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From:   Jann Horn <jannh@google.com>
Date:   Sun, 30 Aug 2020 20:31:36 +0200
Message-ID: <CAG48ez00caDqMomv+PF4dntJkWx7rNYf3E+8gufswis6UFSszw@mail.gmail.com>
Subject: Re: [RESEND PATCH] vfs: add RWF_NOAPPEND flag for pwritev2
To:     Rich Felker <dalias@libc.org>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>
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On Sun, Aug 30, 2020 at 6:36 PM Rich Felker <dalias@libc.org> wrote:
> On Sun, Aug 30, 2020 at 05:05:45PM +0200, Jann Horn wrote:
> > On Sat, Aug 29, 2020 at 4:00 AM Rich Felker <dalias@libc.org> wrote:
> > > The pwrite function, originally defined by POSIX (thus the "p"), is
> > > defined to ignore O_APPEND and write at the offset passed as its
> > > argument. However, historically Linux honored O_APPEND if set and
> > > ignored the offset. This cannot be changed due to stability policy,
> > > but is documented in the man page as a bug.
> > >
> > > Now that there's a pwritev2 syscall providing a superset of the pwrite
> > > functionality that has a flags argument, the conforming behavior can
> > > be offered to userspace via a new flag.
[...]
> > Linux enforces the S_APPEND flag (set by "chattr +a") only at open()
> > time, not at write() time:
[...]
> > It seems to me like your patch will permit bypassing S_APPEND by
> > opening an append-only file with O_WRONLY|O_APPEND, then calling
> > pwritev2() with RWF_NOAPPEND? I think you'll have to add an extra
> > check for IS_APPEND() somewhere.
> >
> >
> > One could also argue that if an O_APPEND file descriptor is handed
> > across privilege boundaries, a programmer might reasonably expect that
> > the recipient will not be able to use the file descriptor for
> > non-append writes; if that is not actually true, that should probably
> > be noted in the open.2 manpage, at the end of the description of
> > O_APPEND.
>
> fcntl F_SETFL can remove O_APPEND, so it is not a security boundary.
> I'm not sure how this interacts with S_APPEND; presumably fcntl
> rechecks it.

Ah, good point, you're right. In fs/fcntl.c:

  35 static int setfl(int fd, struct file * filp, unsigned long arg)
  36 {
  37    struct inode * inode = file_inode(filp);
  38    int error = 0;
  39
  40    /*
  41     * O_APPEND cannot be cleared if the file is marked as append-only
  42     * and the file is open for write.
  43     */
  44    if (((arg ^ filp->f_flags) & O_APPEND) && IS_APPEND(inode))
  45            return -EPERM;

> So just checking IS_APPEND in the code paths used by
> pwritev2 (and erroring out rather than silently writing output at the
> wrong place) should suffice to preserve all existing security
> invariants.

Makes sense.