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SubjectRe: [PATCH] kernel: sys: fix potential Spectre v1
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On 05/15/2018 05:57 PM, Dan Williams wrote:
> On Tue, May 15, 2018 at 3:29 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>> On Tue, 15 May 2018, Andrew Morton wrote:
>>> On Mon, 14 May 2018 22:00:38 -0500 "Gustavo A. R. Silva" <gustavo@embeddedor.com> 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

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