Re: [PATCH] soc: qcom: cmd-db: replace deprecated strncpy with memcpy
From: Justin Stitt
Date: Mon Mar 18 2024 - 18:48:58 EST
On Mon, Mar 18, 2024 at 2:52 PM Kees Cook <keescook@xxxxxxxxxxxx> 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.enhtml [2]
> > Link: https://github.com/KSPP/linux/issues/90
> > Cc: linux-hardening@xxxxxxxxxxxxxxx
> > Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxx>
> >
> >
>
> --
> Kees Cook