The TCP option parsing routines in tcp_parse_options function could
read one byte out of the buffer of the TCP options.
1 while (length > 0) {
2 int opcode = *ptr++;
3 int opsize;
4
5 switch (opcode) {
6 case TCPOPT_EOL:
7 return;
8 case TCPOPT_NOP: /* Ref: RFC 793 section 3.1 */
9 length--;
10 continue;
11 default:
12 opsize = *ptr++; //out of bound access
If length = 1, then there is an access in line2.
And another access is occurred in line 12.
This would lead to out-of-bound access.
Therefore, in the patch we check that the available data length is
larger enough to pase both TCP option code and size.
Signed-off-by: Young Xiao <[email protected]>
---
net/ipv4/tcp_input.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 20f6fac..9775825 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3791,6 +3791,8 @@ void tcp_parse_options(const struct net *net,
length--;
continue;
default:
+ if (length < 2)
+ return;
opsize = *ptr++;
if (opsize < 2) /* "silly options" */
return;
--
2.7.4
On Wed, May 29, 2019 at 1:10 AM Young Xiao <[email protected]> wrote:
>
> The TCP option parsing routines in tcp_parse_options function could
> read one byte out of the buffer of the TCP options.
>
> 1 while (length > 0) {
> 2 int opcode = *ptr++;
> 3 int opsize;
> 4
> 5 switch (opcode) {
> 6 case TCPOPT_EOL:
> 7 return;
> 8 case TCPOPT_NOP: /* Ref: RFC 793 section 3.1 */
> 9 length--;
> 10 continue;
> 11 default:
> 12 opsize = *ptr++; //out of bound access
>
> If length = 1, then there is an access in line2.
> And another access is occurred in line 12.
> This would lead to out-of-bound access.
>
> Therefore, in the patch we check that the available data length is
> larger enough to pase both TCP option code and size.
>
> Signed-off-by: Young Xiao <[email protected]>
> ---
> net/ipv4/tcp_input.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 20f6fac..9775825 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3791,6 +3791,8 @@ void tcp_parse_options(const struct net *net,
> length--;
> continue;
> default:
> + if (length < 2)
> + return;
> opsize = *ptr++;
> if (opsize < 2) /* "silly options" */
> return;
In practice we are good, since we have at least 320 bytes of room there,
and the test done later catches silly options.
if (opsize < 2) /* "silly options" */
return;
if (opsize > length) /* remember, opsize >= 2 here */
return; /* don't parse partial options */
I guess adding yet another conditional will make this code obviously
correct for all eyes
and various tools.
Thanks.
Signed-off-by: Eric Dumazet <[email protected]>
Indeed, condition opsize < 2 and opsize > length can deduce that length >= 2.
However, before the condition (if opsize < 2), there may be one-byte
out-of-bound access in line 12.
I'm not sure whether I have put it very clearly.
On Wed, May 29, 2019 at 10:20 PM Eric Dumazet <[email protected]> wrote:
>
> On Wed, May 29, 2019 at 1:10 AM Young Xiao <[email protected]> wrote:
> >
> > The TCP option parsing routines in tcp_parse_options function could
> > read one byte out of the buffer of the TCP options.
> >
> > 1 while (length > 0) {
> > 2 int opcode = *ptr++;
> > 3 int opsize;
> > 4
> > 5 switch (opcode) {
> > 6 case TCPOPT_EOL:
> > 7 return;
> > 8 case TCPOPT_NOP: /* Ref: RFC 793 section 3.1 */
> > 9 length--;
> > 10 continue;
> > 11 default:
> > 12 opsize = *ptr++; //out of bound access
> >
> > If length = 1, then there is an access in line2.
> > And another access is occurred in line 12.
> > This would lead to out-of-bound access.
> >
> > Therefore, in the patch we check that the available data length is
> > larger enough to pase both TCP option code and size.
> >
> > Signed-off-by: Young Xiao <[email protected]>
> > ---
> > net/ipv4/tcp_input.c | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index 20f6fac..9775825 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -3791,6 +3791,8 @@ void tcp_parse_options(const struct net *net,
> > length--;
> > continue;
> > default:
> > + if (length < 2)
> > + return;
> > opsize = *ptr++;
> > if (opsize < 2) /* "silly options" */
> > return;
>
> In practice we are good, since we have at least 320 bytes of room there,
> and the test done later catches silly options.
