diff mbox series

[net] mISDN: hfcpci: Fix a use after free in hfcmulti_tx()

Message ID 20201022070739.GB2817762@mwanda
State Rejected
Delegated to: David Miller
Headers show
Series [net] mISDN: hfcpci: Fix a use after free in hfcmulti_tx() | expand

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Context Check Description
jkicinski/stable success Stable not CCed
jkicinski/header_inline success Link
jkicinski/build_allmodconfig_warn success Errors and warnings before: 10 this patch: 10
jkicinski/checkpatch success total: 0 errors, 0 warnings, 0 checks, 20 lines checked
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jkicinski/kdoc success Errors and warnings before: 0 this patch: 0
jkicinski/build_32bit success Errors and warnings before: 10 this patch: 10
jkicinski/module_param success Was 0 now: 0
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jkicinski/source_inline success Was 0 now: 0
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jkicinski/tree_selection success Clearly marked for net
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Commit Message

Dan Carpenter Oct. 22, 2020, 7:07 a.m. UTC
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(-)

Comments

Karsten Keil Oct. 22, 2020, 2:24 p.m. UTC | #1
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,
>
Dan Carpenter Oct. 22, 2020, 2:44 p.m. UTC | #2
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 mbox series

Patch

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,