[RFC PATCH 0/2] quota: opt-in strict enforcement of project quota hard limits

[RFC PATCH 0/2] quota: opt-in strict enforcement of project quota hard limits

[Kitae Yoo]

ignore_hardlimit() exempts CAP_SYS_RESOURCE holders from enforcement of
hard limits (and of soft limits past their grace time), regardless of
quota type, so it also covers project quotas.

For user/group quotas this matches the long-standing expectation that an
administrator can act on a full filesystem. For project quotas it is a
poor fit: a project limit bounds the size of a directory tree rather than
restricting a user, and is commonly used for capacity isolation of
container volumes and NFS-exported shares. There the exemption defeats
the purpose. Two concrete cases:

 1. A process holding CAP_SYS_RESOURCE (e.g. a privileged container)
    writes past a project hard limit; accounting keeps rising above it.

 2. knfsd raises CAP_SYS_RESOURCE for requests mapped to root on
    no_root_squash exports (CAP_NFSD_SET in <linux/capability.h>). So on
    an ext4-backed, no_root_squash NFS export any remote root write
    bypasses project hard limits, no matter how confined the client is.
    We hit this while evaluating ext4 project quotas for multi-tenant NFS
    volumes: with a 2 GiB hard limit, remote root writes proceeded well
    past 2 GiB (v6.8, quotaon reporting "enforced"). The same setup on
    XFS, whose enforcement path performs no capability check, stops the
    write at the limit.

DQF_ROOT_SQUASH already disables this exemption, but setting it has been
confined to the old v1 quota format, and commit ca6cb0918e87 ("quota:
Verify flags passed to Q_SETINFO") later made Q_SETINFO reject it
explicitly on other formats, on the grounds that those formats did not
persist the flag and a flag silently lost on remount is confusing. That
confinement predates generic project quota support (commit 847aac644e92
"vfs: Add general support to enforce project quota limits").

This series lifts the restriction and addresses the persistence concern
that motivated it:

  1/2 allows DQF_ROOT_SQUASH to be set through Q_SETINFO on all formats,
      so it can be enabled per quota type (e.g. project only).
  2/2 persists the flag in the v2 on-disk dqi_flags field, which already
      exists, masking on read so only the supported flag is honoured -
      which also keeps the pre-existing unvalidated on-disk bits (the
      reason c119c5b9749e "quota: Don't store flags for v2 quota format"
      stopped storing them) out of the in-memory state.

No behaviour change by default: the flag stays clear unless explicitly
set, and setting it on a non-v1 format previously returned -EINVAL.
Accepting it there is the user-visible ABI change 1/2 makes.

Open questions for reviewers:

 * Is extending DQF_ROOT_SQUASH the right vehicle? Despite its name it
   has always controlled the CAP_SYS_RESOURCE exemption in the generic
   dquot path, not UID squashing, so the semantic already matches. The
   alternative is a newly named per-type flag, at the cost of new uAPI.
   I lean toward reusing the existing flag but defer to your preference;
   the uAPI comment is updated to describe the real semantic either way.

 * Is persisting the flag in the v2 dqi_flags field (2/2) acceptable? An
   older kernel rewriting quota info still clears it, so strict
   enforcement would need to be set up again after booting such a kernel.

 * The NFS case is really a knfsd credential question (should
   no_root_squash grant CAP_SYS_RESOURCE?). Fixing it in quota covers the
   privileged-container case too and needs no per-export policy, which is
   why I bring it here rather than to the nfsd maintainers, but I'm happy
   to pursue that side there if preferred.

Reproducer (ext4, as root on a scratch device $DEV):

  mkfs.ext4 -qF -O project,quota -E quotatype=prjquota $DEV
  mount -o prjquota $DEV /mnt
  mkdir /mnt/vol && chattr +P -p 42 /mnt/vol
  setquota -P 42 0 $((100*1024)) 0 0 /mnt      # 100 MiB project hard limit

  # root write, flag clear: exceeds the limit (the bug)
  dd if=/dev/zero of=/mnt/vol/f bs=1M count=200 oflag=direct
  du -m /mnt/vol/f        # ~200

  ./set_rsquash /mnt      # Q_SETINFO(PRJQUOTA, DQF_ROOT_SQUASH); no CLI for this
  rm /mnt/vol/f
  dd if=/dev/zero of=/mnt/vol/f bs=1M count=200 oflag=direct
  du -m /mnt/vol/f        # ~100, now enforced

  # 2/2: the flag survives a remount
  umount /mnt && mount -o prjquota $DEV /mnt
  dd if=/dev/zero of=/mnt/vol/g bs=1M count=200 oflag=direct
  du -m /mnt/vol/g        # ~100

On XFS the first write already stops at 100 MiB. set_rsquash.c:

  #include <sys/quota.h>
  #include <linux/quota.h>
  int main(int argc, char **argv)
  {
          struct if_dqinfo info;
          if (quotactl(QCMD(Q_GETINFO, PRJQUOTA), argv[1], 0, (void *)&info))
                  return 1;
          info.dqi_flags |= DQF_ROOT_SQUASH;
          info.dqi_valid = IIF_FLAGS;
          return quotactl(QCMD(Q_SETINFO, PRJQUOTA), argv[1], 0, (void *)&info)
                         ? 1 : 0;
  }

I can turn the enforcement check into an fstest.

