linux-kernel - Re: [PATCH v4 43/45] namei: initialize parameters passed to step

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHk-=wjeEre7eeWSwCRy2+ZFH8js4u22+3JTm6n+pY-QHdhbYw@mail.gmail.com>
Date:   Mon, 4 Jul 2022 12:16:24 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Matthew Wilcox <willy@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Vitaly Buka <vitalybuka@...gle.com>,
        linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: Re: [PATCH v4 43/45] namei: initialize parameters passed to step_into()

On Mon, Jul 4, 2022 at 12:03 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Anyway, I've thrown a mount_lock check in there, running xfstests to
> see how it goes...

So my reaction had been that it would be good to just do something like this:

  diff --git a/fs/namei.c b/fs/namei.c
  index 1f28d3f463c3..25c4bcc91142 100644
  --- a/fs/namei.c
  +++ b/fs/namei.c
  @@ -1493,11 +1493,18 @@ static bool __follow_mount_rcu(struct n...
      if (flags & DCACHE_MOUNTED) {
          struct mount *mounted = __lookup_mnt(path->mnt, dentry);
          if (mounted) {
  +           struct dentry *old_dentry = dentry;
  +           unsigned old_seq = *seqp;
  +
              path->mnt = &mounted->mnt;
              dentry = path->dentry = mounted->mnt.mnt_root;
              nd->state |= ND_JUMPED;
              *seqp = read_seqcount_begin(&dentry->d_seq);
              *inode = dentry->d_inode;
  +
  +           if (read_seqcount_retry(&old_dentry->d_seq, old_seq))
  +               return false;
  +
              /*
               * We don't need to re-check ->d_seq after this
               * ->d_inode read - there will be an RCU delay

but the above is just whitespace-damaged random monkey-scribbling by
yours truly.

More like a "shouldn't we do something like this" than a serious
patch, in other words.

IOW, it has *NOT* had a lot of real thought behind it. Purely a
"shouldn't we always clearly check the old sequence number after we've
picked up the new one?"

                   Linus