[PATCH 2/2] f2fs: free the quota file name that remount drops

[Zhan Xusheng <zhanxusheng1024@gmail.com>]

From: Zhan Xusheng <zhanxusheng@xiaomi.com>

Remounting with an empty journalled quota option such as "usrjquota="
clears the name, but f2fs_apply_quota_options() overwrites the old
string without freeing it, so every such remount leaks it.

Free it after a grace period, as f2fs_show_options() may still be
reading it.

Fixes: d18535132523 ("f2fs: separate the options parsing and options checking")
Reported-by: syzbot+6c000dea1a93b4f5905c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6c000dea1a93b4f5905c
Cc: stable@vger.kernel.org
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
---
Tested in QEMU with KASAN and kmemleak, remounting with "usrjquota="
and "usrjquota=f2fs" in a loop while reading /proc/mounts: nothing is
reported.  A plain kfree() here trips KASAN in f2fs_show_options().

 fs/f2fs/super.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 510df7998c18..d86ab25fbcc5 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -1646,7 +1646,7 @@ static void f2fs_apply_quota_options(struct fs_context *fc,
 	struct f2fs_fs_context *ctx = fc->fs_private;
 	struct f2fs_sb_info *sbi = F2FS_SB(sb);
 	bool quota_feature = f2fs_sb_has_quota_ino(sbi);
-	char *qname;
+	char *qname, *old;
 	int i;
 
 	if (quota_feature)
@@ -1662,7 +1662,10 @@ static void f2fs_apply_quota_options(struct fs_context *fc,
 					GFP_KERNEL | __GFP_NOFAIL);
 			set_opt(sbi, QUOTA);
 		}
-		F2FS_OPTION(sbi).s_qf_names[i] = qname;
+		old = 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], qname);
+		kfree_rcu_mightsleep(old);
 	}
 
 	if (ctx->spec_mask & F2FS_SPEC_jqfmt)
-- 
2.43.0