ext4_get_link() follows a symlink that keeps its target in a data block
in rcu pathwalk mode with ext4_getblk(EXT4_GET_BLOCKS_CACHED_NOWAIT).
That reads s_mount_state in the ASSERT() of ext4_getblk() and again in
ext4_es_lookup_extent(), and the lookup bumps the extent status hit or
miss counter, all through sb->s_fs_info.
ext4_put_super() destroys those counters, clears sb->s_fs_info and frees
the sbi without waiting for a grace period. A pathwalk that is still in
ext4_get_link() when the last reference to the mount goes away then
dereferences a NULL s_fs_info, the freed sbi or the destroyed percpu
counters.
With a pathwalk held up in ext4_get_link() while the last reference
goes away:
KASAN: null-ptr-deref in range [0x00000000000000a8-0x00000000000000af]
RIP: 0010:ext4_getblk+0xf0/0x640
Call Trace:
<TASK>
ext4_get_link+0x26c/0x410
pick_link+0xf96/0x1780
step_into_slowpath+0x9b8/0x1250
path_lookupat+0x484/0xb00
filename_lookup+0x1f1/0x5b0
vfs_statx+0xd7/0x3b0
vfs_fstatat+0x4d/0x90
__do_sys_newfstatat+0xa0/0x100
do_syscall_64+0x10e/0x5e0
entry_SYSCALL_64_after_hwframe+0x77/0x7f
So far that has been hard to hit. umount(2) waits for a grace period in
namespace_unlock() before it drops the mount, so only a mount whose last
reference goes away elsewhere, like the close of a file or a chdir out of
a lazily unmounted directory, can race a walker.
Keep sb->s_fs_info and free the sbi together with the extent status
counters after a grace period. Split the counters out of
ext4_es_unregister_shrinker() for that. The shrinker still gets wasted
in ext4_put_super(), the counters go with the sbi.
Fixes: 6493792d3299 ("ext4: convert symlink external data block mapping to
bdev")
Cc: [email protected]
Signed-off-by: Christian Brauner (Amutable) <[email protected]>
---
Reproducer: delay.c holds a task named walker in ext4_get_link() in rcu
pathwalk mode for two seconds and repro.c drops the last reference to a
lazily unmounted ext4 meanwhile. Use an ext4 without a journal or a
kernel without lockdep: jbd2_journal_destroy() calls
lockdep_unregister_key(), which waits for a grace period on lockdep
kernels and hides this for a journaled ext4.
mkfs.ext4 -O ^has_journal /dev/vdb
mount -o noatime /dev/vdb /mnt
ln -s /$(printf 'x%.0s' $(seq 100)) /mnt/link
insmod delay.ko
./repro
The symlink target is too long for the inode, so ext4_get_link() reads it
from its block. Without this patch that gives the splat above, with it
it runs clean.
delay.c:
#include <linux/delay.h>
#include <linux/kprobes.h>
#include <linux/module.h>
/* keep a task named "walker" in ext4_get_link() in rcu pathwalk */
static int pre(struct kprobe *p, struct pt_regs *regs)
{
if (!regs_get_kernel_argument(regs, 0) && !strcmp(current->comm,
"walker"))
mdelay(2000);
return 0;
}
static struct kprobe kp = {
.symbol_name = "ext4_get_link",
.pre_handler = pre,
};
static int __init delay_init(void)
{
return register_kprobe(&kp);
}
static void __exit delay_exit(void)
{
unregister_kprobe(&kp);
}
module_init(delay_init);
module_exit(delay_exit);
MODULE_LICENSE("GPL");
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 with preemption disabled. Empty this cpu's lru
* batches so that lru_add_drain_all() from invalidate_bdev() 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("link", &st);
return NULL;
}
int main(void)
{
struct stat st;
pthread_t t;
pin(0);
chdir("/mnt");
stat("link", &st);
/* the cwd now holds the last reference to the mount */
umount2("/mnt", MNT_DETACH);
pthread_create(&t, NULL, walker, NULL);
sleep(1);
/* drop it, ->kill_sb() runs before chdir() returns */
chdir("/");
pthread_join(t, NULL);
return 0;
}
---
fs/ext4/ext4.h | 2 ++
fs/ext4/extents-test.c | 2 ++
fs/ext4/extents.c | 1 +
fs/ext4/extents_status.c | 7 ++++++-
fs/ext4/extents_status.h | 1 +
fs/ext4/super.c | 13 +++++++++++--
6 files changed, 23 insertions(+), 3 deletions(-)
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 724a27e8be61..af70bb26bb54 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1774,6 +1774,8 @@ struct ext4_sb_info {
struct list_head s_es_list; /* List of inodes with reclaimable
extents */
long s_es_nr_inode;
