linux-hardening - Re: [PATCH] ipmi: refactor deprecated strncpy

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Date:   Tue, 12 Sep 2023 17:58:36 -0700
From:   Justin Stitt <justinstitt@...gle.com>
To:     minyard@....org
Cc:     openipmi-developer@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] ipmi: refactor deprecated strncpy

On Tue, Sep 12, 2023 at 5:55 PM Justin Stitt <justinstitt@...gle.com> wrote:
>
> On Tue, Sep 12, 2023 at 5:19 PM Corey Minyard <minyard@....org> wrote:
> >
> > On Tue, Sep 12, 2023 at 11:43:05PM +0000, Justin Stitt wrote:
> > > `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> > >
> > > In this case, strncpy is being used specifically for its NUL-padding
> > > behavior (and has been commented as such). We can use a more robust and
> > > less ambiguous interface in `strscpy_pad` which makes the code more
> > > readable and even eliminates the need for that comment.
> > >
> > > Let's also use `strnlen` instead of `strlen()` with an upper-bounds
> > > check as this is intrinsically a part of `strnlen`.
> > >
> > > Also included in this patch is a simple 1:1 change of `strncpy` to
> > > `strscpy` for ipmi_ssif.c. If NUL-padding is wanted here as well then we
> > > should opt again for `strscpy_pad`.
> > >
> > > Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> > > Link: https://github.com/KSPP/linux/issues/90
> > > Cc: linux-hardening@...r.kernel.org
> > > Cc: Kees Cook <keescook@...omium.org>
> > > Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> > > ---
> > >  drivers/char/ipmi/ipmi_msghandler.c | 11 +++--------
> > >  drivers/char/ipmi/ipmi_ssif.c       |  2 +-
> > >  2 files changed, 4 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/drivers/char/ipmi/ipmi_msghandler.c b/drivers/char/ipmi/ipmi_msghandler.c
> > > index 186f1fee7534..04f7622cb703 100644
> > > --- a/drivers/char/ipmi/ipmi_msghandler.c
> > > +++ b/drivers/char/ipmi/ipmi_msghandler.c
> > > @@ -5377,20 +5377,15 @@ static void send_panic_events(struct ipmi_smi *intf, char *str)
> > >
> > >       j = 0;
> > >       while (*p) {
> > > -             int size = strlen(p);
> > > +             int size = strnlen(p, 11);
> > >
> > > -             if (size > 11)
> > > -                     size = 11;
> > >               data[0] = 0;
> > >               data[1] = 0;
> > >               data[2] = 0xf0; /* OEM event without timestamp. */
> > >               data[3] = intf->addrinfo[0].address;
> > >               data[4] = j++; /* sequence # */
> > > -             /*
> > > -              * Always give 11 bytes, so strncpy will fill
> > > -              * it with zeroes for me.
> > > -              */
> > > -             strncpy(data+5, p, 11);
> > > +
> > > +             strscpy_pad(data+5, p, 11);
> >
> > This is incorrect, the destination should *not* be nil terminated if the
> > destination is full.  strncpy does exactly what is needed here.
>
> Could we use `memcpy_and_pad()` as this matches the behavior of
> strncpy in this case? I understand strncpy works here but I'm really
> keen on snuffing out all its uses -- treewide.

^ I mean something like the following:
|    memcpy_and_pad(data+5, 11, p, size, '\0');

as this is explicit in its behavior.

>
> >
> > A comment should be added here, this is not the first time this has been
> > brought up.
> >
> > >               p += size;
> > >
> > >               ipmi_panic_request_and_wait(intf, &addr, &msg);
> > > diff --git a/drivers/char/ipmi/ipmi_ssif.c b/drivers/char/ipmi/ipmi_ssif.c
> > > index 3b921c78ba08..edcb83765dce 100644
> > > --- a/drivers/char/ipmi/ipmi_ssif.c
> > > +++ b/drivers/char/ipmi/ipmi_ssif.c
> > > @@ -1940,7 +1940,7 @@ static int new_ssif_client(int addr, char *adapter_name,
> > >               }
> > >       }
> > >
> > > -     strncpy(addr_info->binfo.type, DEVICE_NAME,
> > > +     strscpy(addr_info->binfo.type, DEVICE_NAME,
> > >               sizeof(addr_info->binfo.type));
> >
> > This one is good.
> >
> > -corey
> >
> > >       addr_info->binfo.addr = addr;
> > >       addr_info->binfo.platform_data = addr_info;
> > >
> > > ---
> > > base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
> > > change-id: 20230912-strncpy-drivers-char-ipmi-ipmi-dda47b3773fd
> > >
> > > Best regards,
> > > --
> > > Justin Stitt <justinstitt@...gle.com>
> > >