[PATCH 8/8] ntfs3: free replaced mount options after a grace period
Christian Brauner
brauner at kernel.org
Fri Oct 9 14:51:58 PDT 2026
ntfs_fs_reconfigure() replaces the old mount options with new mount
options in sbi->options. The old mount options land in fs_context and
are freed as soon as remount returns. With a pathwalk held up in
ntfs_nls_to_utf16() between loading sbi->options and dereferencing it
while a remount replaces them:
BUG: KASAN: slab-use-after-free in ntfs_nls_to_utf16+0x7ae/0x7c0
Read of size 8 at addr ff110001044fd448 by task walker/98
Call Trace:
<TASK>
ntfs_nls_to_utf16+0x7ae/0x7c0
ntfs_d_hash+0x22b/0x500
link_path_walk+0x11ad/0x1d30
path_lookupat+0x63/0xb00
filename_lookup+0x1f1/0x5b0
vfs_statx+0xd7/0x3b0
[...]
Allocated by task 95:
__kmalloc_node_track_caller_noprof+0x2a2/0x730
kmemdup_noprof+0x25/0x50
ntfs_fill_super+0x1e5/0x4550
[...]
Freed by task 97:
kfree+0x1db/0x5b0
put_fs_context+0x13c/0x7d0
path_mount+0x54a/0x2130
__x64_sys_mount+0x20c/0x270
That info is needed in rcu lookup in the dentry hash and dentry compary
methods for ntfs3 and causes a UAF today. Delay freeing with an rcu
grace period.
ntfs_nls_to_utf16() reads ->nls and ->ads once under rcu_read_lock() so
the hooks pair with the release in ntfs_fs_reconfigure(). That also
covers callers outside of rcu pathwalk such as ntfs_lookup(). They don't
hold rcu_read_lock() themselves and wouldn't be protected by the grace
period otherwise. Remount refuses to change the iocharset so ->nls stays
valid after rcu_read_unlock().
Fixes: a3a956c78efa ("fs/ntfs3: Add option "nocase"")
Cc: stable at vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner at kernel.org>
---
Reproducer: the window is the load of sbi->options in
ntfs_nls_to_utf16() and the dereference right after it, so widen it in
the kernel: split
struct nls_table *nls = sbi->options->nls;
into a load of sbi->options, an mdelay(2000) when current->comm is
"walker", and the load of ->nls from the options read before the delay.
repro.c then remounts while a task named walker sits in that delay. The
name is not ascii, so ntfs_d_hash() goes through the options.
mkntfs -F -f -Q /dev/vdb
mount -t ntfs3 -o nocase,noatime /dev/vdb /mnt
echo hello > /mnt/$(printf '\303\244')
./repro
Without this patch that gives the splat above, with it (and the same
delay between the READ_ONCE() and ->nls) it runs clean.
repro.c:
#define _GNU_SOURCE
#include <pthread.h>
#include <sched.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/mount.h>
#include <sys/prctl.h>
#include <sys/stat.h>
#include <unistd.h>
static void pin(int cpu)
{
cpu_set_t set;
CPU_ZERO(&set);
CPU_SET(cpu, &set);
sched_setaffinity(0, sizeof(set), &set);
}
static void *walker(void *arg)
{
struct stat st;
char *p;
/*
* The delay spins. Empty this cpu's lru batches so that an
* lru_add_drain_all() on the other side has no work to wait for here.
