[Devel] [PATCH v7 VZ10] fs: enforce container device-mount policy in the common mount path
Vasileios Almpanis
vasileios.almpanis at virtuozzo.com
Thu Jul 23 16:49:51 MSK 2026
In a container the per-device ve_devmnt policy restricts which options
a device may be mounted with and force-inserts a set of hidden options.
The check used to run inside the option-string parser
(vfs_parse_monolithic_sep) and, for remount, in a separate helper. Two
things escaped it:
- MS_* flags from the legacy mount(2)/fsconfig(2) API are folded into
fc->sb_flags and never appear in the option string, so a container
could set MS_RDONLY, MS_SYNCHRONOUS, MS_MANDLOCK, ... outside its
allowed set.
- The check sat in filesystem-selectable callbacks (->parse_monolithic,
->mount), so a filesystem not routing through them evaded the policy,
and a skipped hidden-option insertion dropped a container's mandated
options without error.
Enforce the policy in the fs-agnostic common mount path instead:
vfs_get_tree() for a new mount and reconfigure_super() for a remount.
The device is taken from the mounted superblock, so fc->source cannot be
raced to target another device, and fc->sb_flags is vetted alongside the
option string. On a new mount the forced options must also be present,
so a filesystem that skipped inserting them has its mount refused rather
than silently losing them.
The parse-time ve_devmnt_process() call is kept as a best-effort early
reject, so a disallowed device or option is refused before the
filesystem's fill_super() runs.
Fix a bug where a containers that use the legacy mount(2) syscall are able
to reconfigure a mount and change the superblock flags, for example
from RO to RW. Compute the effective superblock flags and emit rw incase
SB_RDONLY is missing in vfs_format_sb_flags.
https://virtuozzo.atlassian.net/browse/VSTOR-132330
Fixes: 263467c864c5 ("ve/fs/devmnt: process mount options")
Signed-off-by: Vasileios Almpanis <vasileios.almpanis at virtuozzo.com>
Co-developed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
Feature: ve: ve generic structures
---
Changes since v6:
- Enforce sb-flag changes on remount: emit "rw" when SB_RDONLY is cleared
and vet the effective flags,
(sb->s_flags & ~fc->sb_flags_mask) | (fc->sb_flags & fc->sb_flags_mask),
so a legacy mount(2) remount can no longer flip RO->RW unchecked.
- Constrain only devices listed in ve.mount_opts: an unlisted device
mounted with no userspace options is allowed rather than refused by the
synthesized ro/rw token (tracked via have_user_opts).
- Reset fc->ve_final_opts in vfs_dup_fs_context() to avoid a double-free.
- Factor per-option emit into __vfs_emit_flag(); checkpatch/style fixes.
Changes since v5:
- Fix use-after-free: do_new_mount() freed the merged page that
legacy_get_tree()->mount() dereferences later; the fs now sees only
the caller-owned original data.
- Fix remount,dirsync failing with -EINVAL in containers: stop re-parsing
the formatted flag names (SB_DIRSYNC is not in MS_RMT_MASK).
- Check policy only: legacy fs parsers no longer get synthesized ro/sync
tokens, and the flag check reads fc->sb_flags symmetrically for mount
and fsconfig.
- Keep ve_check_mount_options() only on the remount path (pinned sb
device, covers legacy ->reconfigure); drop legacy_merge_mount_data()
and ve_prepare_mount_options(); add Fixes: tag and rewrite the message.
Changes since v4:
- Emit only the positive sb-flag names, not the clear names (rw/async/
...); on legacy remount sb_flags_mask is MS_RMT_MASK, so the clear
names had wrongly rejected ordinary in-container remounts.
- NUL-terminate the options page when data is empty and no flags emitted.
- Fix __vfs_format_flags() comment (-E2BIG, not -ENOSPC).
Changes since v3:
- Drop excess length check in legacy_merge_mount_data().
Changes since v2:
- Remove the legacy_merge_mount_data guard in fs/internal.h.
- Add __vfs_format_flags() helper, used by vfs_format_sb_flags().
