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Date:   Fri, 7 Aug 2020 10:42:03 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Chris Palmer <palmer@google.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Christoph Hellwig <hch@lst.de>,
        Matt Denton <mpdenton@google.com>,
        Robert Sesek <rsesek@google.com>,
        Sargun Dhillon <sargun@sargun.me>,
        Shuah Khan <shuah@kernel.org>, Tycho Andersen <tycho@tycho.ws>,
        Will Deacon <will@kernel.org>, Will Drewry <wad@chromium.org>,
        Yonghong Song <yhs@fb.com>
Subject: Re: wine fails to start with seccomp updates for v5.9-rc1
Message-ID: <202008071038.3F308DEEE@keescook>
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On Fri, Aug 07, 2020 at 02:36:09PM -0300, Thadeu Lima de Souza Cascardo wrote:
> On Fri, Aug 07, 2020 at 08:48:46AM -0700, Linus Torvalds wrote:
> > On Fri, Aug 7, 2020 at 8:19 AM Alex Xu (Hello71) <alex_y_xu@yahoo.ca> wrote:
> > >
> > > On Linus' master, wine fails to start with the following error:
> > >
> > > wine client error:0: write: Bad file descriptor
> > >
> > > This issue is not present on 5.8. It appears to be caused by failure to
> > > write to a pipe FD received via SCM_RIGHTS. Therefore, I tried reverting
> > > 9ecc6ea491f0, which resolved the issue.
> > 
> > Would you mind trying to bisect exactly where it happens?
> > 
> 
> This report [1] seemed related and pointed out at c0029de50982 ("net/scm:
> Regularize compat handling of scm_detach_fds()"). The use of CMSG_USER_DATA
> instead of CMSG_COMPAT_DATA seems fishy.

Argh; yes. Thank you for finding that! That's what I get for trying to
regularize the compat path. :(

> Alex, can you try applying the patch below?
> 
> Cascardo.
> 
> [1] https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-August/216156.html
> 
> > I don't think any of the commits in that pull are supposed to change
> > semantics, and while reverting the whole merge shows that yes, that's
> > what brought in the problems, it would be good to pinpoint just which
> > change breaks so that we can fix just that thing.
> > 
> > Kees, ideas?
> > 
> >                  Linus
> 
> ---
> diff --git a/net/compat.c b/net/compat.c
> index 703acb51c698..95ce707a30a3 100644
> --- a/net/compat.c
> +++ b/net/compat.c
> @@ -294,7 +294,7 @@ void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm)
>  		(struct compat_cmsghdr __user *)msg->msg_control;
>  	unsigned int o_flags = (msg->msg_flags & MSG_CMSG_CLOEXEC) ? O_CLOEXEC : 0;
>  	int fdmax = min_t(int, scm_max_fds_compat(msg), scm->fp->count);
> -	int __user *cmsg_data = CMSG_USER_DATA(cm);
> +	int __user *cmsg_data = CMSG_COMPAT_DATA(cm);
>  	int err = 0, i;
>  
>  	for (i = 0; i < fdmax; i++) {

That almost certainly will fix the problem.

-- 
Kees Cook