>
> if (opsize < 2) /* "silly options" */
> return;
> if (opsize > length) /* remember, opsize >= 2 here */
> return; /* don't parse partial options */
>
> I guess adding yet another conditional will make this code obviously
> correct for all eyes
> and various tools.
>
> Thanks.
>
> Signed-off-by: Eric Dumazet <[email protected]>
--
Best regards!
Young
-----------------------------------------------------------
On Wed, May 29, 2019 at 8:11 AM Yang Xiao <[email protected]> wrote:
>
> Indeed, condition opsize < 2 and opsize > length can deduce that length >= 2.
> However, before the condition (if opsize < 2), there may be one-byte
> out-of-bound access in line 12.
> I'm not sure whether I have put it very clearly.
Maybe I should have been clear about the 320 bytes we have at the end
of skb->head
This is the struct skb_shared_info
So reading one byte, 'out-of-bound' here is harmless.
Whatever value is read, we will return early without ever looking at a
following byte.
>
> On Wed, May 29, 2019 at 10:20 PM Eric Dumazet <[email protected]> wrote:
> >
> > On Wed, May 29, 2019 at 1:10 AM Young Xiao <[email protected]> wrote:
> > >
> > > The TCP option parsing routines in tcp_parse_options function could
> > > read one byte out of the buffer of the TCP options.
> > >
> > > 1 while (length > 0) {
> > > 2 int opcode = *ptr++;
> > > 3 int opsize;
> > > 4
> > > 5 switch (opcode) {
> > > 6 case TCPOPT_EOL:
> > > 7 return;
> > > 8 case TCPOPT_NOP: /* Ref: RFC 793 section 3.1 */
> > > 9 length--;
> > > 10 continue;
> > > 11 default:
> > > 12 opsize = *ptr++; //out of bound access
> > >
> > > If length = 1, then there is an access in line2.
> > > And another access is occurred in line 12.
> > > This would lead to out-of-bound access.
> > >
> > > Therefore, in the patch we check that the available data length is
> > > larger enough to pase both TCP option code and size.
> > >
> > > Signed-off-by: Young Xiao <[email protected]>
> > > ---
> > > net/ipv4/tcp_input.c | 2 ++
> > > 1 file changed, 2 insertions(+)
> > >
> > > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > > index 20f6fac..9775825 100644
> > > --- a/net/ipv4/tcp_input.c
> > > +++ b/net/ipv4/tcp_input.c
> > > @@ -3791,6 +3791,8 @@ void tcp_parse_options(const struct net *net,
> > > length--;
> > > continue;
> > > default:
> > > + if (length < 2)
> > > + return;
> > > opsize = *ptr++;
> > > if (opsize < 2) /* "silly options" */
> > > return;
> >
> > In practice we are good, since we have at least 320 bytes of room there,
> > and the test done later catches silly options.
> >
> > if (opsize < 2) /* "silly options" */
> > return;
> > if (opsize > length) /* remember, opsize >= 2 here */
> > return; /* don't parse partial options */
> >
> > I guess adding yet another conditional will make this code obviously
> > correct for all eyes
> > and various tools.
> >
> > Thanks.
> >
> > Signed-off-by: Eric Dumazet <[email protected]>
>
>
>
> --
> Best regards!
>
> Young
> -----------------------------------------------------------
From: Young Xiao <[email protected]>
Date: Wed, 29 May 2019 16:10:59 +0800
> The TCP option parsing routines in tcp_parse_options function could
> read one byte out of the buffer of the TCP options.
>
> 1 while (length > 0) {
> 2 int opcode = *ptr++;
> 3 int opsize;
> 4
> 5 switch (opcode) {
> 6 case TCPOPT_EOL:
> 7 return;
> 8 case TCPOPT_NOP: /* Ref: RFC 793 section 3.1 */
> 9 length--;
> 10 continue;
> 11 default:
> 12 opsize = *ptr++; //out of bound access
>
> If length = 1, then there is an access in line2.
> And another access is occurred in line 12.
> This would lead to out-of-bound access.
>
> Therefore, in the patch we check that the available data length is
> larger enough to pase both TCP option code and size.
>
> Signed-off-by: Young Xiao <[email protected]>
Applied.