ int ima_collect_measurement(struct ima_iint_cache *iint, struct file *file,
> > >  	int length;
> > >  	void *tmpbuf;
> > >  	u64 i_version = 0;
> > > +	struct timespec64 ctime = {0};
> > >  
> > >  	/*
> > >  	 * Always collect the modsig, because IMA might have already collected
> > > @@ -272,10 +273,15 @@ 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;
> > > +		if (stat.result_mask & STATX_CTIME)
> > > +			ctime = stat.ctime;
> > > +	}
> > >  	hash.hdr.algo = algo;
> > >  	hash.hdr.length = hash_digest_size[algo];
> > >  
> > > @@ -305,11 +311,13 @@ int ima_collect_measurement(struct ima_iint_cache *iint, struct file *file,
> > >  
> > >  	iint->ima_hash = tmpbuf;
> > >  	memcpy(iint->ima_hash, &hash, length);
> > > -	if (real_inode == inode)
> > > +	if (real_inode == inode) {
> > >  		iint->real_inode.version = i_version;
> > > -	else
> > > +		iint->real_inode.ctime = ctime;
> > > +	} else {
> > >  		integrity_inode_attrs_store(&iint->real_inode, i_version,
> > > -					    real_inode);
> > > +					    ctime, real_inode);
> > > +	}
> > >  
> > >  	/* Possibly temporary failure due to type of read (eg. O_DIRECT) */
> > >  	if (!result)
> > > diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> > > index 5770cf691912aa912fc65280c59f5baac35dd725..54b638663c9743d39e5fb65711dbd9698b38e39b 100644
> > > --- a/security/integrity/ima/ima_main.c
> > > +++ b/security/integrity/ima/ima_main.c
> > > @@ -22,12 +22,14 @@
> > >  #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>
> > >  #include <linux/iversion.h>
> > >  #include <linux/evm.h>
> > >  #include <linux/crash_dump.h>
> > > +#include <linux/time64.h>
> > >  
> > >  #include "ima.h"
> > >  
> > > @@ -199,10 +201,13 @@ static void ima_check_last_writer(struct ima_iint_cache *iint,
> > >  					    &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) {
> > > +			    STATX_CHANGE_COOKIE | STATX_CTIME,
> > > +			    AT_STATX_SYNC_AS_STAT) ||
> > > +		    ((stat.result_mask & STATX_CHANGE_COOKIE) ?
> > > +		      stat.change_cookie != iint->real_inode.version :
> > > +		      (!(stat.result_mask & STATX_CTIME) ||
> > > +			!timespec64_equal(&stat.ctime,
> > > +					  &iint->real_inode.ctime)))) {
> > >  			iint->flags &= ~(IMA_DONE_MASK | IMA_NEW_FILE);
> > >  			iint->measured_pcrs = 0;
> > >  			if (update)
> > > @@ -328,9 +333,8 @@ 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,
> > 
> > Looks like a good change overall. Thanks for fixing this up. I don't
> > know IMA or EVM very well, so FWIW:
> 
> Thanks! Fun investigation & fix.
> 
> > 
> > Acked-by: Jeff Layton <jlayton@...nel.org>

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

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