[RFCv2,2/4] Documentation: document nospec helpers

Message ID 20180105145750.53294-3-mark.rutland@arm.com
State New
Headers show
Series
  • API for inhibiting speculative arbitrary read primitives
Related show

Commit Message

Mark Rutland Jan. 5, 2018, 2:57 p.m.
Document the rationale and usage of the new nospec*() helpers.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>

Signed-off-by: Will Deacon <will.deacon@arm.com>

Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Peter Zijlstra <peterz@infradead.org>
---
 Documentation/speculation.txt | 166 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 166 insertions(+)
 create mode 100644 Documentation/speculation.txt

-- 
2.11.0

Comments

Randy Dunlap Jan. 7, 2018, 5:20 a.m. | #1
On 01/05/18 06:57, Mark Rutland wrote:
> Document the rationale and usage of the new nospec*() helpers.

> 

> Signed-off-by: Mark Rutland <mark.rutland@arm.com>

> Signed-off-by: Will Deacon <will.deacon@arm.com>

> Cc: Dan Williams <dan.j.williams@intel.com>

> Cc: Jonathan Corbet <corbet@lwn.net>

> Cc: Peter Zijlstra <peterz@infradead.org>

> ---

>  Documentation/speculation.txt | 166 ++++++++++++++++++++++++++++++++++++++++++

>  1 file changed, 166 insertions(+)

>  create mode 100644 Documentation/speculation.txt

> 

> diff --git a/Documentation/speculation.txt b/Documentation/speculation.txt

> new file mode 100644

> index 000000000000..748fcd4dcda4

> --- /dev/null

> +++ b/Documentation/speculation.txt

> @@ -0,0 +1,166 @@

> +

> +Typically speculative execution cannot be observed from architectural state,

> +such as the contents of registers. However, in some cases it is possible to

> +observe its impact on microarchitectural state, such as the presence or

> +absence of data in caches. Such state may form side-channels which can be

> +observed to extract secret information.


I'm curious about what it takes to observe this...

or is that covered in the exploit papers?

thanks,
-- 
~Randy
Geert Uytterhoeven Jan. 7, 2018, 10:27 a.m. | #2
Hi Mark,

On Fri, Jan 5, 2018 at 3:57 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> Document the rationale and usage of the new nospec*() helpers.

>

> Signed-off-by: Mark Rutland <mark.rutland@arm.com>

> Signed-off-by: Will Deacon <will.deacon@arm.com>

> Cc: Dan Williams <dan.j.williams@intel.com>

> Cc: Jonathan Corbet <corbet@lwn.net>

> Cc: Peter Zijlstra <peterz@infradead.org>


I love your patch! Yet something to improve:
(borrowed from another Intel division)

> --- /dev/null

> +++ b/Documentation/speculation.txt

> @@ -0,0 +1,166 @@

> +This document explains potential effects of speculation, and how undesirable

> +effects can be mitigated portably using common APIs.

> +

> +===========

> +Speculation

> +===========

> +

> +To improve performance and minimize average latencies, many contemporary CPUs

> +employ speculative execution techniques such as branch prediction, performing

> +work which may be discarded at a later stage.

> +

> +Typically speculative execution cannot be observed from architectural state,

> +such as the contents of registers. However, in some cases it is possible to

> +observe its impact on microarchitectural state, such as the presence or

> +absence of data in caches. Such state may form side-channels which can be

> +observed to extract secret information.

> +

> +For example, in the presence of branch prediction, it is possible for bounds

> +checks to be ignored by code which is speculatively executed. Consider the

> +following code:

> +

> +       int load_array(int *array, unsigned int idx) {


"{" on next line?

> +               if (idx >= MAX_ARRAY_ELEMS)

> +                       return 0;

> +               else

> +                       return array[idx];

> +       }

> +

> +Which, on arm64, may be compiled to an assembly sequence such as:

> +

> +       CMP     <idx>, #MAX_ARRAY_ELEMS

> +       B.LT    less

> +       MOV     <returnval>, #0

> +       RET

> +  less:

> +       LDR     <returnval>, [<array>, <idx>]

> +       RET

> +

> +It is possible that a CPU mis-predicts the conditional branch, and

> +speculatively loads array[idx], even if idx >= MAX_ARRAY_ELEMS. This value

> +will subsequently be discarded, but the speculated load may affect

> +microarchitectural state which can be subsequently measured.

