linux-hardening - Re: [PATCH] PNP: replace deprecated strncpy with memcpy

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Date: Fri, 20 Oct 2023 19:51:46 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Kees Cook <keescook@...omium.org>, Justin Stitt <justinstitt@...gle.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>, linux-acpi@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] PNP: replace deprecated strncpy with memcpy

On Fri, Oct 20, 2023 at 2:31 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Thu, Oct 19, 2023 at 11:28:32PM +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
> > interfaces.
> >
> > After having precisely calculated the lengths and ensuring we don't
> > overflow the buffer, this really decays to just a memcpy. Let's not use
> > a C string api as it makes the intention of the code confusing.
>
> This is another case where we're building a C string from a byte array.
>
> > It'd be nice to use strscpy() in this case (as we clearly want
> > NUL-termination) because it'd clean up the code a bit. However, I don't
> > quite know enough about what is going on here to justify a drop-in
> > replacement -- too much bit magic and why (PNP_NAME_LEN - 2)? I'm afraid
> > using strscpy() may result in copying too many or too few bytes into our
> > dev->name buffer resulting in different behavior. At least using
> > memcpy() we can ensure the behavior is exactly the same.
> >
> > Side note:
> > NUL-padding is not required because insert_device() calls
> > pnpbios_parse_data_stream() with a zero-allocated `dev`:
> > 299 |  static int __init insert_device(struct pnp_bios_node *node) {
> > ...
> > 312 |  dev = pnp_alloc_dev(&pnpbios_protocol, node->handle, id);
> > ...
> > 316 |  pnpbios_parse_data_stream(dev, node);
> >
> > then pnpbios_parse_data_stream() calls pnpbios_parse_compatible_ids().
> >
> > 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
> > Signed-off-by: Justin Stitt <justinstitt@...gle.com>
>
> tl;dr:
>
> Reviewed-by: Kees Cook <keescook@...omium.org>
>
> My ramblings below...
>
> > ---
> > Note: build-tested only.
> >
> > Found with: $ rg "strncpy\("
> > ---
> >  drivers/pnp/pnpbios/rsparser.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/pnp/pnpbios/rsparser.c b/drivers/pnp/pnpbios/rsparser.c
> > index 2f31b212b1a5..70af7821d3fa 100644
> > --- a/drivers/pnp/pnpbios/rsparser.c
> > +++ b/drivers/pnp/pnpbios/rsparser.c
> > @@ -454,8 +454,8 @@ static unsigned char *pnpbios_parse_compatible_ids(unsigned char *p,
> >               switch (tag) {
>
> So we've got a fixed-sized C string as a destination:
>
> struct pnp_dev {
>         ...
>         char name[PNP_NAME_LEN];        /* contains a human-readable name */
>
> include/linux/pnp.h:#define PNP_NAME_LEN                50
>
> And a funky "source length" calculation, which appears to be effectively
> a u16 (it's either the low 3 bits of a u8, or a full u16);
>
>         int len ...
>
>                 /* determine the type of tag */
>                 if (p[0] & LARGE_TAG) { /* large tag */
>                         len = (p[2] << 8) | p[1];
>                         tag = p[0];
>                 } else {        /* small tag */
>                         len = p[0] & 0x07;
>                         tag = ((p[0] >> 3) & 0x0f);
>                 }
>
> The old code was doing:
>
>                 case LARGE_TAG_ANSISTR:
>                         strncpy(dev->name, p + 3,
>                                 len >= PNP_NAME_LEN ? PNP_NAME_LEN - 2 : len);
>                         dev->name[len >=
>                                   PNP_NAME_LEN ? PNP_NAME_LEN - 1 : len] = '\0';
>                         break;
>
> The two conditionals are not the same -- the first is -2, the latter is
> -1, but only when len >= PNP_NAME_LEN. This smells like a bug? For the
> len >= PNP_NAME_LEN case, it will copy 48 bytes and then write a %NUL to
> index 49 (byte 50). ... ... source byte 49 is ignored for no reason I
> can see.
>
> Regardless, the point is to copy no more than min(len, PNP_NAME_LEN - 1)
> from "p + 3" to not overflow dev->name, and leaving it %NUL terminated.
>
> So, I think what you have is identical behavior, and likely still
> contains the 1 byte short bug, which I think is fine to keep as-is since
> it's been like this forever and it's PNP...

And so applied as 6.7 material, thanks!