[PATCH 1/2] f2fs: fix use-after-free of quota file names on failed remount

[Zhan Xusheng]

From: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>

f2fs_show_options() prints the journalled quota file names without any
lock that excludes remount, and a failed remount frees the current names
before restoring the old ones.  Reading /proc/mounts at that moment is a
use-after-free.

Read the names under rcu_read_lock() and free them only after a grace
period.

Fixes: 4b2414d04e99 ("f2fs: support journalled quota")
Reported-by: syzbot+e818dd42ada3b583bed1@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=e818dd42ada3b583bed1
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
---
The names are not marked __rcu: every other access holds s_umount,
except is_journalled_quota(), which only tests them for NULL.

 fs/f2fs/super.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 1071059f2cc2..510df7998c18 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -2325,6 +2325,7 @@ static inline void f2fs_show_quota_options(struct seq_file *seq,
 {
 #ifdef CONFIG_QUOTA
 	struct f2fs_sb_info *sbi = F2FS_SB(sb);
+	char *qf_name;
 
 	if (F2FS_OPTION(sbi).s_jquota_fmt) {
 		char *fmtname = "";
@@ -2343,17 +2344,20 @@ static inline void f2fs_show_quota_options(struct seq_file *seq,
 		seq_printf(seq, ",jqfmt=%s", fmtname);
 	}
 
-	if (F2FS_OPTION(sbi).s_qf_names[USRQUOTA])
-		seq_show_option(seq, "usrjquota",
-			F2FS_OPTION(sbi).s_qf_names[USRQUOTA]);
+	/* Remount frees replaced names only after a grace period */
+	rcu_read_lock();
+	qf_name = READ_ONCE(F2FS_OPTION(sbi).s_qf_names[USRQUOTA]);
+	if (qf_name)
+		seq_show_option(seq, "usrjquota", qf_name);
 
-	if (F2FS_OPTION(sbi).s_qf_names[GRPQUOTA])
-		seq_show_option(seq, "grpjquota",
-			F2FS_OPTION(sbi).s_qf_names[GRPQUOTA]);
+	qf_name = READ_ONCE(F2FS_OPTION(sbi).s_qf_names[GRPQUOTA]);
+	if (qf_name)
+		seq_show_option(seq, "grpjquota", qf_name);
 
-	if (F2FS_OPTION(sbi).s_qf_names[PRJQUOTA])
-		seq_show_option(seq, "prjjquota",
-			F2FS_OPTION(sbi).s_qf_names[PRJQUOTA]);
+	qf_name = READ_ONCE(F2FS_OPTION(sbi).s_qf_names[PRJQUOTA]);
+	if (qf_name)
+		seq_show_option(seq, "prjjquota", qf_name);
+	rcu_read_unlock();
 #endif
 }
 
@@ -3151,8 +3155,12 @@ static int __f2fs_remount(struct fs_context *fc, struct super_block *sb)
 #ifdef CONFIG_QUOTA
 	F2FS_OPTION(sbi).s_jquota_fmt = org_mount_opt.s_jquota_fmt;
 	for (i = 0; i < MAXQUOTAS; i++) {
-		kfree(F2FS_OPTION(sbi).s_qf_names[i]);
-		F2FS_OPTION(sbi).s_qf_names[i] = org_mount_opt.s_qf_names[i];
+		char *qf_name = F2FS_OPTION(sbi).s_qf_names[i];
+
+		/* Pairs with READ_ONCE() in f2fs_show_quota_options() */
+		smp_store_release(&F2FS_OPTION(sbi).s_qf_names[i],
+				  org_mount_opt.s_qf_names[i]);
+		kfree_rcu_mightsleep(qf_name);
 	}
 #endif
 	sbi->mount_opt = org_mount_opt;

base-commit: c339f4010dea8b0d754f84ace70cd1b089878ef8
-- 
2.43.0