> +

> +More complex sequences involving multiple dependent memory accesses may result

> +in sensitive information being leaked. Consider the following code, building on

> +the prior example:

> +

> +       int load_dependent_arrays(int *arr1, int *arr2, int idx) {


"{" on next line

> +               int val1, val2,

> +

> +               val1 = load_array(arr1, idx);

> +               val2 = load_array(arr2, val1);

> +

> +               return val2;

> +       }

> +

> +Under speculation, the first call to load_array() may return the value of an

> +out-of-bounds address, while the second call will influence microarchitectural

> +state dependent on this value. This may provide an arbitrary read primitive.

> +

> +====================================

> +Mitigating speculation side-channels

> +====================================

> +

> +The kernel provides a generic API to ensure that bounds checks are respected

> +even under speculation. Architectures which are affected by speculation-based

> +side-channels are expected to implement these primitives.

> +

> +The following helpers found in <asm/barrier.h> can be used to prevent

> +information from being leaked via side-channels.

> +

> +* nospec_ptr(ptr, lo, hi)

> +

> +  Returns a sanitized pointer that is bounded by the [lo, hi) interval. When

> +  ptr < lo, or ptr >= hi, NULL is returned. Prevents an out-of-bounds pointer

> +  being propagated to code which is speculatively executed.

> +

> +  This is expected to be used by code which computes pointers to data

> +  structures, where part of the address (such as an array index) may be

> +  user-controlled.

> +

> +  This can be used to protect the earlier load_array() example:

> +

> +  int load_array(int *array, unsigned int idx)

> +  {

> +       int *elem;

> +

> +       if ((elem = nospec_ptr(array + idx, array, array + MAX_ARRAY_ELEMS)))


elem = nospec_ptr(array + idx, array, array + MAX_ARRAY_ELEMS);
if (elem)

> +               return *elem;

> +       else

> +               return 0;

> +  }

> +

> +  This can also be used in situations where multiple fields on a structure are

> +  accessed:

> +

> +       struct foo array[SIZE];

> +       int a, b;

> +

> +       void do_thing(int idx)

> +       {

> +               struct foo *elem;

> +

> +               if ((elem = nospec_ptr(array + idx, array, array + SIZE)) {


elem = nospec_ptr(array + idx, array, array + SIZE;
if (elem) {

> +                       a = elem->field_a;

> +                       b = elem->field_b;

> +               }

> +       }

> +

> +  It is imperative that the returned pointer is used. Pointers which are

> +  generated separately are subject to a number of potential CPU and compiler

> +  optimizations, and may still be used speculatively. For example, this means

> +  that the following sequence is unsafe:

> +

> +       struct foo array[SIZE];

> +       int a, b;

> +

> +       void do_thing(int idx)

> +       {

> +               if (nospec_ptr(array + idx, array, array + SIZE) != NULL) {

> +                       // unsafe as wrong pointer is used

> +                       a = array[idx].field_a;

> +                       b = array[idx].field_b;

> +               }

> +       }

> +

> +  Similarly, it is unsafe to compare the returned pointer with other pointers,

> +  as this may permit the compiler to substitute one pointer with another,

> +  permitting speculation. For example, the following sequence is unsafe:

> +

> +       struct foo array[SIZE];

> +       int a, b;

> +

> +       void do_thing(int idx)

> +       {

> +               struct foo *elem = nospec_ptr(array + idx, array, array + size);

> +

> +               // unsafe due to pointer substitution

> +               if (elem == &array[idx]) {

> +                       a = elem->field_a;

> +                       b = elem->field_b;

> +               }

> +       }

> +

> +* nospec_array_ptr(arr, idx, sz)

> +

> +  Returns a sanitized pointer to arr[idx] only if idx falls in the [0, sz)

> +  interval. When idx < 0 or idx > sz, NULL is returned. Prevents an

> +  out-of-bounds pointer being propagated to code which is speculatively

> +  executed.