- Use -E2BIG (not -ENOSPC) for the buffer-too-small case.
Changes since v1:
- Unify the comma-insert-copy pattern across call sites via an
append_entry() helper.
- Rework legacy_merge_mount_data() to allocate the page upfront and
append sb flags via vfs_format_sb_flags(), dropping flags_buf[128]
and the size arithmetic.
- Use -ENOSPC (not -EINVAL) for buffer-too-small; comment
FS_BINARY_MOUNTDATA; minor blank-line cleanups.
fs/fs_context.c | 159 +++++++++++++++++++++++++++++++++++++
fs/internal.h | 1 +
fs/namespace.c | 110 +++++++++++++++++++------
fs/super.c | 12 +++
include/linux/fs_context.h | 2 +
include/linux/mount.h | 2 +
6 files changed, 263 insertions(+), 23 deletions(-)
diff --git a/fs/fs_context.c b/fs/fs_context.c
index 76f34f3d468e..8cf06e2ac800 100644
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -81,6 +81,70 @@ static int vfs_parse_sb_flag(struct fs_context *fc, const char *key)
return -ENOPARAM;
}
+/*
+ * Emit option @name into @buff at *@off, prefixed with ',' if the buffer
+ * already holds text. Advances *@off. Returns 0 or -E2BIG if @buff is full.
+ */
+static int __vfs_emit_flag(const char *name, char *buff,
+ size_t size, size_t *off)
+{
+ ssize_t ret;
+
+ if (*off) {
+ if (*off + 1 >= size)
+ return -E2BIG;
+ buff[(*off)++] = ',';
+ }
+
+ ret = strscpy(buff + *off, name, size - *off);
+ if (ret < 0)
+ return -E2BIG;
+ *off += ret;
+ return 0;
+}
+
+static int __vfs_format_flags(const struct constant_table *p, unsigned int flags,
+ char *buff, size_t size, size_t *off)
+{
+ for (; p->name; p++) {
+ int ret;
+
+ if (!(flags & p->value))
+ continue;
+ ret = __vfs_emit_flag(p->name, buff, size, off);
+ if (ret)
+ return ret;
+ }
+ return 0;
+}
+
+static int vfs_format_sb_flags(char *buff, size_t size, size_t *off,
+ unsigned int sb_flags)
+{
+ int err;
+
+ err = __vfs_format_flags(common_set_sb_flag, sb_flags, buff, size, off);
+ if (err)
+ return err;
+
+ /*
+ * "rw" has no flag bit of its own - it is simply the absence of
+ * SB_RDONLY. Emit it explicitly so the ve_devmnt policy can allow or
+ * deny read-write access as a first-class option; otherwise a mount or
+ * remount that leaves the superblock read-write carries no token and
+ * slips past the "every option must be allowed" check.
+ *
+ * @sb_flags is the effective post-operation flag word, so this reflects
+ * the state the superblock actually ends up in. A remount that only
+ * touches an unrelated flag (e.g. "sync") keeps its current SB_RDONLY
+ * and so does not emit "rw".
+ */
+ if (!(sb_flags & SB_RDONLY))
+ return __vfs_emit_flag("rw", buff, size, off);
+
+ return 0;
+}
+
/**
* vfs_parse_fs_param_source - Handle setting "source" via parameter
* @fc: The filesystem context to modify
@@ -224,6 +288,85 @@ static inline int fscontext_lookup_bdev(struct fs_context *fc, dev_t *s_dev)
return -ENODEV;
}
+/*
+ * ve_devmnt_verify_fc - check a mount against the container device-mount policy
+ * @fc: the mount context, with fc->root set
+ * @new_mount: true at vfs_get_tree() (new mount), false at reconfigure_super()
+ *
+ * Vets the stashed userspace option string plus the synthesized SB_* flag
+ * names against the mounted superblock's device. A device absent from the
+ * policy that is mounted with no userspace options is allowed. Returns 0 when
+ * permitted (or no check applies), or a negative errno.
