Re: [PATCH 3.4 022/214] filter: prevent nla extensions to peek beyond the end of the message

[Luis Henriques]

On Thu, Jun 05, 2014 at 02:50:24PM +0100, Ben Hutchings wrote:
> On Thu, 2014-06-05 at 14:33 +0100, Luis Henriques wrote:
> > On Wed, Jun 04, 2014 at 09:16:25PM -0700, Greg Kroah-Hartman wrote:
> > > 3.4-stable review patch.  If anyone has any objections, please let me know.
> > > 
> > > ------------------
> > > 
> > > From: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> > > 
> > > [ Upstream commit 05ab8f2647e4221cbdb3856dd7d32bd5407316b3 ]
> > > 
> > > The BPF_S_ANC_NLATTR and BPF_S_ANC_NLATTR_NEST extensions fail to check
> > > for a minimal message length before testing the supplied offset to be
> > > within the bounds of the message. This allows the subtraction of the nla
> > > header to underflow and therefore -- as the data type is unsigned --
> > > allowing far to big offset and length values for the search of the
> > > netlink attribute.
> > > 
> > > The remainder calculation for the BPF_S_ANC_NLATTR_NEST extension is
> > > also wrong. It has the minuend and subtrahend mixed up, therefore
> > > calculates a huge length value, allowing to overrun the end of the
> > > message while looking for the netlink attribute.
> > > 
> > > The following three BPF snippets will trigger the bugs when attached to
> > > a UNIX datagram socket and parsing a message with length 1, 2 or 3.
> > > 
> > >  ,-[ PoC for missing size check in BPF_S_ANC_NLATTR ]--
> > >  | ld	#0x87654321
> > >  | ldx	#42
> > >  | ld	#nla
> > >  | ret	a
> > >  `---
> > > 
> > >  ,-[ PoC for the same bug in BPF_S_ANC_NLATTR_NEST ]--
> > >  | ld	#0x87654321
> > >  | ldx	#42
> > >  | ld	#nlan
> > >  | ret	a
> > >  `---
> > > 
> > >  ,-[ PoC for wrong remainder calculation in BPF_S_ANC_NLATTR_NEST ]--
> > >  | ; (needs a fake netlink header at offset 0)
> > >  | ld	#0
> > >  | ldx	#42
> > >  | ld	#nlan
> > >  | ret	a
> > >  `---
> > > 
> > > Fix the first issue by ensuring the message length fulfills the minimal
> > > size constrains of a nla header. Fix the second bug by getting the math
> > > for the remainder calculation right.
> > > 
> > > Fixes: 4738c1db15 ("[SKFILTER]: Add SKF_ADF_NLATTR instruction")
> > > Fixes: d214c7537b ("filter: add SKF_AD_NLATTR_NEST to look for nested..")
> > > Cc: Patrick McHardy <kaber@xxxxxxxxx>
> > > Cc: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
> > > Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> > > Acked-by: Daniel Borkmann <dborkman@xxxxxxxxxx>
> > > Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > ---
> > >  net/core/filter.c |    6 +++++-
> > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > > 
> > > --- a/net/core/filter.c
> > > +++ b/net/core/filter.c
> > > @@ -338,11 +338,15 @@ load_b:
> > >  
> > >  			if (skb_is_nonlinear(skb))
> > >  				return 0;
> > > +			if (skb->len < sizeof(struct nlattr))
> > > +				return 0;
> > > +			if (skb->len < sizeof(struct nlattr))
> > > +				return 0;
> > 
> > There above code is duplicated this backport.  The same comment
> > applies to the 3.2.y backport (I've added Ben to the CC list).
> 
> This seems to be due to a bug in patch <https://bugs.debian.org/717782>.
> I had the same problem when initially trying to fix this in 3.14.  The
> first two inserted lines should be 16 lines further up.
> 
> Ben.
> 
> -- 
> Ben Hutchings
> I say we take off; nuke the site from orbit.  It's the only way to be sure.

Awesome!  Thank you both for clarifying that.

Cheers,
--
Luís
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