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

[Fedor Pchelkin]

On 23/12/05 01:29PM, Christian Schoenebeck wrote:
> 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 <pchelkin@xxxxxxxxx>
> > ---
> > 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 ...
> 

The hassle with indices in this code can be eliminated with using
kcalloc() instead of kmalloc_array(). It would initialize all the members
to zero and later we can use the fact that kfree() is a no-op for NULL
args and iterate over all the elements - this trick is ubiquitous in
kernel AFAIK.

But when trying to do such kind of changes, I wonder whether it would
impact performance (I'm not able to test this fully) or related issues as
for some reason an unsafe kmalloc_array() was originally used.

If you have no objections, then I'll better prepare a new patch with
this in mind. That will make the code less prone to potential errors in
future.

> >  
> >  				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 <linux_oss@xxxxxxxxxxxxx>
> 
> > -					kfree(*wnames);
> > -					*wnames = NULL;
> >  				}
> >  			}
> >  			break;
> > 
> 
>