+ */
+int ve_devmnt_verify_fc(struct fs_context *fc, bool new_mount)
+{
+ struct ve_struct *ve = get_exec_env();
+ unsigned int sb_flags;
+ bool have_user_opts;
+ size_t off = 0;
+ char *page;
+ int err;
+
+ if (ve_is_super(ve))
+ return 0;
+
+ if (!fc->fs_type || !(fc->fs_type->fs_flags & FS_REQUIRES_DEV))
+ return 0;
+
+ /*
+ * Filesystems with binary mount data (e.g. btrfs) bypass option
+ * string parsing entirely, so our checks cannot apply here.
+ */
+ if (fc->fs_type->fs_flags & FS_BINARY_MOUNTDATA)
+ return 0;
+
+ if (WARN_ON_ONCE(!fc->root))
+ return -EINVAL;
+
+ page = (char *)__get_free_page(GFP_KERNEL);
+ if (!page)
+ return -ENOMEM;
+
+ /*
+ * Track whether userspace actually supplied options. @page below also
+ * gets the synthesized ro/rw flag token, so its length cannot answer
+ * this; ve_final_opts holds only the userspace string.
+ */
+ have_user_opts = fc->ve_final_opts && *fc->ve_final_opts;
+ if (have_user_opts) {
+ ssize_t ret = strscpy(page, fc->ve_final_opts, PAGE_SIZE);
+
+ if (ret < 0) {
+ err = -E2BIG;
+ goto out;
+ }
+ off = ret;
+ }
+
+ /*
+ * On a remount fc->sb_flags holds only the bits being changed, so
+ * combine them with the current superblock flags to get the state the
+ * sb will actually have - the same value reconfigure_super() writes
+ * back. On a new mount fc->sb_flags is already the full flag word.
+ */
+ sb_flags = fc->sb_flags;
+ if (!new_mount)
+ sb_flags = (fc->root->d_sb->s_flags & ~fc->sb_flags_mask) |
+ (fc->sb_flags & fc->sb_flags_mask);
+
+ err = vfs_format_sb_flags(page, PAGE_SIZE, &off, sb_flags);
+ if (err)
+ goto out;
+
+ page[off] = '\0';
+ err = ve_devmnt_verify(ve, fc->root->d_sb->s_dev, page, new_mount,
+ have_user_opts);
+
+out:
+ free_page((unsigned long)page);
+ return err;
+}
+
static int fscontext_init_lazy_opts(struct fs_context *fc)
{
struct ve_struct *ve = get_exec_env();
@@ -389,10 +532,22 @@ int vfs_parse_monolithic_sep(struct fs_context *fc, void *data,
return -ENODEV;
}
+ /* Early reject and hidden-option insertion; verified for real later. */
ret = ve_devmnt_process(ve, bd_dev, (void **) &options,
fc->purpose == FS_CONTEXT_FOR_RECONFIGURE);
if (ret)
return ret;
+
+ /* Stash what the filesystem parses; checked in the common mount path. */
+ if (options) {
+ kfree(fc->ve_final_opts);
+ fc->ve_final_opts = kstrdup(options, GFP_KERNEL);
+ if (!fc->ve_final_opts) {
+ if (options != options_orig)
+ free_page((unsigned long)options);
+ return -ENOMEM;
+ }
+ }
}
/*
@@ -614,6 +769,7 @@ struct fs_context *vfs_dup_fs_context(struct fs_context *src_fc)
fc->s_fs_info = NULL;
fc->source = NULL;
fc->security = NULL;
+ fc->ve_final_opts = NULL;
get_filesystem(fc->fs_type);
get_net(fc->net_ns);
get_user_ns(fc->user_ns);
@@ -742,6 +898,7 @@ void put_fs_context(struct fs_context *fc)
put_filesystem(fc->fs_type);
if (fc->lazy_opts)
free_page((unsigned long)fc->lazy_opts);
+ kfree(fc->ve_final_opts);
kfree(fc->source);
kfree(fc);
}