Kitae Yoo (2):
  quota: allow DQF_ROOT_SQUASH on all quota formats
  quota_v2: persist DQF_ROOT_SQUASH

 fs/quota/dquot.c           | 8 +-------
 fs/quota/quota_v2.c        | 6 ++----
 include/uapi/linux/quota.h | 2 +-
 3 files changed, 4 insertions(+), 12 deletions(-)

-- 
2.50.1 (Apple Git-155)

[RFC PATCH 1/2] quota: allow DQF_ROOT_SQUASH on all quota formats

[Kitae Yoo]

ignore_hardlimit() exempts CAP_SYS_RESOURCE holders from enforcement of
hard limits and of soft limits whose grace time expired.
DQF_ROOT_SQUASH disables that exemption, but it has been confined to the
old v1 quota format since the quota format abstraction was introduced,
and commit ca6cb0918e87 ("quota: Verify flags passed to Q_SETINFO")
later made Q_SETINFO reject it explicitly on other formats.

That confinement predates generic project quota support. Project limits
bound the size of a directory tree rather than restrict a user, and are
commonly used for capacity isolation of container volumes and NFS
exports. There the exemption defeats the purpose: knfsd raises
CAP_SYS_RESOURCE for requests mapped to root on no_root_squash exports
(CAP_NFSD_SET), so any remote root write silently exceeds project hard
limits. XFS enforces project limits regardless of capabilities.

Lift the format restriction so the flag can be set per quota type
through Q_SETINFO on journaled quota as well. Existing setups keep their
behaviour: the flag stays clear unless explicitly set, and setting it
was previously rejected with -EINVAL on anything but QFMT_VFS_OLD -
accepting it there is the user-visible ABI change this patch makes.

Signed-off-by: Kitae Yoo <kitaeyoo777@gmail.com>
---
 fs/quota/dquot.c           | 8 +-------
 include/uapi/linux/quota.h | 2 +-
 2 files changed, 2 insertions(+), 8 deletions(-)

diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
index 9850de3955..e431e72dfe 100644
--- a/fs/quota/dquot.c
+++ b/fs/quota/dquot.c
@@ -1309,8 +1309,7 @@ static int ignore_hardlimit(struct dquot *dquot)
 	struct mem_dqinfo *info = &sb_dqopt(dquot->dq_sb)->info[dquot->dq_id.type];
 
 	return capable(CAP_SYS_RESOURCE) &&
-	       (info->dqi_format->qf_fmt_id != QFMT_VFS_OLD ||
-		!(info->dqi_flags & DQF_ROOT_SQUASH));
+	       !(info->dqi_flags & DQF_ROOT_SQUASH);
 }
 
 static int dquot_add_inodes(struct dquot *dquot, qsize_t inodes,
@@ -2900,11 +2899,6 @@ int dquot_set_dqinfo(struct super_block *sb, int type, struct qc_info *ii)
 	if (!sb_has_quota_active(sb, type))
 		return -ESRCH;
 	mi = sb_dqopt(sb)->info + type;
-	if (ii->i_fieldmask & QC_FLAGS) {
-		if ((ii->i_flags & QCI_ROOT_SQUASH &&
-		     mi->dqi_format->qf_fmt_id != QFMT_VFS_OLD))
-			return -EINVAL;
-	}
 	spin_lock(&dq_data_lock);
 	if (ii->i_fieldmask & QC_SPC_TIMER)
 		mi->dqi_bgrace = ii->i_spc_timelimit;
diff --git a/include/uapi/linux/quota.h b/include/uapi/linux/quota.h
index 52090105b8..a34f43519a 100644
--- a/include/uapi/linux/quota.h
+++ b/include/uapi/linux/quota.h
@@ -149,7 +149,7 @@ enum {
 	DQF_PRIVATE
 };
 
-/* Root squash enabled (for v1 quota format) */
+/* Enforce limits also for CAP_SYS_RESOURCE processes */
 #define DQF_ROOT_SQUASH	(1 << DQF_ROOT_SQUASH_B)
 /* Quota stored in a system file */
 #define DQF_SYS_FILE	(1 << DQF_SYS_FILE_B)
-- 
2.50.1 (Apple Git-155)

[RFC PATCH 2/2] quota_v2: persist DQF_ROOT_SQUASH

[Kitae Yoo]

Commit c119c5b9749e ("quota: Don't store flags for v2 quota format")
stopped persisting dqi_flags because no v2 flag was supported and
on-disk flags could contain unvalidated garbage. Now that
DQF_ROOT_SQUASH is settable on v2, losing it across quotaoff/quotaon or
remount would silently re-enable the CAP_SYS_RESOURCE exemption - a poor
property for an enforcement policy.

