| Message ID | 20201022070739.GB2817762@mwanda |
|---|---|
| State | New |
| Headers | show |
| Series | [net] mISDN: hfcpci: Fix a use after free in hfcmulti_tx() | expand |
Hi Dan, that looks wrong to me and never was a use after free. sp is set either to the address containing the pointer to the actual D-channel SKB or to the actual B-channel SKB. This address is not freed and will not change in this context. The dev_kfree(*sp) will delete the old SKB and the call to get_next_[bd]frame(), if returning true, will place a new SKB into this address, so (*sp) point to this new SKB. The len of course need to be the length of the new SKB, not the old one, which would be the result of this patch. Best regards Karsten On 10/22/20 9:07 AM, Dan Carpenter wrote: > This frees "*sp" before dereferencing it to get "len = (*sp)->len;". > > Fixes: af69fb3a8ffa ("Add mISDN HFC multiport driver") > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com> > --- > drivers/isdn/hardware/mISDN/hfcmulti.c | 10 ++++------ > 1 file changed, 4 insertions(+), 6 deletions(-) > > diff --git a/drivers/isdn/hardware/mISDN/hfcmulti.c b/drivers/isdn/hardware/mISDN/hfcmulti.c > index 7013a3f08429..ce6c160e0df4 100644 > --- a/drivers/isdn/hardware/mISDN/hfcmulti.c > +++ b/drivers/isdn/hardware/mISDN/hfcmulti.c > @@ -2152,16 +2152,14 @@ hfcmulti_tx(struct hfc_multi *hc, int ch) > HFC_wait_nodebug(hc); > } > > + len = (*sp)->len; > dev_kfree_skb(*sp); > /* check for next frame */ > - if (bch && get_next_bframe(bch)) { > - len = (*sp)->len; > + if (bch && get_next_bframe(bch)) > goto next_frame; > - } > - if (dch && get_next_dframe(dch)) { > - len = (*sp)->len; > + > + if (dch && get_next_dframe(dch)) > goto next_frame; > - } > > /* > * now we have no more data, so in case of transparent, >
On Thu, Oct 22, 2020 at 04:24:00PM +0200, isdn@linux-pingi.de wrote: > Hi Dan, > > that looks wrong to me and never was a use after free. > > sp is set either to the address containing the pointer to the actual > D-channel SKB or to the actual B-channel SKB. This address is not freed > and will not change in this context. The dev_kfree(*sp) will delete the > old SKB and the call to get_next_[bd]frame(), if returning true, will > place a new SKB into this address, so (*sp) point to this new SKB. > The len of course need to be the length of the new SKB, not the old one, > which would be the result of this patch. > Oh, wow. You're absolutely right. That's pretty subtle. Thanks for catching it. regards, dan carpenter > Best regards > Karsten > > On 10/22/20 9:07 AM, Dan Carpenter wrote: > > This frees "*sp" before dereferencing it to get "len = (*sp)->len;". > > > > Fixes: af69fb3a8ffa ("Add mISDN HFC multiport driver") > > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com> > > --- > > drivers/isdn/hardware/mISDN/hfcmulti.c | 10 ++++------ > > 1 file changed, 4 insertions(+), 6 deletions(-) > > > > diff --git a/drivers/isdn/hardware/mISDN/hfcmulti.c b/drivers/isdn/hardware/mISDN/hfcmulti.c > > index 7013a3f08429..ce6c160e0df4 100644 > > --- a/drivers/isdn/hardware/mISDN/hfcmulti.c > > +++ b/drivers/isdn/hardware/mISDN/hfcmulti.c > > @@ -2152,16 +2152,14 @@ hfcmulti_tx(struct hfc_multi *hc, int ch) > > HFC_wait_nodebug(hc); > > } > > > > + len = (*sp)->len; > > dev_kfree_skb(*sp); > > /* check for next frame */ > > - if (bch && get_next_bframe(bch)) { > > - len = (*sp)->len; > > + if (bch && get_next_bframe(bch)) > > goto next_frame; > > - } > > - if (dch && get_next_dframe(dch)) { > > - len = (*sp)->len; > > + > > + if (dch && get_next_dframe(dch)) > > goto next_frame; > > - } > > > > /* > > * now we have no more data, so in case of transparent, > >
diff --git a/drivers/isdn/hardware/mISDN/hfcmulti.c b/drivers/isdn/hardware/mISDN/hfcmulti.c index 7013a3f08429..ce6c160e0df4 100644 --- a/drivers/isdn/hardware/mISDN/hfcmulti.c +++ b/drivers/isdn/hardware/mISDN/hfcmulti.c @@ -2152,16 +2152,14 @@ hfcmulti_tx(struct hfc_multi *hc, int ch) HFC_wait_nodebug(hc); } + len = (*sp)->len; dev_kfree_skb(*sp); /* check for next frame */ - if (bch && get_next_bframe(bch)) { - len = (*sp)->len; + if (bch && get_next_bframe(bch)) goto next_frame; - } - if (dch && get_next_dframe(dch)) { - len = (*sp)->len; + + if (dch && get_next_dframe(dch)) goto next_frame; - } /* * now we have no more data, so in case of transparent,
This frees "*sp" before dereferencing it to get "len = (*sp)->len;". Fixes: af69fb3a8ffa ("Add mISDN HFC multiport driver") Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com> --- drivers/isdn/hardware/mISDN/hfcmulti.c | 10 ++++------ 1 file changed, 4 insertions(+), 6 deletions(-)