struct ext4_es_stats s_es_stats;
+ /* ext4_get_link() reads the sbi in rcu pathwalk */
+ struct rcu_head s_rcu;
struct mb_cache *s_ea_block_cache;
struct mb_cache *s_ea_inode_cache;
diff --git a/fs/ext4/extents-test.c b/fs/ext4/extents-test.c
index c3836ecb89f9..2e997f228330 100644
--- a/fs/ext4/extents-test.c
+++ b/fs/ext4/extents-test.c
@@ -151,6 +151,7 @@ static void extents_kunit_exit(struct kunit *test)
sbi = k_ctx.k_ei->vfs_inode.i_sb->s_fs_info;
ext4_es_unregister_shrinker(sbi);
+ ext4_es_destroy_stats(sbi);
deactivate_super(sbi->s_sb);
kfree(sbi);
kfree(k_ctx.k_ei);
@@ -340,6 +341,7 @@ static int extents_kunit_init(struct kunit *test)
k_ctx.k_data = NULL;
ext4_es_unregister_shrinker(sbi);
+ ext4_es_destroy_stats(sbi);
out_deactivate:
deactivate_locked_super(sb);
kfree(sbi);
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 76038b6c3655..3bf0a6cdbec2 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -6409,6 +6409,7 @@ EXPORT_SYMBOL_FOR_EXT4_TEST(__ext4_ext_dirty);
EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_ext_zeroout);
EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_es_register_shrinker);
EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_es_unregister_shrinker);
+EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_es_destroy_stats);
EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_map_create_blocks);
EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_es_init_tree);
EXPORT_SYMBOL_FOR_EXT4_TEST(ext4_es_lookup_extent);
diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
index 6e4a191e8219..018b6146b9d8 100644
--- a/fs/ext4/extents_status.c
+++ b/fs/ext4/extents_status.c
@@ -1881,12 +1881,17 @@ int ext4_es_register_shrinker(struct ext4_sb_info *sbi)
}
void ext4_es_unregister_shrinker(struct ext4_sb_info *sbi)
+{
+ shrinker_free(sbi->s_es_shrinker);
+}
+
+/* ext4_get_link() bumps the hit and miss counters in rcu pathwalk */
+void ext4_es_destroy_stats(struct ext4_sb_info *sbi)
{
percpu_counter_destroy(&sbi->s_es_stats.es_stats_cache_hits);
percpu_counter_destroy(&sbi->s_es_stats.es_stats_cache_misses);
percpu_counter_destroy(&sbi->s_es_stats.es_stats_all_cnt);
percpu_counter_destroy(&sbi->s_es_stats.es_stats_shk_cnt);
- shrinker_free(sbi->s_es_shrinker);
}
/*
diff --git a/fs/ext4/extents_status.h b/fs/ext4/extents_status.h
index f3396cf32b44..01cddcaaa68e 100644
--- a/fs/ext4/extents_status.h
+++ b/fs/ext4/extents_status.h
@@ -238,6 +238,7 @@ static inline void ext4_es_store_pblock_status(struct
extent_status *es,
extern int ext4_es_register_shrinker(struct ext4_sb_info *sbi);
extern void ext4_es_unregister_shrinker(struct ext4_sb_info *sbi);
+extern void ext4_es_destroy_stats(struct ext4_sb_info *sbi);
extern int ext4_seq_es_shrinker_info_show(struct seq_file *seq, void *v);
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index bca0dc87d0b7..f028c5835c5d 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1279,6 +1279,14 @@ static void ext4_flex_groups_free(struct ext4_sb_info
*sbi)
}
}
+static void ext4_put_sbi_rcu(struct rcu_head *head)
+{
+ struct ext4_sb_info *sbi = container_of(head, struct ext4_sb_info,
s_rcu);
+
+ ext4_es_destroy_stats(sbi);
+ kfree(sbi);
+}
+
static void ext4_put_super(struct super_block *sb)
{
struct ext4_sb_info *sbi = EXT4_SB(sb);
@@ -1374,7 +1382,6 @@ static void ext4_put_super(struct super_block *sb)
ext4_stop_mmpd(sbi);
brelse(sbi->s_sbh);
- sb->s_fs_info = NULL;
/*
* Now that we are completely done shutting down the
* superblock, we need to actually destroy the kobject.
@@ -1387,7 +1394,8 @@ static void ext4_put_super(struct super_block *sb)
#if IS_ENABLED(CONFIG_UNICODE)
utf8_unload(sb->s_encoding);
#endif
- kfree(sbi);
+ /* ext4_get_link() reads the sbi and the counters in rcu pathwalk */
+ call_rcu(&sbi->s_rcu, ext4_put_sbi_rcu);
}
static struct kmem_cache *ext4_inode_cachep;
@@ -5805,6 +5813,7 @@ failed_mount8: __maybe_unused
/* Drain deferred EA inode iputs from journal replay */
flush_delayed_work(&sbi->s_ea_inode_work);
ext4_es_unregister_shrinker(sbi);
+ ext4_es_destroy_stats(sbi);
failed_mount3:
/* flush s_sb_upd_work before sbi destroy */
flush_work(&sbi->s_sb_upd_work);
--
2.53.0
