linux-kernel - Re: [PATCH] scsi: lpfc: replace deprecated strncpy with strscpy

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Message-ID: <CAFhGd8rTBqqQvr9vhpbfcSv6cpj5JSPVPSftqOPyWkZEWBP1AQ@mail.gmail.com>
Date: Fri, 23 Feb 2024 12:02:22 -0800
From: Justin Stitt <justinstitt@...gle.com>
To: Kees Cook <kees@...nel.org>
Cc: James Smart <james.smart@...adcom.com>, Dick Kennedy <dick.kennedy@...adcom.com>, 
	"James E.J. Bottomley" <jejb@...ux.ibm.com>, "Martin K. Petersen" <martin.petersen@...cle.com>, 
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] scsi: lpfc: replace deprecated strncpy with strscpy

Hi,

On Wed, Feb 21, 2024 at 6:38 PM Kees Cook <kees@...nel.org> wrote:
>
>
>
> On February 21, 2024 4:41:52 PM PST, Justin Stitt <justinstitt@...gle.com> 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.
> >
> >We expect ae->value_string to be NUL-terminated because there's a
> >comment that says as much; these attr strings are also used with other
> >string APIs, further cementing the fact.
> >
> >Now, the question of whether or not to NUL-pad the destination buffer:
> >lpfc_fdmi_rprt_defer() initializes vports (all zero-initialized), then
> >we call lpfc_fdmi_cmd() with each vport and a mask. Then, inside of
> >lpfc_fdmi_cmd() we check each bit in the mask to invoke the proper
> >callback. Importantly, the zero-initialized vport is passed in as the
> >"attr" parameter. Seeing this:
> >|      struct lpfc_fdmi_attr_string *ae = attr;
> >... we can tell that ae->value_string is entirely zero-initialized. Due
> >to this, NUL-padding is _not_ required as it would be redundant.
> >
> >Conveniently, strscpy also returns the number of bytes copied into the
> >destination buffer, eliminating the need for strnlen!
> >
> >Considering the above, a suitable replacement is `strscpy` [2].
> >
> >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>
> >---
> > drivers/scsi/lpfc/lpfc_ct.c | 5 ++---
> > 1 file changed, 2 insertions(+), 3 deletions(-)
> >
> >diff --git a/drivers/scsi/lpfc/lpfc_ct.c b/drivers/scsi/lpfc/lpfc_ct.c
> >index baae1f8279e0..42594ec87290 100644
> >--- a/drivers/scsi/lpfc/lpfc_ct.c
> >+++ b/drivers/scsi/lpfc/lpfc_ct.c
> >@@ -2569,9 +2569,8 @@ lpfc_fdmi_set_attr_string(void *attr, uint16_t attrtype, char *attrstring)
> >        * 64 bytes or less.
> >        */
> >
> >-      strncpy(ae->value_string, attrstring, sizeof(ae->value_string));
> >-      len = strnlen(ae->value_string, sizeof(ae->value_string));
> >-      /* round string length to a 32bit boundary. Ensure there's a NULL */
> >+      len = strscpy(ae->value_string, attrstring, sizeof(ae->value_string));
>
> This could be < 0 on error, and at least lpfc_fdmi_hba_attr_os_ver() may present more than 64 bytes...

Am I putting too much faith in this comment?

static inline int lpfc_fdmi_set_attr_string(void *attr, uint16_t
attrtype, char *attrstring)
..
/*
* We are trusting the caller that if a fdmi string field
* is capped at 64 bytes, the caller passes in a string of
* 64 bytes or less.
*/
..

I see lpfc_fdmi_hba_attr_os_ver() calls lpfc_fdmi_set_attr_string()
with an attrstring sized at 256 bytes:
char buf[256] = { 0 };

Can we really return -E2BIG from strscpy() if the dest buffer is the
same size as the source buffer?

I'm happy to just make the standard strncpy -> strscpy replacement and
drop the len assignment. Let me know what you think, Kees.

>
> -Kees
>
>
> >+      /* round string length to a 32bit boundary */
> >       len += (len & 3) ? (4 - (len & 3)) : 4;
> >       /* size is Type/Len (4 bytes) plus string length */
> >       size = FOURBYTES + len;
> >
> >---
> >base-commit: 39133352cbed6626956d38ed72012f49b0421e7b
> >change-id: 20240222-strncpy-drivers-scsi-lpfc-lpfc_ct-c-f54b67eeeb68
> >
> >Best regards,
> >--
> >Justin Stitt <justinstitt@...gle.com>
> >
> >
>
> --
> Kees Cook

Thanks
Justin