From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Google-Smtp-Source: ACJfBovmH1svHhH1Lo3R8m9LU2rNYUWERoyaihJCaaPBpPU0edIfExb2R7ETqIauoSJCFTbbSx21 ARC-Seal: i=1; a=rsa-sha256; t=1516320649; cv=none; d=google.com; s=arc-20160816; b=IeK2gaGo5zW3uxuMQVOsVKzQQb0dJKeR7KoLT7LopmEB+bDilnRpF7wRtHj0x8g3Fn 3Tfd9U9gMDVeeZacn6Pv6Y0Rcu5e64K6XZwZ5Wby2CNJfws+kCgf26IrsK5hxpLSE1GI lAu36umkU/pEXeuerdWse6sZollWGUlN8VhwWE6FKaD+EOSfYwOugMi1Dm46PK3pDeGx RVxC3rSmWYMAdDlbb5YTxmd8ZxZjFgm3sq5cadNvrfo3IbdPcJw1ctqg9GjTGxiPwJEj lig8ozJ+7EmMUDzeXvyqw3CHhpyYlTwRAJSpLihMepQljHHBmrtzp72tOq+Lu7c2lumE LMtw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=content-transfer-encoding:mime-version:user-agent:references :in-reply-to:message-id:date:cc:to:from:subject :arc-authentication-results; bh=+CooYn4aNXo6cXY7BajUtDDyIICrD8ZQqtKRxShlTL8=; b=ONYZJxt4MmsptXosZgoBAQMMDtVLy6cABdowGgzWcmYp2IFqtm2CSHS0cEiDAxGncL dYTYOf8fZUR0J3sJ/NK+xTHYokKW9ORP2O5/jw3c+65UFq8V8ofp11in2buT5wHh0Aas /5ROO5Y1DEPsfSOVV2bc4eC84QbnKnf27fkfcCiMMK0zqlkoJHWQGEyb/4Yj5C5zvyy7 LiMTKJJQkiUSqEkgX2KoM7T0HzLcmtQ5QtTQ4SFRRk7bWmUsD2DJB3AjMuoZw2ACN1uL 40FSnVnE5D3WnW5m7yaagMpCK5p9G61YIS12HmTTMOpjJZzc7OVJxohhXFWDOubjXKcJ yGGA== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: domain of dan.j.williams@intel.com designates 134.134.136.31 as permitted sender) smtp.mailfrom=dan.j.williams@intel.com Authentication-Results: mx.google.com; spf=pass (google.com: domain of dan.j.williams@intel.com designates 134.134.136.31 as permitted sender) smtp.mailfrom=dan.j.williams@intel.com X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.46,379,1511856000"; d="scan'208";a="27657720" Subject: [PATCH v4 01/10] Documentation: document array_ptr From: Dan Williams To: linux-kernel@vger.kernel.org Cc: Mark Rutland , linux-arch@vger.kernel.org, Kees Cook , kernel-hardening@lists.openwall.com, Peter Zijlstra , gregkh@linuxfoundation.org, Jonathan Corbet , Will Deacon , tglx@linutronix.de, torvalds@linux-foundation.org, akpm@linux-foundation.org, alan@linux.intel.com Bcc: dan.j.williams@intel.com Date: Thu, 18 Jan 2018 16:01:41 -0800 Message-ID: <151632010172.21271.13545507895393426065.stgit@dwillia2-desk3.amr.corp.intel.com> In-Reply-To: <151632009605.21271.11304291057104672116.stgit@dwillia2-desk3.amr.corp.intel.com> References: <151632009605.21271.11304291057104672116.stgit@dwillia2-desk3.amr.corp.intel.com> User-Agent: StGit/0.17.1-9-g687f MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1589977441536307433?= X-GMAIL-MSGID: =?utf-8?q?1589977441536307433?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: From: Mark Rutland Document the rationale and usage of the new array_ptr() helper. Signed-off-by: Mark Rutland Signed-off-by: Will Deacon Cc: Dan Williams Cc: Jonathan Corbet Cc: Peter Zijlstra Reviewed-by: Kees Cook Signed-off-by: Dan Williams --- Documentation/speculation.txt | 143 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 143 insertions(+) create mode 100644 Documentation/speculation.txt diff --git a/Documentation/speculation.txt b/Documentation/speculation.txt new file mode 100644 index 000000000000..a47fbffe0dab --- /dev/null +++ b/Documentation/speculation.txt @@ -0,0 +1,143 @@ +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 , #MAX_ARRAY_ELEMS + B.LT less + MOV , #0 + RET + less: + LDR , [, ] + 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 array_ptr() helper in can be used to prevent +information from being leaked via side-channels. + +A call to 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. Additionally, array_ptr() of an out-of-bounds pointer is +not propagated to code which is speculatively executed. + +This can be used to protect the earlier load_array() example: + + int load_array(int *array, unsigned int idx) + { + int *elem; + + elem = array_ptr(array, idx, 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; + + elem = array_ptr(array, idx, 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 (array_ptr(array, idx, 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 = array_ptr(array, idx, size); + + // unsafe due to pointer substitution + if (elem == &array[idx]) { + a = elem->field_a; + b = elem->field_b; + } + } + From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Williams Date: Thu, 18 Jan 2018 16:01:41 -0800 Message-ID: <151632010172.21271.13545507895393426065.stgit@dwillia2-desk3.amr.corp.intel.com> In-Reply-To: <151632009605.21271.11304291057104672116.stgit@dwillia2-desk3.amr.corp.intel.com> References: <151632009605.21271.11304291057104672116.stgit@dwillia2-desk3.amr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Subject: [kernel-hardening] [PATCH v4 01/10] Documentation: document array_ptr To: linux-kernel@vger.kernel.org Cc: Mark Rutland , linux-arch@vger.kernel.org, Kees Cook , kernel-hardening@lists.openwall.com, Peter Zijlstra , gregkh@linuxfoundation.org, Jonathan Corbet , Will Deacon , tglx@linutronix.de, torvalds@linux-foundation.org, akpm@linux-foundation.org, alan@linux.intel.com List-ID: From: Mark Rutland Document the rationale and usage of the new array_ptr() helper. Signed-off-by: Mark Rutland Signed-off-by: Will Deacon Cc: Dan Williams Cc: Jonathan Corbet Cc: Peter Zijlstra Reviewed-by: Kees Cook Signed-off-by: Dan Williams --- Documentation/speculation.txt | 143 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 143 insertions(+) create mode 100644 Documentation/speculation.txt diff --git a/Documentation/speculation.txt b/Documentation/speculation.txt new file mode 100644 index 000000000000..a47fbffe0dab --- /dev/null +++ b/Documentation/speculation.txt @@ -0,0 +1,143 @@ +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 , #MAX_ARRAY_ELEMS + B.LT less + MOV , #0 + RET + less: + LDR , [, ] + 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 array_ptr() helper in can be used to prevent +information from being leaked via side-channels. + +A call to 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. Additionally, array_ptr() of an out-of-bounds pointer is +not propagated to code which is speculatively executed. + +This can be used to protect the earlier load_array() example: + + int load_array(int *array, unsigned int idx) + { + int *elem; + + elem = array_ptr(array, idx, 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; + + elem = array_ptr(array, idx, 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 (array_ptr(array, idx, 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 = array_ptr(array, idx, size); + + // unsafe due to pointer substitution + if (elem == &array[idx]) { + a = elem->field_a; + b = elem->field_b; + } + } +