[PATCH kernel] cherry-pick fix for NULL pointer dereference in ceph_mds_auth_match()

Fiona Ebner
2026-03-31T15:04:51Z

As reported in the enterprise support, there is a regression between
kernels 6.17.4-2-pve and 6.17.13-2-pve that might result in a NULL
pointer dereference in the ceph_mds_auth_match() function when using
an external CephFS file system. The only interesting commit touching
fs/ceph/ between those kernels is a backport of 22c73d52a6d0 ("ceph:
fix multifs mds auth caps issue"), namely f26ac354dcb3f ("ceph: fix
multifs mds auth caps issue"). There is an explicit fix for that
commit/issue upstream, namely 7987cce375ac8 ("ceph: fix NULL pointer
dereference in ceph_mds_auth_match()"). Pick it up.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---

Based on 5b5bf10 ("update ABI file for 6.17.13-2-pve (amd64)") rather
than current master which is already using 7.0 as a base.

Did not reproduce the original issue, but did a quick smoke test with
an external CephFS file system.

...inter-dereference-in-ceph_mds_auth_m.patch | 189 ++++++++++++++++++
1 file changed, 189 insertions(+)
create mode 100644 patches/kernel/0035-ceph-fix-NULL-pointer-dereference-in-ceph_mds_auth_m.patch