Store the flag in the existing on-disk dqi_flags field and mask on read
so only the supported flag is ever accepted from disk, which also keeps
pre-existing garbage bits out of the in-memory flags.

An older kernel rewriting quota info still clears the stored flag;
strict enforcement then needs to be set up again after booting back.

Signed-off-by: Kitae Yoo <kitaeyoo777@gmail.com>
---
 fs/quota/quota_v2.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/fs/quota/quota_v2.c b/fs/quota/quota_v2.c
index a24fab5f9f..814db61681 100644
--- a/fs/quota/quota_v2.c
+++ b/fs/quota/quota_v2.c
@@ -142,8 +142,7 @@ static int v2_read_file_info(struct super_block *sb, int type)
 	}
 	info->dqi_bgrace = le32_to_cpu(dinfo.dqi_bgrace);
 	info->dqi_igrace = le32_to_cpu(dinfo.dqi_igrace);
-	/* No flags currently supported */
-	info->dqi_flags = 0;
+	info->dqi_flags = le32_to_cpu(dinfo.dqi_flags) & DQF_ROOT_SQUASH;
 	qinfo->dqi_sb = sb;
 	qinfo->dqi_type = type;
 	qinfo->dqi_blocks = le32_to_cpu(dinfo.dqi_blocks);
@@ -209,8 +208,7 @@ static int v2_write_file_info(struct super_block *sb, int type)
 	info->dqi_flags &= ~DQF_INFO_DIRTY;
 	dinfo.dqi_bgrace = cpu_to_le32(info->dqi_bgrace);
 	dinfo.dqi_igrace = cpu_to_le32(info->dqi_igrace);
-	/* No flags currently supported */
-	dinfo.dqi_flags = cpu_to_le32(0);
+	dinfo.dqi_flags = cpu_to_le32(info->dqi_flags & DQF_ROOT_SQUASH);
 	spin_unlock(&dq_data_lock);
 	dinfo.dqi_blocks = cpu_to_le32(qinfo->dqi_blocks);
 	dinfo.dqi_free_blk = cpu_to_le32(qinfo->dqi_free_blk);
-- 
2.50.1 (Apple Git-155)

Re: [RFC PATCH 2/2] quota_v2: persist DQF_ROOT_SQUASH

[sashiko-bot]

> Commit c119c5b9749e ("quota: Don't store flags for v2 quota format")
> stopped persisting dqi_flags because no v2 flag was supported and
> on-disk flags could contain unvalidated garbage. Now that
> DQF_ROOT_SQUASH is settable on v2, losing it across quotaoff/quotaon or
> remount would silently re-enable the CAP_SYS_RESOURCE exemption - a poor
> property for an enforcement policy.
> 
> Store the flag in the existing on-disk dqi_flags field and mask on read
> so only the supported flag is ever accepted from disk, which also keeps
> pre-existing garbage bits out of the in-memory flags.
> 
> An older kernel rewriting quota info still clears the stored flag;
> strict enforcement then needs to be set up again after booting back.
> 
> Signed-off-by: Kitae Yoo <kitaeyoo777@gmail.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260814114649.51253-1-kitaeyoo777@gmail.com?part=2

Re: [RFC PATCH 1/2] quota: allow DQF_ROOT_SQUASH on all quota formats

[sashiko-bot]

> ignore_hardlimit() exempts CAP_SYS_RESOURCE holders from enforcement of
> hard limits and of soft limits whose grace time expired.
> DQF_ROOT_SQUASH disables that exemption, but it has been confined to the
> old v1 quota format since the quota format abstraction was introduced,
> and commit ca6cb0918e87 ("quota: Verify flags passed to Q_SETINFO")
> later made Q_SETINFO reject it explicitly on other formats.
> 
> That confinement predates generic project quota support. Project limits
> bound the size of a directory tree rather than restrict a user, and are
> commonly used for capacity isolation of container volumes and NFS
> exports. There the exemption defeats the purpose: knfsd raises
> CAP_SYS_RESOURCE for requests mapped to root on no_root_squash exports
> (CAP_NFSD_SET), so any remote root write silently exceeds project hard
> limits. XFS enforces project limits regardless of capabilities.
> 
> Lift the format restriction so the flag can be set per quota type
> through Q_SETINFO on journaled quota as well. Existing setups keep their
> behaviour: the flag stays clear unless explicitly set, and setting it
> was previously rejected with -EINVAL on anything but QFMT_VFS_OLD -
> accepting it there is the user-visible ABI change this patch makes.
> 
> Signed-off-by: Kitae Yoo <kitaeyoo777@gmail.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260814114649.51253-1-kitaeyoo777@gmail.com?part=1