Re: [PATCH] kernel: sys: fix potential Spectre v1

[Gustavo A. R. Silva]

On 05/15/2018 05:57 PM, Dan Williams wrote:
> On Tue, May 15, 2018 at 3:29 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > On Tue, 15 May 2018, Andrew Morton wrote:
> > > On Mon, 14 May 2018 22:00:38 -0500 "Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx> wrote:
> > >
> > > > resource can be controlled by user-space, hence leading to a
> > > > potential exploitation of the Spectre variant 1 vulnerability.
> > > >
> > > > This issue was detected with the help of Smatch:
> > > >
> > > > kernel/sys.c:1474 __do_compat_sys_old_getrlimit() warn: potential
> > > > spectre issue 'get_current()->signal->rlim' (local cap)
> > > > kernel/sys.c:1455 __do_sys_old_getrlimit() warn: potential spectre issue
> > > > 'get_current()->signal->rlim' (local cap)
> > > >
> > > > Fix this by sanitizing *resource* before using it to index
> > > > current->signal->rlim
> > > >
> > > > Notice that given that speculation windows are large, the policy is
> > > > to kill the speculation on the first load and not worry if it can be
> > > > completed with a dependent load/store [1].
> > >
> > > hm.  Not my area, but I'm always willing to learn ;)
> > >
> > > > --- a/kernel/sys.c
> > > > +++ b/kernel/sys.c
> > > > @@ -69,6 +69,9 @@
> > > >   #include <asm/io.h>
> > > >   #include <asm/unistd.h>
> > > >
> > > > +/* Hardening for Spectre-v1 */
> > > > +#include <linux/nospec.h>
> > > > +
> > > >   #include "uid16.h"
> > > >
> > > >   #ifndef SET_UNALIGN_CTL
> > > > @@ -1451,6 +1454,7 @@ SYSCALL_DEFINE2(old_getrlimit, unsigned int, resource,
> > > >      if (resource >= RLIM_NLIMITS)
> > > >              return -EINVAL;
> > > >
> > > > +   resource = array_index_nospec(resource, RLIM_NLIMITS);
> > > >      task_lock(current->group_leader);
> > > >      x = current->signal->rlim[resource];
> > >
> > > Can the speculation proceed past the task_lock()?  Or is the policy to
> > > ignore such happy happenstances even if they are available?
> >
> > Locks are not in the way of speculation. Speculation has almost no limits
> > except serializing instructions. At least they respect the magic AND
> > limitation in array_index_nospec().
>
> I'd say it another way, because they don't respect the magic AND, we
> just make the result in the speculation path safe. So, it's controlled
> speculation.

Dan,

What do you think about adding the following function to the nospec API:

diff --git a/include/linux/nospec.h b/include/linux/nospec.h
index e791ebc..81e9a77 100644
--- a/include/linux/nospec.h
+++ b/include/linux/nospec.h
@@ -55,4 +55,17 @@ static inline unsigned long 
array_index_mask_nospec(unsigned long index,
                                                                        \
        (typeof(_i)) (_i & _mask);                                      \
 })
+
+
+#ifndef sanitize_index_nospec
+inline bool sanitize_index_nospec(unsigned long index,
+                                 unsigned long size)
+{
+       if (index >= size)
+               return false;
+       index = array_index_nospec(index, size);
+
+       return true;
+}
+#endif
 #endif /* _LINUX_NOSPEC_H */


And here is an example of its use:

diff --git a/drivers/media/i2c/tvp7002.c b/drivers/media/i2c/tvp7002.c
index 4599b7e..27b39c0 100644
--- a/drivers/media/i2c/tvp7002.c
+++ b/drivers/media/i2c/tvp7002.c
@@ -35,6 +35,8 @@
 #include <media/v4l2-ctrls.h>
 #include <media/v4l2-fwnode.h>

+#include <linux/nospec.h>
+
 #include "tvp7002_reg.h"

 MODULE_DESCRIPTION("TI TVP7002 Video and Graphics Digitizer driver");
@@ -784,7 +786,7 @@ static int tvp7002_enum_dv_timings(struct 
v4l2_subdev *sd,
                return -EINVAL;

        /* Check requested format index is within range */
-       if (timings->index >= NUM_TIMINGS)
+       if (!sanitize_index_nospec(timings->index, NUM_TIMINGS))
                return -EINVAL;

        timings->timings = tvp7002_timings[timings->index].timings;

This patter is very common. So, it may be a good idea to unify both 
bounds checking and the serialization of instructions into a single 
function.

What do you think?

Thanks
--
Gustavo