It is possible to indirectly invoke functions with prototypes that do not
match those of the respectively used function pointers by using void types.
Despite widely used as a feature for relaxing function invocation, this
should be avoided when possible as it may prevent the use of heuristics
such as prototype matching-based Control-Flow Integrity, which can be used
to prevent ROP-based attacks.
Given the above, the current efforts to improve the Linux security, and the
upcoming kernel support to compilers with CFI features, fix prototypes in
vgacon console driver.
Another similar fix can be seen in [1].
[1] https://android-review.googlesource.com/c/kernel/common/+/602010
Signed-off-by: João Moreira <[email protected]>
---
drivers/video/console/vgacon.c | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)
diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
index a17ba1465815..f00b630f6839 100644
--- a/drivers/video/console/vgacon.c
+++ b/drivers/video/console/vgacon.c
@@ -1407,21 +1407,29 @@ static bool vgacon_scroll(struct vc_data *c, unsigned int t, unsigned int b,
* The console `switch' structure for the VGA based console
*/
-static int vgacon_dummy(struct vc_data *c)
+static int vgacon_clear(struct vc_data *c)
{
return 0;
}
-#define DUMMY (void *) vgacon_dummy
+static void vgacon_putc(struct vc_data *c, int a, int b, int d)
+{
+ return;
+}
+
+static void vgacon_putcs(struct vc_data *c, ushort *s, int a, int b, int d)
+{
+ return;
+}
const struct consw vga_con = {
.owner = THIS_MODULE,
.con_startup = vgacon_startup,
.con_init = vgacon_init,
.con_deinit = vgacon_deinit,
- .con_clear = DUMMY,
- .con_putc = DUMMY,
- .con_putcs = DUMMY,
+ .con_clear = vgacon_clear,
+ .con_putc = vgacon_putc,
+ .con_putcs = vgacon_putcs,
.con_cursor = vgacon_cursor,
.con_scroll = vgacon_scroll,
.con_switch = vgacon_switch,
--
2.13.6
On Fri, Mar 9, 2018 at 10:40 PM, Joao Moreira <[email protected]> wrote:
> It is possible to indirectly invoke functions with prototypes that do not
> match those of the respectively used function pointers by using void types.
> Despite widely used as a feature for relaxing function invocation, this
> should be avoided when possible as it may prevent the use of heuristics
> such as prototype matching-based Control-Flow Integrity, which can be used
> to prevent ROP-based attacks.
>
> Given the above, the current efforts to improve the Linux security, and the
> upcoming kernel support to compilers with CFI features, fix prototypes in
> vgacon console driver.
>
> Another similar fix can be seen in [1].
>
> [1] https://android-review.googlesource.com/c/kernel/common/+/602010
>
> Signed-off-by: João Moreira <[email protected]>
Whoops, I missed this one. :) Thanks! Greg, do you need this resent
directly to you?
Acked-by: Kees Cook <[email protected]>
-Kees
> ---
> drivers/video/console/vgacon.c | 18 +++++++++++++-----
> 1 file changed, 13 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
> index a17ba1465815..f00b630f6839 100644
> --- a/drivers/video/console/vgacon.c
> +++ b/drivers/video/console/vgacon.c
> @@ -1407,21 +1407,29 @@ static bool vgacon_scroll(struct vc_data *c, unsigned int t, unsigned int b,
> * The console `switch' structure for the VGA based console
> */
>
> -static int vgacon_dummy(struct vc_data *c)
> +static int vgacon_clear(struct vc_data *c)
> {
> return 0;
> }
>
> -#define DUMMY (void *) vgacon_dummy
> +static void vgacon_putc(struct vc_data *c, int a, int b, int d)
> +{
> + return;
> +}
> +
> +static void vgacon_putcs(struct vc_data *c, ushort *s, int a, int b, int d)
> +{
> + return;
> +}
>
> const struct consw vga_con = {
> .owner = THIS_MODULE,
> .con_startup = vgacon_startup,
> .con_init = vgacon_init,
> .con_deinit = vgacon_deinit,
> - .con_clear = DUMMY,
> - .con_putc = DUMMY,
> - .con_putcs = DUMMY,
> + .con_clear = vgacon_clear,
> + .con_putc = vgacon_putc,
> + .con_putcs = vgacon_putcs,
> .con_cursor = vgacon_cursor,
> .con_scroll = vgacon_scroll,
> .con_switch = vgacon_switch,
> --
> 2.13.6
>
--
Kees Cook
Pixel Security
Hi,
On Saturday, March 10, 2018 07:27:21 AM Kees Cook wrote:
> On Fri, Mar 9, 2018 at 10:40 PM, Joao Moreira <[email protected]> wrote:
> > It is possible to indirectly invoke functions with prototypes that do not
> > match those of the respectively used function pointers by using void types.