@@ -962,6 +1119,8 @@ void vfs_clean_context(struct fs_context *fc)
free_page((unsigned long)fc->lazy_opts);
fc->lazy_opts = NULL;
}
+ kfree(fc->ve_final_opts);
+ fc->ve_final_opts = NULL;
kfree(fc->source);
fc->source = NULL;
fc->exclusive = false;
diff --git a/fs/internal.h b/fs/internal.h
index 3647ce69b2c7..e33d3ae70cce 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -46,6 +46,7 @@ extern void __init chrdev_init(void);
*/
extern const struct fs_context_operations legacy_fs_context_ops;
extern int parse_monolithic_mount_data(struct fs_context *, void *);
+extern int ve_devmnt_verify_fc(struct fs_context *fc, bool new_mount);
extern void vfs_clean_context(struct fs_context *fc);
extern int finish_clean_context(struct fs_context *fc);
diff --git a/fs/namespace.c b/fs/namespace.c
index 0f4a3668e558..0b46158852e6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3258,6 +3258,92 @@ int ve_devmnt_process(struct ve_struct *ve, dev_t dev, void **data_pp, int remou
return err;
}
+/* Return 0 if every option in @options is listed in @a or @b, else -EPERM. */
+static int ve_devmnt_options_subset(char *options, char *a, char *b)
+{
+ char *copy, *cur, *p;
+ int err = 0;
+
+ if (!options || !*options)
+ return 0;
+ if (!a && !b)
+ return -EPERM;
+
+ copy = cur = kstrdup(options, GFP_KERNEL);
+ if (!copy)
+ return -ENOMEM;
+
+ while ((p = strsep(&cur, ",")) != NULL) {
+ if (!*p)
+ continue;
+ if ((!a || !strstr_separated(a, p, ',')) &&
+ (!b || !strstr_separated(b, p, ','))) {
+ err = -EPERM;
+ break;
+ }
+ }
+
+ kfree(copy);
+ return err;
+}
+
+/*
+ * ve_devmnt_verify - enforce the container device-mount policy for @dev
+ * @ve: the container
+ * @dev: device taken from the mounted superblock (not from a raceable path)
+ * @opts: mount options plus the SB_* flag names to vet
+ * @new_mount: true for a new mount, false for a remount
+ * @have_user_opts: true if userspace supplied any mount options. @opts always
+ * carries the kernel-synthesized ro/rw flag token, so it is never empty
+ * and cannot answer this on its own.
+ *
+ * Every supplied option must be allowed or forced. On a new mount the forced
+ * ("hidden") options must also be present: a filesystem that skipped inserting
+ * them is refused rather than silently dropping a container's mandated option
+ */
+int ve_devmnt_verify(struct ve_struct *ve, dev_t dev, char *opts, bool new_mount,
+ bool have_user_opts)
+{
+ struct ve_devmnt *devmnt;
+ char *allowed = NULL, *hidden = NULL;
+ bool found = false;
+ int err = 0;
+
+ if (ve->is_pseudosuper)
+ return 0;
+
+ mutex_lock(&ve->devmnt_mutex);
+ list_for_each_entry(devmnt, &ve->devmnt_list, link) {
+ if (devmnt->dev == dev) {
+ allowed = devmnt->allowed_options;
+ hidden = devmnt->hidden_options;
+ found = true;
+ break;
+ }
+ }
+
+ /*
+ * Enforce for a listed device, or for any mount carrying userspace
+ * options. An unlisted device with no userspace options is unconstrained
+ * here, so the synthesized ro/rw token in @opts does not deny it.
