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

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Message-ID: <20231019-strncpy-drivers-pnp-pnpbios-rsparser-c-v1-1-e47d93b52e3e@google.com>
Date:   Thu, 19 Oct 2023 23:28:32 +0000
From:   Justin Stitt <justinstitt@...gle.com>
To:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Cc:     linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org,
        Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] PNP: replace deprecated strncpy with memcpy

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.

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>
---
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) {

 		case LARGE_TAG_ANSISTR:
-			strncpy(dev->name, p + 3,
-				len >= PNP_NAME_LEN ? PNP_NAME_LEN - 2 : len);
+			memcpy(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;

---
base-commit: dab3e01664eaddae965699f1fec776609db0ea9d
change-id: 20231019-strncpy-drivers-pnp-pnpbios-rsparser-c-5d7343441cac

Best regards,
--
Justin Stitt <justinstitt@...gle.com>