> > Despite widely used as a feature for relaxing function invocation, this
> > should be avoided when possible as it may prevent the use of heuristics
> > such as prototype matching-based Control-Flow Integrity, which can be used
> > to prevent ROP-based attacks.
> >
> > Given the above, the current efforts to improve the Linux security, and the
> > upcoming kernel support to compilers with CFI features, fix prototypes in
> > vgacon console driver.
> >
> > Another similar fix can be seen in [1].
> >
> > [1] https://android-review.googlesource.com/c/kernel/common/+/602010
> >
> > Signed-off-by: Jo?o Moreira <[email protected]>
>
> Whoops, I missed this one. :) Thanks! Greg, do you need this resent
> directly to you?
I would prefer for drivers/video/console/ changes to go through
fbdev tree (like suggested by scripts/get_maintainers.pl)..
However since Greg has already merged your CFI patches:
Acked-by: Bartlomiej Zolnierkiewicz <[email protected]>
> Acked-by: Kees Cook <[email protected]>
>
> -Kees
>
> > ---
> > drivers/video/console/vgacon.c | 18 +++++++++++++-----
> > 1 file changed, 13 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/video/console/vgacon.c b/drivers/video/console/vgacon.c
> > index a17ba1465815..f00b630f6839 100644
> > --- a/drivers/video/console/vgacon.c
> > +++ b/drivers/video/console/vgacon.c
> > @@ -1407,21 +1407,29 @@ static bool vgacon_scroll(struct vc_data *c, unsigned int t, unsigned int b,
> > * The console `switch' structure for the VGA based console
> > */
> >
> > -static int vgacon_dummy(struct vc_data *c)
> > +static int vgacon_clear(struct vc_data *c)
> > {
> > return 0;
> > }
> >
> > -#define DUMMY (void *) vgacon_dummy
> > +static void vgacon_putc(struct vc_data *c, int a, int b, int d)
> > +{
> > + return;
> > +}
> > +
> > +static void vgacon_putcs(struct vc_data *c, ushort *s, int a, int b, int d)
> > +{
> > + return;
> > +}
> >
> > const struct consw vga_con = {
> > .owner = THIS_MODULE,
> > .con_startup = vgacon_startup,
> > .con_init = vgacon_init,
> > .con_deinit = vgacon_deinit,
> > - .con_clear = DUMMY,
> > - .con_putc = DUMMY,
> > - .con_putcs = DUMMY,
> > + .con_clear = vgacon_clear,
> > + .con_putc = vgacon_putc,
> > + .con_putcs = vgacon_putcs,
> > .con_cursor = vgacon_cursor,
> > .con_scroll = vgacon_scroll,
> > .con_switch = vgacon_switch,
> > --
> > 2.13.6
Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics
On Sat, Mar 10, 2018 at 03:40:59AM -0300, Joao Moreira wrote:
> It is possible to indirectly invoke functions with prototypes that do not
> match those of the respectively used function pointers by using void types.
> Despite widely used as a feature for relaxing function invocation, this
> should be avoided when possible as it may prevent the use of heuristics
> such as prototype matching-based Control-Flow Integrity, which can be used
> to prevent ROP-based attacks.
>
> Given the above, the current efforts to improve the Linux security, and the
> upcoming kernel support to compilers with CFI features, fix prototypes in
> vgacon console driver.
>
> Another similar fix can be seen in [1].
>
> [1] https://android-review.googlesource.com/c/kernel/common/+/602010
>
> Signed-off-by: Jo?o Moreira <[email protected]>
> Acked-by: Kees Cook <[email protected]>
> Acked-by: Bartlomiej Zolnierkiewicz <[email protected]>
> ---
> drivers/video/console/vgacon.c | 18 +++++++++++++-----
> 1 file changed, 13 insertions(+), 5 deletions(-)
This fails the build :(
drivers/video/console/vgacon.c:1432:15: error: initialization from incompatible pointer type [-Werror=incompatible-pointer-types]
.con_putcs = vgacon_putcs,
^~~~~~~~~~~~
What tree should this go to? It doesn't apply at all to my tty tree,
which is where I thought it should go, so I tried a different one and
got this mess :(
confused,
greg k-h