*/
pin(1);
p = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
memset(p, 1, 4096);
madvise(p, 4096, MADV_COLD);
prctl(PR_SET_NAME, "walker");
stat("\xc3\xa4", &st);
return NULL;
}
int main(void)
{
struct stat st;
pthread_t t;
pin(0);
chdir("/mnt");
stat("\xc3\xa4", &st);
pthread_create(&t, NULL, walker, NULL);
sleep(1);
/* replaces sbi->options and frees the old ones */
mount("none", "/mnt", NULL, MS_REMOUNT, "nocase");
pthread_join(t, NULL);
return 0;
}
---
fs/ntfs3/dir.c | 13 +++++++++++--
fs/ntfs3/ntfs_fs.h | 2 ++
fs/ntfs3/super.c | 16 +++++++++++++++-
3 files changed, 28 insertions(+), 3 deletions(-)
diff --git a/fs/ntfs3/dir.c b/fs/ntfs3/dir.c
index eb9152e9fa22..14ca54e54578 100644
--- a/fs/ntfs3/dir.c
+++ b/fs/ntfs3/dir.c
@@ -186,11 +186,20 @@ int ntfs_nls_to_utf16(struct ntfs_sb_info *sbi, const u8 *name, u32 name_len,
{
int ret, slen, i;
const u8 *end;
- struct nls_table *nls = sbi->options->nls;
+ struct ntfs_mount_options *opts;
+ struct nls_table *nls;
+ bool ads;
u16 *uname = uni->name;
static_assert(sizeof(wchar_t) == sizeof(u16));
+ /* ntfs_fs_reconfigure() may replace the options, but not ->nls */
+ rcu_read_lock();
+ opts = READ_ONCE(sbi->options);
+ nls = opts->nls;
+ ads = opts->ads;
+ rcu_read_unlock();
+
if (!nls) {
/* utf8 -> utf16 */
ret = _utf8s_to_utf16s(name, name_len, endian, uname, max_ulen);
@@ -230,7 +239,7 @@ int ntfs_nls_to_utf16(struct ntfs_sb_info *sbi, const u8 *name, u32 name_len,
uni->len = ret;
uni->ads_len = 0;
- if (ret > 0 && sbi->options->ads) {
+ if (ret > 0 && ads) {
uname = uni->name;
/* Find delimiter in range [1 : ret-2). */
for (i = 1; i + 1 < ret; i++) {
diff --git a/fs/ntfs3/ntfs_fs.h b/fs/ntfs3/ntfs_fs.h
index bbcd2e02bf63..44d59b444648 100644
--- a/fs/ntfs3/ntfs_fs.h
+++ b/fs/ntfs3/ntfs_fs.h
@@ -91,6 +91,8 @@ enum utf16_endian;
struct ntfs_mount_options {
char *nls_name;
struct nls_table *nls;
+ /* ->d_hash() and ->d_compare() read the options in rcu pathwalk */
+ struct rcu_head rcu;
kuid_t fs_uid;
kgid_t fs_gid;
diff --git a/fs/ntfs3/super.c b/fs/ntfs3/super.c
index 841f9068d82f..58ef6be3286b 100644
--- a/fs/ntfs3/super.c
+++ b/fs/ntfs3/super.c
@@ -251,6 +251,11 @@ static inline void put_mount_options(struct ntfs_mount_options *options)
kfree(options);
}
+static void put_mount_options_rcu(struct rcu_head *head)
+{
+ put_mount_options(container_of(head, struct ntfs_mount_options, rcu));
+}
+
enum Opt {
Opt_uid,
Opt_gid,
@@ -444,6 +449,7 @@ static int ntfs_fs_reconfigure(struct fs_context *fc)
struct super_block *sb = fc->root->d_sb;
struct ntfs_sb_info *sbi = sb->s_fs_info;
struct ntfs_mount_options *new_opts = fc->fs_private;
+ struct ntfs_mount_options *old_opts;
int ro_rw;
ro_rw = sb_rdonly(sb) && !(fc->sb_flags & SB_RDONLY);
@@ -473,7 +479,15 @@ static int ntfs_fs_reconfigure(struct fs_context *fc)
}
sync_filesystem(sb);
- swap(sbi->options, fc->fs_private);
+ old_opts = sbi->options;
+ /* pairs with the READ_ONCE() in ntfs_nls_to_utf16() */
+ smp_store_release(&sbi->options, new_opts);
+ fc->fs_private = NULL;
+ /*
+ * ntfs_d_hash() and ntfs_d_compare() of a pathwalk in rcu mode may
+ * still read the old options through the sbi.
+ */
+ call_rcu(&old_opts->rcu, put_mount_options_rcu);
return 0;
}
--
2.53.0
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