diff --git a/patches/kernel/0035-ceph-fix-NULL-pointer-dereference-in-ceph_mds_auth_m.patch b/patches/kernel/0035-ceph-fix-NULL-pointer-dereference-in-ceph_mds_auth_m.patch
new file mode 100644
index 0000000..d8115eb
--- /dev/null
+++ b/patches/kernel/0035-ceph-fix-NULL-pointer-dereference-in-ceph_mds_auth_m.patch
@@ -0,0 +1,189 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
+Date: Tue, 3 Feb 2026 14:54:46 -0800
+Subject: [PATCH] ceph: fix NULL pointer dereference in ceph_mds_auth_match()
+
+The CephFS kernel client has regression starting from 6.18-rc1.
+We have issue in ceph_mds_auth_match() if fs_name == NULL:
+
+ const char fs_name = mdsc->fsc->mount_options->mds_namespace;
+ ...
+ if (auth->match.fs_name && strcmp(auth->match.fs_name, fs_name)) {
+ / fsname mismatch, try next one */
+ return 0;
+ }
+
+Patrick Donnelly suggested that: In summary, we should definitely start
+decoding `fs_name` from the MDSMap and do strict authorizations checks
+against it. Note that the `-o mds_namespace=foo` should only be used for
+selecting the file system to mount and nothing else. It's possible
+no mds_namespace is specified but the kernel will mount the only
+file system that exists which may have name "foo".
+
+This patch reworks ceph_mdsmap_decode() and namespace_equals() with
+the goal of supporting the suggested concept. Now struct ceph_mdsmap
+contains m_fs_name field that receives copy of extracted FS name
+by ceph_extract_encoded_string(). For the case of "old" CephFS file
+systems, it is used "cephfs" name.
+
+[ idryomov: replace redundant %*pE with %s in ceph_mdsmap_decode(),
+ get rid of a series of strlen() calls in ceph_namespace_match(),
+ drop changes to namespace_equals() body to avoid treating empty
+ mds_namespace as equal, drop changes to ceph_mdsc_handle_fsmap()
+ as namespace_equals() isn't an equivalent substitution there ]
+
+Cc: stable@vger.kernel.org
+Fixes: 22c73d52a6d0 ("ceph: fix multifs mds auth caps issue")
+Link: https://tracker.ceph.com/issues/73886
+Signed-off-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
+Reviewed-by: Patrick Donnelly <pdonnell@ibm.com>
+Tested-by: Patrick Donnelly <pdonnell@ibm.com>
+Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
+(cherry picked from commit 7987cce375ac8ce98e170a77aa2399f2cf6eb99f)
+Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
+---
+ fs/ceph/mds_client.c | 5 +++--
+ fs/ceph/mdsmap.c | 26 +++++++++++++++++++-------
+ fs/ceph/mdsmap.h | 1 +
+ fs/ceph/super.h | 16 ++++++++++++++--
+ include/linux/ceph/ceph_fs.h | 6 ++++++
+ 5 files changed, 43 insertions(+), 11 deletions(-)
+
+diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
+index 3efbc11596e003193021f08b4f0d75ef36a7da7a..2d0bdb223db6714d253d76b8c0fd1a8347da33e5 100644
+--- a/fs/ceph/mds_client.c
++++ b/fs/ceph/mds_client.c
+@@ -5649,7 +5649,7 @@ static int ceph_mds_auth_match(struct ceph_mds_client *mdsc,
+ u32 caller_uid = from_kuid(&init_user_ns, cred->fsuid);
+ u32 caller_gid = from_kgid(&init_user_ns, cred->fsgid);
+ struct ceph_client *cl = mdsc->fsc->client;
+- const char *fs_name = mdsc->fsc->mount_options->mds_namespace;
++ const char *fs_name = mdsc->mdsmap->m_fs_name;
+ const char *spath = mdsc->fsc->mount_options->server_path;
+ bool gid_matched = false;
+ u32 gid, tlen, len;
+@@ -5657,7 +5657,8 @@ static int ceph_mds_auth_match(struct ceph_mds_client *mdsc,
+
+ doutc(cl, "fsname check fs_name=%s match.fs_name=%s\n",
+ fs_name, auth->match.fs_name ? auth->match.fs_name : "");
+- if (auth->match.fs_name && strcmp(auth->match.fs_name, fs_name)) {
++
++ if (!ceph_namespace_match(auth->match.fs_name, fs_name)) {
+ /* fsname mismatch, try next one */
+ return 0;
+ }
+diff --git a/fs/ceph/mdsmap.c b/fs/ceph/mdsmap.c
+index e82f09e50a8e7f7026b0602e8799f93f4a112692..dbe9f40cb262763c7877dbd73da9e97c7d5182b7 100644
+--- a/fs/ceph/mdsmap.c
++++ b/fs/ceph/mdsmap.c
+@@ -354,22 +354,33 @@ struct ceph_mdsmap *ceph_mdsmap_decode(struct ceph_mds_client *mdsc, void **p,
+ __decode_and_drop_type(p, end, u8, bad_ext);
+ }
+ if (mdsmap_ev >= 8) {
+- u32 fsname_len;
++ size_t fsname_len;
++
+ /* enabled */
+ ceph_decode_8_safe(p, end, m->m_enabled, bad_ext);
++
+ /* fs_name */
+- ceph_decode_32_safe(p, end, fsname_len, bad_ext);
++ m->m_fs_name = ceph_extract_encoded_string(p, end,
++ &fsname_len,
++ GFP_NOFS);
++ if (IS_ERR(m->m_fs_name)) {
++ m->m_fs_name = NULL;
++ goto nomem;
++ }
+
+ /* validate fsname against mds_namespace */
+- if (!namespace_equals(mdsc->fsc->mount_options, *p,
++ if (!namespace_equals(mdsc->fsc->mount_options, m->m_fs_name,
+ fsname_len)) {
+- pr_warn_client(cl, "fsname %*pE doesn't match mds_namespace %s\n",
+- (int)fsname_len, (char *)*p,
++ pr_warn_client(cl, "fsname %s doesn't match mds_namespace %s\n",
++ m->m_fs_name,
+ mdsc->fsc->mount_options->mds_namespace);
+ goto bad;
+ }
+- /* skip fsname after validation */
+- ceph_decode_skip_n(p, end, fsname_len, bad);
++ } else {
++ m->m_enabled = false;
++ m->m_fs_name = kstrdup(CEPH_OLD_FS_NAME, GFP_NOFS);
++ if (!m->m_fs_name)
++ goto nomem;
+ }
+ /* damaged */
+ if (mdsmap_ev >= 9) {
+@@ -431,6 +442,7 @@ void ceph_mdsmap_destroy(struct ceph_mdsmap *m)
+ kfree(m->m_info);
+ }
+ kfree(m->m_data_pg_pools);
++ kfree(m->m_fs_name);
+ kfree(m);
+ }
+
+diff --git a/fs/ceph/mdsmap.h b/fs/ceph/mdsmap.h
+index 1f2171dd01bfa34a404eef00113646bdcb978980..d48d07c3516d447a8f3e684f6ded1f1ca1674417 100644
+--- a/fs/ceph/mdsmap.h
++++ b/fs/ceph/mdsmap.h
+@@ -45,6 +45,7 @@ struct ceph_mdsmap {
+ bool m_enabled;
+ bool m_damaged;
+ int m_num_laggy;
++ char *m_fs_name;
+ };
+
+ static inline struct ceph_entity_addr *
+diff --git a/fs/ceph/super.h b/fs/ceph/super.h
+index 4ac6561285b18a48c1e24f0752296b88e51c7971..7b67853c0ffe5e391e9098a76ced3b0a4fda94cc 100644
+--- a/fs/ceph/super.h
++++ b/fs/ceph/super.h
+@@ -104,14 +104,26 @@ struct ceph_mount_options {
+ struct fscrypt_dummy_policy dummy_enc_policy;
+ };
+
++#define CEPH_NAMESPACE_WILDCARD "*"
++
++static inline bool ceph_namespace_match(const char *pattern,
++ const char *target)
++{
++ if (!pattern || !pattern[0] ||
++ !strcmp(pattern, CEPH_NAMESPACE_WILDCARD))
++ return true;
++
++ return !strcmp(pattern, target);
++}
++
+ /*
+ * Check if the mds namespace in ceph_mount_options matches
+ * the passed in namespace string. First time match (when
+ * ->mds_namespace is NULL) is treated specially, since
+ * ->mds_namespace needs to be initialized by the caller.
+ */
+-static inline int namespace_equals(struct ceph_mount_options *fsopt,
+- const char *namespace, size_t len)
++static inline bool namespace_equals(struct ceph_mount_options *fsopt,
++ const char *namespace, size_t len)
+ {
+ return !(fsopt->mds_namespace &&
+ (strlen(fsopt->mds_namespace) != len ||
+diff --git a/include/linux/ceph/ceph_fs.h b/include/linux/ceph/ceph_fs.h
+index c7f2c63b3bc3fb3c6a1ec0d114fcaf9cd7489de3..08e5dbe15ca4446081779f035d64f0a6bc34b382 100644
+--- a/include/linux/ceph/ceph_fs.h
++++ b/include/linux/ceph/ceph_fs.h
+@@ -31,6 +31,12 @@
+ #define CEPH_INO_CEPH 2 /* hidden .ceph dir */
+ #define CEPH_INO_GLOBAL_SNAPREALM 3 /* global dummy snaprealm */
+
++/*
++ * name for "old" CephFS file systems,
++ * see ceph.git e2b151d009640114b2565c901d6f41f6cd5ec652
++ */
++#define CEPH_OLD_FS_NAME "cephfs"
++
+ /* arbitrary limit on max # of monitors (cluster of 3 is typical) */
+ #define CEPH_MAX_MON 31
+
--
2.47.3