linux-hardening - Re: [PATCH] SUNRPC: Replace strlcpy with strscpy

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Date:   Tue, 13 Jun 2023 12:42:13 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Chuck Lever III <chuck.lever@...cle.com>
Cc:     Azeem Shaikh <azeemshaikh38@...il.com>,
        Jeff Layton <jlayton@...nel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        Neil Brown <neilb@...e.de>,
        Olga Kornievskaia <kolga@...app.com>,
        Dai Ngo <dai.ngo@...cle.com>, Tom Talpey <tom@...pey.com>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] SUNRPC: Replace strlcpy with strscpy

On Tue, Jun 13, 2023 at 02:18:06PM +0000, Chuck Lever III wrote:
> 
> 
> > On Jun 12, 2023, at 8:40 PM, Azeem Shaikh <azeemshaikh38@...il.com> wrote:
> > 
> > strlcpy() reads the entire source buffer first.
> > This read may exceed the destination size limit.
> > This is both inefficient and can lead to linear read
> > overflows if a source string is not NUL-terminated [1].
> > In an effort to remove strlcpy() completely [2], replace
> > strlcpy() here with strscpy().
> 
> Using sprintf() seems cleaner to me: it would get rid of
> the undocumented naked integer. Would that work for you?

This is changing the "get" routine for reporting module parameters out
of /sys. I think the right choice here is sysfs_emit(), as it performs
the size tracking correctly. (Even the "default" sprintf() call should
be replaced too, IMO.)

> 
> 
> > Direct replacement is safe here since the getter in kernel_params_ops
> > handles -errorno return [3].
> 
> s/errorno/errno/
> 
> 
> > [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> > [2] https://github.com/KSPP/linux/issues/89
> > [3] https://elixir.bootlin.com/linux/v6.4-rc6/source/include/linux/moduleparam.h#L52
> > 
> > Signed-off-by: Azeem Shaikh <azeemshaikh38@...il.com>
> > ---
> > net/sunrpc/svc.c |    8 ++++----
> > 1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> > index e6d4cec61e47..e5f379c4fdb3 100644
> > --- a/net/sunrpc/svc.c
> > +++ b/net/sunrpc/svc.c
> > @@ -109,13 +109,13 @@ param_get_pool_mode(char *buf, const struct kernel_param *kp)
> > switch (*ip)
> > {
> > case SVC_POOL_AUTO:
> > - return strlcpy(buf, "auto\n", 20);
> > + return strscpy(buf, "auto\n", 20);

e.g.
	return sysfs_emit(buf, "auto\n");
...

> > case SVC_POOL_GLOBAL:
> > - return strlcpy(buf, "global\n", 20);
> > + return strscpy(buf, "global\n", 20);
> > case SVC_POOL_PERCPU:
> > - return strlcpy(buf, "percpu\n", 20);
> > + return strscpy(buf, "percpu\n", 20);
> > case SVC_POOL_PERNODE:
> > - return strlcpy(buf, "pernode\n", 20);
> > + return strscpy(buf, "pernode\n", 20);
> > default:
> > return sprintf(buf, "%d\n", *ip);

and:

	return sysfs_emit(buf, "%d\n", *ip);


-Kees

-- 
Kees Cook