> +

> +  This is a convenience function which wraps nospec_ptr(), and has the same

> +  caveats w.r.t. the use of the returned pointer.

> +

> +  For example, this may be used as follows:

> +

> +  int load_array(int *array, unsigned int idx)

> +  {

> +       int *elem;

> +

> +       if ((elem = nospec_array_ptr(array, idx, MAX_ARRAY_ELEMS)))


elem = nospec_array_ptr(array, idx, MAX_ARRAY_ELEMS);
if (elem)

> +               return *elem;

> +       else

> +               return 0;

> +  }

> +


Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
Mark Rutland Jan. 7, 2018, 1:06 p.m. | #3
On Sat, Jan 06, 2018 at 09:20:59PM -0800, Randy Dunlap wrote:
> On 01/05/18 06:57, Mark Rutland wrote:

> > Document the rationale and usage of the new nospec*() helpers.

> > 

> > Signed-off-by: Mark Rutland <mark.rutland@arm.com>

> > Signed-off-by: Will Deacon <will.deacon@arm.com>

> > Cc: Dan Williams <dan.j.williams@intel.com>

> > Cc: Jonathan Corbet <corbet@lwn.net>

> > Cc: Peter Zijlstra <peterz@infradead.org>

> > ---

> >  Documentation/speculation.txt | 166 ++++++++++++++++++++++++++++++++++++++++++

> >  1 file changed, 166 insertions(+)

> >  create mode 100644 Documentation/speculation.txt

> > 

> > diff --git a/Documentation/speculation.txt b/Documentation/speculation.txt

> > new file mode 100644

> > index 000000000000..748fcd4dcda4

> > --- /dev/null

> > +++ b/Documentation/speculation.txt

> > @@ -0,0 +1,166 @@

> > +

> > +Typically speculative execution cannot be observed from architectural state,

> > +such as the contents of registers. However, in some cases it is possible to

> > +observe its impact on microarchitectural state, such as the presence or

> > +absence of data in caches. Such state may form side-channels which can be

> > +observed to extract secret information.

> 

> I'm curious about what it takes to observe this...

> 

> or is that covered in the exploit papers?


That's covered elsewhere, e.g.

https://googleprojectzero.blogspot.co.uk/2018/01/reading-privileged-memory-with-side.html

I'll add some references.

Thanks,
Mark.

