| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <aW_4V93kczhzFjQ3@CMGLRV3>
Date: Tue, 20 Jan 2026 15:49:11 -0600
From: Frederick Lawler <fred@...udflare.com>
To: Jeff Layton <jlayton@...nel.org>
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, 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 could be out of date depending on how file systems set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.
Alternatively, create security_get/setxattr() equivalent hooks, or
have an internal mapping of sorts.
>
> > 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>
Powered by blists - more mailing lists