+ */
+ if (found || have_user_opts) {
+ /* every supplied option must be either allowed or forced */
+ err = ve_devmnt_options_subset(opts, allowed, hidden);
+
+ /* on a new mount every forced option must have reached the fs */
+ if (!err && new_mount)
+ err = ve_devmnt_options_subset(hidden, opts, NULL);
+ }
+ mutex_unlock(&ve->devmnt_mutex);
+
+ if (err == -EPERM)
+ ve_pr_warn_ratelimited(VE_LOG_BOTH,
+ "VE%s: mount options not permitted for device %u:%u\n",
+ ve_name(ve), MAJOR(dev), MINOR(dev));
+ return err;
+}
+
static inline int ve_mount_allowed(void)
{
struct ve_struct *ve = get_exec_env();
@@ -3308,23 +3394,6 @@ static inline void ve_mount_nr_inc(struct mount *mnt, struct ve_struct *ve) { }
static inline void ve_mount_nr_dec(struct mount *mnt) { }
#endif /* CONFIG_VE */
-static int ve_prepare_mount_options(struct fs_context *fc, void *data)
-{
-#ifdef CONFIG_VE
- struct super_block *sb = fc->root->d_sb;
- struct ve_struct *ve = get_exec_env();
-
- if (sb->s_bdev && data && !ve_is_super(ve)) {
- int err;
-
- err = ve_devmnt_process(ve, sb->s_bdev->bd_dev, &data, 1);
- if (err)
- return err;
- }
-#endif
- return 0;
-}
-
/*
* change filesystem flags. dir should be a physical root of filesystem.
* If you've mounted a non-root directory somewhere and want to do remount
@@ -3357,12 +3426,6 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
*/
fc->oldapi = true;
- err = ve_prepare_mount_options(fc, data);
- if (err) {
- put_fs_context(fc);
- return err;
- }
-
err = parse_monolithic_mount_data(fc, data);
if (!err) {
down_write(&sb->s_umount);
@@ -3816,6 +3879,7 @@ static int do_new_mount(struct path *path, const char *fstype, int sb_flags,
subtype, strlen(subtype));
if (!err && name)
err = vfs_parse_fs_string(fc, "source", name, strlen(name));
+ /* Container device-mount policy is enforced later, in vfs_get_tree(). */
if (!err)
err = parse_monolithic_mount_data(fc, data);
if (!err && !mount_capable(fc))
diff --git a/fs/super.c b/fs/super.c
index 1adebbf35803..c0c067eb2d8e 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1085,6 +1085,11 @@ int reconfigure_super(struct fs_context *fc)
if (retval)
return retval;
+ /* Enforce the container device-mount policy on the remount options. */
+ retval = ve_devmnt_verify_fc(fc, false);
+ if (retval)
+ return retval;
+
if (fc->sb_flags_mask & SB_RDONLY) {
#ifdef CONFIG_BLOCK
if (!(fc->sb_flags & SB_RDONLY) && sb->s_bdev &&
@@ -1924,6 +1929,13 @@ int vfs_get_tree(struct fs_context *fc)
return error;
}
+ /* Enforce the container device-mount policy against the real device. */
+ error = ve_devmnt_verify_fc(fc, true);
+ if (unlikely(error)) {
+ fc_drop_locked(fc);
+ return error;
+ }
+
/*
* filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
* but s_maxbytes was an unsigned long long for many releases. Throw
diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
index 1801aed1da67..2ca586e2cc2a 100644
--- a/include/linux/fs_context.h
+++ b/include/linux/fs_context.h
@@ -93,6 +93,8 @@ struct fs_context {
struct file_system_type *fs_type;
void *fs_private; /* The filesystem's context */
void *lazy_opts; /* mount options which can't be checked at fsconfig() time */
+ /* option string handed to the fs, for the ve_devmnt policy check */
+ char *ve_final_opts;
void *sget_key;
struct dentry *root; /* The root and superblock */
struct user_namespace *user_ns; /* The user namespace for this mount */
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 0cbc6f6893c0..ab0ea4f7afc6 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -127,5 +127,7 @@ extern int cifs_root_data(char **dev, char **opts);
struct ve_struct;
extern int ve_devmnt_process(struct ve_struct *, dev_t, void **, int);
+extern int ve_devmnt_verify(struct ve_struct *ve, dev_t dev, char *opts,
+ bool new_mount, bool have_user_opts);
#endif /* _LINUX_MOUNT_H */
--
2.43.0
More information about the Devel
mailing list