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

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Date: Thu, 19 Oct 2023 17:04:17 -0700
From: Kees Cook <keescook@...omium.org>
To: 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 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...

-Kees

-- 
Kees Cook