linux-hardening - Re: [PATCH] soc: qcom: cmd-db: replace deprecated strncpy with memcpy

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Message-ID: <CAFhGd8qJ+Sks33cgEie_cvj-YrAUUiLrA7wY42eWQ2Xr++C4Pw@mail.gmail.com>
Date: Mon, 18 Mar 2024 15:47:38 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: Bjorn Andersson <andersson@...nel.org>, Konrad Dybcio <konrad.dybcio@...aro.org>, 
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] soc: qcom: cmd-db: replace deprecated strncpy with memcpy

On Mon, Mar 18, 2024 at 2:52 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Thu, Mar 14, 2024 at 10:29:37PM +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.
> >
> > @query is already marked as __nonstring and doesn't need to be
> > NUL-terminated. Due to this, we don't need to use a string API here
> > (especially a deprecated one). Let's have our stack allocation also
> > zero-initialize so that we can just perform a standard memcpy. Since the
> > code now speaks for itself we can drop the comment. A memcpy on a
> > __nonstring buffer explains everything that this comment talks about.
> >
> > Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> > Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> > Link: https://github.com/KSPP/linux/issues/90
> > Cc: linux-hardening@...r.kernel.org
> > Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> > ---
> > Note: build-tested only.
> >
> > Found with: $ rg "strncpy\("
> > ---
> >  drivers/soc/qcom/cmd-db.c | 9 ++-------
> >  1 file changed, 2 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/soc/qcom/cmd-db.c b/drivers/soc/qcom/cmd-db.c
> > index a5fd68411bed..512556366a3e 100644
> > --- a/drivers/soc/qcom/cmd-db.c
> > +++ b/drivers/soc/qcom/cmd-db.c
> > @@ -141,18 +141,13 @@ static int cmd_db_get_header(const char *id, const struct entry_header **eh,
> >       const struct rsc_hdr *rsc_hdr;
> >       const struct entry_header *ent;
> >       int ret, i, j;
> > -     u8 query[sizeof(ent->id)] __nonstring;
> > +     u8 query[sizeof(ent->id)] __nonstring = { 0 };
> >
> >       ret = cmd_db_ready();
> >       if (ret)
> >               return ret;
> >
> > -     /*
> > -      * Pad out query string to same length as in DB. NOTE: the output
> > -      * query string is not necessarily '\0' terminated if it bumps up
> > -      * against the max size. That's OK and expected.
> > -      */
> > -     strncpy(query, id, sizeof(query));
> > +     memcpy(query, id, sizeof(query));
>
> Hm, no, this isn't right. We do want to stop copying at the first NUL
> character, but we don't care about truncation. e.g. imagine if "id" was
> a 3 character string followed by other bytes in memory. We'd copy beyond
> the end of "id" into query, and the later memcmp()s would start failing.
> I think what you want here is:
>
>         strtomem(query, id);

Gotcha, I was operating under the assumption that we needed to know
the size of id at compile time. Apparently __builtin_object_size(_, 1)
will return SIZE_T_MAX if we don't know the size of something. Sending
a v2.

>
> -Kees
>
> >
> >       for (i = 0; i < MAX_SLV_ID; i++) {
> >               rsc_hdr = &cmd_db_header->header[i];
> >
> > ---
> > base-commit: fe46a7dd189e25604716c03576d05ac8a5209743
> > change-id: 20240314-strncpy-drivers-soc-qcom-cmd-db-c-284f3abaabb8
> >
> > Best regards,
> > --
> > Justin Stitt <justinstitt@...gle.com>
> >
> >
>
> --
> Kees Cook