Subject: Re: [PATCH v2] net: 9p: avoid freeing uninit memory in p9pdu_vreadf

From: Christian Schoenebeck <[email-masked]>
Date: Tue, 05 Dec 2023 13:29:49 +0100

On Tuesday, December 5, 2023 10:19:50 AM CET Fedor Pchelkin wrote:
> If an error occurs while processing an array of strings in p9pdu_vreadf
> then uninitialized members of *wnames array are freed.
> 
> Fix this by iterating over only lower indices of the array. Also handle
> possible uninit *wnames usage if first p9pdu_readf() call inside 'T' case
> fails.
> 
> Found by Linux Verification Center (linuxtesting.org).
> 
> Fixes: ace51c4dd2f9 ("9p: add new protocol support code")
> Signed-off-by: Fedor Pchelkin <[email-masked]>
> ---
> v2: I've missed that *wnames can also be left uninitialized. Please
> ignore the patch v1. As an answer to Dominique's comment: my
> organization marks this statement in all commits.
> 
>  net/9p/protocol.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/net/9p/protocol.c b/net/9p/protocol.c
> index 4e3a2a1ffcb3..043b621f8b84 100644
> --- a/net/9p/protocol.c
> +++ b/net/9p/protocol.c
> @@ -393,6 +393,8 @@ p9pdu_vreadf(struct p9_fcall *pdu, int proto_version, const char *fmt,
>  		case 'T':{
>  				uint16_t *nwname = va_arg(ap, uint16_t *);
>  				char ***wnames = va_arg(ap, char ***);
> +				int i;
> +				*wnames = NULL;

Consider also initializing `int i = 0;` here. Because ...

>  
>  				errcode = p9pdu_readf(pdu, proto_version,
>  								"w", nwname);
> @@ -406,8 +408,6 @@ p9pdu_vreadf(struct p9_fcall *pdu, int proto_version, const char *fmt,
>  				}
>  
>  				if (!errcode) {
> -					int i;
> -
>  					for (i = 0; i < *nwname; i++) {

... this block that initializes `i` is conditional. I mean it does work right
now as-is, because ...

>  						errcode =
>  						    p9pdu_readf(pdu,
> @@ -421,13 +421,11 @@ p9pdu_vreadf(struct p9_fcall *pdu, int proto_version, const char *fmt,
>  
>  				if (errcode) {
>  					if (*wnames) {
> -						int i;
> -
> -						for (i = 0; i < *nwname; i++)
> +						while (--i >= 0)
>  							kfree((*wnames)[i]);
> +						kfree(*wnames);
> +						*wnames = NULL;
>  					}

... this is wrapped into `if (*wnames) {` and you initialized *wnames with
NULL, but it just feels like a potential future trap somehow.

Anyway, at least it looks like correct behaviour (ATM), so:

Reviewed-by: Christian Schoenebeck <[email-masked]>

> -					kfree(*wnames);
> -					*wnames = NULL;
>  				}
>  			}
>  			break;