linux-kernel - [PATCH] Re: KASAN: use-after-free Read in path

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Date:   Wed, 12 Aug 2020 05:15:18 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     arnd@...db.de, christian.brauner@...ntu.com, hch@....de,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux@...inikbrodowski.net,
        syzkaller-bugs@...glegroups.com,
        syzbot <syzbot+bbeb1c88016c7db4aa24@...kaller.appspotmail.com>
Subject: [PATCH] Re: KASAN: use-after-free Read in path_init (2)

On Tue, Aug 11, 2020 at 08:17:16PM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    5631c5e0 Merge tag 'xfs-5.9-merge-7' of git://git.kernel.o..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=17076984900000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=afba7c06f91e56eb
> dashboard link: https://syzkaller.appspot.com/bug?extid=bbeb1c88016c7db4aa24
> compiler:       gcc (GCC) 10.1.0-syz 20200507
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1502ce02900000

fix breakage in do_rmdir()

putname() should happen only after we'd *not* branched to
retry, same as it's done in do_unlinkat().

Fixes: e24ab0ef689d "fs: push the getname from do_rmdir into the callers"
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
diff --git a/fs/namei.c b/fs/namei.c
index fde8fe086c09..9fa10c614de7 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3770,11 +3770,11 @@ long do_rmdir(int dfd, struct filename *name)
 	mnt_drop_write(path.mnt);
 exit1:
 	path_put(&path);
-	putname(name);
 	if (retry_estale(error, lookup_flags)) {
 		lookup_flags |= LOOKUP_REVAL;
 		goto retry;
 	}
+	putname(name);
 	return error;
 }