linux-hardening - Re: [PATCH] IB/hfi1: replace deprecated strncpy

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Date: Fri, 29 Sep 2023 12:56:22 -0700
From: Kees Cook <keescook@...omium.org>
To: Dean Luick <dean.luick@...nelisnetworks.com>
Cc: Leon Romanovsky <leon@...nel.org>,
	Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
	Jason Gunthorpe <jgg@...pe.ca>,
	Justin Stitt <justinstitt@...gle.com>, linux-rdma@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] IB/hfi1: replace deprecated strncpy

On Tue, Sep 26, 2023 at 07:56:34AM -0500, Dean Luick wrote:
> On 9/23/2023 10:20 PM, Kees Cook wrote:
> > On Fri, Sep 22, 2023 at 09:25:39AM -0500, Dean Luick wrote:
> >> On 9/22/2023 5:29 AM, Leon Romanovsky wrote:
> >>>
> >>> On Thu, 21 Sep 2023 07:17:47 +0000, Justin Stitt wrote:
> >>>> `strncpy` is deprecated for use on NUL-terminated destination strings
> >>>> [1] and as such we should prefer more robust and less ambiguous string
> >>>> interfaces.
> >>>>
> >>>> We see that `buf` is expected to be NUL-terminated based on it's use
> >>>> within a trace event wherein `is_misc_err_name` and `is_various_name`
> >>>> map to `is_name` through `is_table`:
> >>>> | TRACE_EVENT(hfi1_interrupt,
> >>>> |        TP_PROTO(struct hfi1_devdata *dd, const struct is_table *is_entry,
> >>>> |                 int src),
> >>>> |        TP_ARGS(dd, is_entry, src),
> >>>> |        TP_STRUCT__entry(DD_DEV_ENTRY(dd)
> >>>> |                         __array(char, buf, 64)
> >>>> |                         __field(int, src)
> >>>> |                         ),
> >>>> |        TP_fast_assign(DD_DEV_ASSIGN(dd);
> >>>> |                       is_entry->is_name(__entry->buf, 64,
> >>>> |                                         src - is_entry->start);
> >>>> |                       __entry->src = src;
> >>>> |                       ),
> >>>> |        TP_printk("[%s] source: %s [%d]", __get_str(dev), __entry->buf,
> >>>> |                  __entry->src)
> >>>> | );
> >>>>
> >>>> [...]
> >>>
> >>> Applied, thanks!
> >>
> >> It is unfortunate that this and the qib patch was accepted so quickly.  The replacement is functionally correct.  However, I was going to suggest using strscpy() since the return value is never looked at and all use cases only require a NUL-terminated string.  Padding is not needed.
> >
> > Is the trace buffer already guaranteed to be zeroed? Since this is
> > defined as a fixed-size string in the buffer, it made sense to me to be
> > sure that the unused bytes were 0 before copying them to userspace.
> 
> I was not aware that binary trace records were exposed to user space.  If so, and the event records are not zeroed (either the buffer as a whole, or individual records), then strscpy_pad() is the correct solution.  My quick review of the tracing system suggests that nothing is zeroed and the record is embedded in a larger structure.  However, this begs the question for all users of tracing: Aren't alignment holes in the fast assign record a leak?

I thought they were passed over direct to userspace somehow, but I
haven't looked at the details in a long time. I could very well be
misunderstanding it.

-- 
Kees Cook