Re: [PATCH] FS, NET: Fix KMSAN uninit-value in vfs_write

[Oliver Hartkopp]

On 14.03.23 16:37, Ivan Orlov wrote:
> On 3/14/23 18:38, Oliver Hartkopp wrote:
> > Hello Ivan,
> >
> > besides the fact that we would read some uninitialized value the 
> > outcome of the original implementation would have been an error and a 
> > termination of the copy process too. Maybe throwing a different error 
> > number.
> >
> > But it is really interesting to see what KMSAN is able to detect these 
> > days! Many thanks for the finding and your effort to contribute this fix!
> >
> > Best regards,
> > Oliver
> >
> >
> > On 14.03.23 13:04, Ivan Orlov wrote:
> > > Syzkaller reported the following issue:
> >
> > (..)
> >
> > > Reported-by: syzbot+c9bfd85eca611ebf5db1@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Link: 
> > > https://syzkaller.appspot.com/bug?id=47f897f8ad958bbde5790ebf389b5e7e0a345089
> > > Signed-off-by: Ivan Orlov <ivan.orlov0322@xxxxxxxxx>
> >
> > Acked-by: Oliver Hartkopp <socketcan@xxxxxxxxxxxx>
> >
> >
> > > ---
> > >   net/can/bcm.c | 16 ++++++++++------
> > >   1 file changed, 10 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/net/can/bcm.c b/net/can/bcm.c
> > > index 27706f6ace34..a962ec2b8ba5 100644
> > > --- a/net/can/bcm.c
> > > +++ b/net/can/bcm.c
> > > @@ -941,6 +941,8 @@ static int bcm_tx_setup(struct bcm_msg_head 
> > > *msg_head, struct msghdr *msg,
> > >               cf = op->frames + op->cfsiz * i;
> > >               err = memcpy_from_msg((u8 *)cf, msg, op->cfsiz);
> > > +            if (err < 0)
> > > +                goto free_op;
> > >               if (op->flags & CAN_FD_FRAME) {
> > >                   if (cf->len > 64)
> > > @@ -950,12 +952,8 @@ static int bcm_tx_setup(struct bcm_msg_head 
> > > *msg_head, struct msghdr *msg,
> > >                       err = -EINVAL;
> > >               }
> > > -            if (err < 0) {
> > > -                if (op->frames != &op->sframe)
> > > -                    kfree(op->frames);
> > > -                kfree(op);
> > > -                return err;
> > > -            }
> > > +            if (err < 0)
> > > +                goto free_op;
> > >               if (msg_head->flags & TX_CP_CAN_ID) {
> > >                   /* copy can_id into frame */
> > > @@ -1026,6 +1024,12 @@ static int bcm_tx_setup(struct bcm_msg_head 
> > > *msg_head, struct msghdr *msg,
> > >           bcm_tx_start_timer(op);
> > >       return msg_head->nframes * op->cfsiz + MHSIZ;
> > > +
> > > +free_op:
> > > +    if (op->frames != &op->sframe)
> > > +        kfree(op->frames);
> > > +    kfree(op);
> > > +    return err;
> > >   }
> > >   /*
>
> Thank you for the quick answer! I totally agree that this patch will not 
> change the behavior a lot. However, I think a little bit more error 
> processing will not be bad (considering this will not bring any 
> performance overhead). If someone in the future tries to use the "cf" 
> object right after "memcpy_from_msg" call without proper error 
> processing it will lead to a bug (which will be hard to trigger). Maybe 
> fixing it now to avoid possible future mistakes in the future makes sense?

Yes! Definitely!

Therefore I added my Acked-by: tag. Marc will likely pick this patch for 
upstream.

Best regards,
Oliver