| Message ID | 20180105145750.53294-3-mark.rutland@arm.com |
|---|---|
| State | New |
| Headers | show |
| Series | API for inhibiting speculative arbitrary read primitives | expand |
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
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
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.
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; + } +