linux-kernel - Re: [PATCH] caif: fix integer underflow in cffrml

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Message-ID:
 <SYBPR01MB78810A9BF01E403BFD95F871AFADA@SYBPR01MB7881.ausprd01.prod.outlook.com>
Date: Mon, 15 Dec 2025 11:33:37 +0000
From: Junrui Luo <moonafterrain@...look.com>
To: David Laight <david.laight.linux@...il.com>
CC: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Sjur Braendeland
	<sjur.brandeland@...ricsson.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Yuhao Jiang <danisjiang@...il.com>
Subject: Re: [PATCH] caif: fix integer underflow in cffrml_receive()

On Thu, Dec 11, 2025 at 01:26:16PM +0000, David Laight wrote:
> On Thu, 04 Dec 2025 21:30:47 +0800
> Junrui Luo <moonafterrain@...look.com> wrote:
> 
> > The cffrml_receive() function extracts a length field from the packet
> > header and, when FCS is disabled, subtracts 2 from this length without
> > validating that len >= 2.
> > 
> > If an attacker sends a malicious packet with a length field of 0 or 1
> > to an interface with FCS disabled, the subtraction causes an integer
> > underflow.
> > 
> > This can lead to memory exhaustion and kernel instability, potential
> > information disclosure if padding contains uninitialized kernel memory.
> > 
> > Fix this by validating that len >= 2 before performing the subtraction.
> > 
> > Reported-by: Yuhao Jiang <danisjiang@...il.com>
> > Reported-by: Junrui Luo <moonafterrain@...look.com>
> > Fixes: b482cd2053e3 ("net-caif: add CAIF core protocol stack")
> > Signed-off-by: Junrui Luo <moonafterrain@...look.com>
> > ---
> >  net/caif/cffrml.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/caif/cffrml.c b/net/caif/cffrml.c
> > index 6651a8dc62e0..d4d63586053a 100644
> > --- a/net/caif/cffrml.c
> > +++ b/net/caif/cffrml.c
> > @@ -92,8 +92,15 @@ static int cffrml_receive(struct cflayer *layr, struct cfpkt *pkt)
> >  	len = le16_to_cpu(tmp);
> >  
> >  	/* Subtract for FCS on length if FCS is not used. */
> > -	if (!this->dofcs)
> > +	if (!this->dofcs) {
> > +		if (len < 2) {
> > +			++cffrml_rcv_error;
> > +			pr_err("Invalid frame length (%d)\n", len);
> 
> Doesn't that let the same remote attacker flood the kernel message buffer?

Thanks for the review and suggestion.  Please let me know if you'd like me to
repost a patch removing pr_err(), or if there is a preferred alternative.

> 
> 	David
> 
> > +			cfpkt_destroy(pkt);
> > +			return -EPROTO;
> > +		}
> >  		len -= 2;
> > +	}
> >  
> >  	if (cfpkt_setlen(pkt, len) < 0) {
> >  		++cffrml_rcv_error;
> > 
> > ---
> > base-commit: 559e608c46553c107dbba19dae0854af7b219400
> > change-id: 20251204-fixes-23393d72bfc8
> > 
> > Best regards,
>

Thanks,
Junrui Luo