Patch

diff --git a/Documentation/speculation.txt b/Documentation/speculation.txt
new file mode 100644
index 000000000000..748fcd4dcda4
--- /dev/null
+++ b/Documentation/speculation.txt
@@ -0,0 +1,166 @@ 
+This document explains potential effects of speculation, and how undesirable
+effects can be mitigated portably using common APIs.
+
+===========
+Speculation
+===========
+
+To improve performance and minimize average latencies, many contemporary CPUs
+employ speculative execution techniques such as branch prediction, performing
+work which may be discarded at a later stage.
+
+Typically speculative execution cannot be observed from architectural state,
+such as the contents of registers. However, in some cases it is possible to
+observe its impact on microarchitectural state, such as the presence or
+absence of data in caches. Such state may form side-channels which can be
+observed to extract secret information.
+
+For example, in the presence of branch prediction, it is possible for bounds
+checks to be ignored by code which is speculatively executed. Consider the
+following code:
+
+	int load_array(int *array, unsigned int idx) {
+		if (idx >= MAX_ARRAY_ELEMS)
+			return 0;
+		else
+			return array[idx];
+	}
+
+Which, on arm64, may be compiled to an assembly sequence such as:
+
+	CMP	<idx>, #MAX_ARRAY_ELEMS
+	B.LT	less
+	MOV	<returnval>, #0
+	RET
+  less:
+	LDR	<returnval>, [<array>, <idx>]
+	RET
+
+It is possible that a CPU mis-predicts the conditional branch, and
+speculatively loads array[idx], even if idx >= MAX_ARRAY_ELEMS. This value
+will subsequently be discarded, but the speculated load may affect
+microarchitectural state which can be subsequently measured.
+
+More complex sequences involving multiple dependent memory accesses may result
+in sensitive information being leaked. Consider the following code, building on
+the prior example:
+
+	int load_dependent_arrays(int *arr1, int *arr2, int idx) {
+		int val1, val2,
+
+		val1 = load_array(arr1, idx);
+		val2 = load_array(arr2, val1);
+
+		return val2;
+	}
+
+Under speculation, the first call to load_array() may return the value of an
+out-of-bounds address, while the second call will influence microarchitectural
+state dependent on this value. This may provide an arbitrary read primitive.
+
+====================================
+Mitigating speculation side-channels
+====================================
+
+The kernel provides a generic API to ensure that bounds checks are respected
+even under speculation. Architectures which are affected by speculation-based
+side-channels are expected to implement these primitives.
+
+The following helpers found in <asm/barrier.h> can be used to prevent
+information from being leaked via side-channels.
+
+* nospec_ptr(ptr, lo, hi)
+
+  Returns a sanitized pointer that is bounded by the [lo, hi) interval. When
+  ptr < lo, or ptr >= hi, NULL is returned. Prevents an out-of-bounds pointer
+  being propagated to code which is speculatively executed.
+
+  This is expected to be used by code which computes pointers to data
+  structures, where part of the address (such as an array index) may be
+  user-controlled.
+
+  This can be used to protect the earlier load_array() example:
+
+  int load_array(int *array, unsigned int idx)
+  {
+	int *elem;
+
+	if ((elem = nospec_ptr(array + idx, array, array + MAX_ARRAY_ELEMS)))
+		return *elem;
+	else
+		return 0;
+  }
+
+  This can also be used in situations where multiple fields on a structure are
+  accessed:
+
+	struct foo array[SIZE];
+	int a, b;
+
+	void do_thing(int idx)
+	{
+		struct foo *elem;
+
+		if ((elem = nospec_ptr(array + idx, array, array + SIZE)) {
+			a = elem->field_a;
+			b = elem->field_b;
+		}
+	}
+
+  It is imperative that the returned pointer is used. Pointers which are
+  generated separately are subject to a number of potential CPU and compiler
+  optimizations, and may still be used speculatively. For example, this means
+  that the following sequence is unsafe:
+
+	struct foo array[SIZE];
+	int a, b;
+
+	void do_thing(int idx)
+	{
+		if (nospec_ptr(array + idx, array, array + SIZE) != NULL) {
+			// unsafe as wrong pointer is used
+			a = array[idx].field_a;
+			b = array[idx].field_b;
+		}
+	}
+
+  Similarly, it is unsafe to compare the returned pointer with other pointers,
+  as this may permit the compiler to substitute one pointer with another,
+  permitting speculation. For example, the following sequence is unsafe:
+
+	struct foo array[SIZE];
+	int a, b;
+
+	void do_thing(int idx)
+	{
+		struct foo *elem = nospec_ptr(array + idx, array, array + size);
+
+		// unsafe due to pointer substitution
+		if (elem == &array[idx]) {
+			a = elem->field_a;
+			b = elem->field_b;
+		}
+	}
+
+* nospec_array_ptr(arr, idx, sz)
+
+  Returns a sanitized pointer to arr[idx] only if idx falls in the [0, sz)
+  interval. When idx < 0 or idx > sz, NULL is returned. Prevents an
+  out-of-bounds pointer being propagated to code which is speculatively
+  executed.
+
+  This is a convenience function which wraps nospec_ptr(), and has the same
+  caveats w.r.t. the use of the returned pointer.
+
+  For example, this may be used as follows:
+
+  int load_array(int *array, unsigned int idx)
+  {
+	int *elem;
+
+	if ((elem = nospec_array_ptr(array, idx, MAX_ARRAY_ELEMS)))
+		return *elem;
+	else
+		return 0;
+  }
+