From mboxrd@z Thu Jan 1 00:00:00 1970 From: Greg Kroah-Hartman Subject: [PATCH 4.4 177/193] x86/get_user: Use pointer masking to limit speculation Date: Fri, 23 Feb 2018 19:26:50 +0100 Message-ID: <20180223170353.836877903@linuxfoundation.org> References: <20180223170325.997716448@linuxfoundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Return-path: List-Post: List-Help: List-Unsubscribe: List-Subscribe: In-Reply-To: <20180223170325.997716448@linuxfoundation.org> To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Dan Williams , Thomas Gleixner , linux-arch@vger.kernel.org, Kees Cook , kernel-hardening@lists.openwall.com, Al Viro , Andy Lutomirski , torvalds@linux-foundation.org, alan@linux.intel.com, David Woodhouse , Jack Wang List-Id: linux-arch.vger.kernel.org 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Dan Williams (cherry picked from commit c7f631cb07e7da06ac1d231ca178452339e32a94) Quoting Linus: I do think that it would be a good idea to very expressly document the fact that it's not that the user access itself is unsafe. I do agree that things like "get_user()" want to be protected, but not because of any direct bugs or problems with get_user() and friends, but simply because get_user() is an excellent source of a pointer that is obviously controlled from a potentially attacking user space. So it's a prime candidate for then finding _subsequent_ accesses that can then be used to perturb the cache. Unlike the __get_user() case get_user() includes the address limit check near the pointer de-reference. With that locality the speculation can be mitigated with pointer narrowing rather than a barrier, i.e. array_index_nospec(). Where the narrowing is performed by: cmp %limit, %ptr sbb %mask, %mask and %mask, %ptr With respect to speculation the value of %ptr is either less than %limit or NULL. Co-developed-by: Linus Torvalds Signed-off-by: Dan Williams Signed-off-by: Thomas Gleixner Cc: linux-arch@vger.kernel.org Cc: Kees Cook Cc: kernel-hardening@lists.openwall.com Cc: gregkh@linuxfoundation.org Cc: Al Viro Cc: Andy Lutomirski Cc: torvalds@linux-foundation.org Cc: alan@linux.intel.com Link: https://lkml.kernel.org/r/151727417469.33451.11804043010080838495.stgit@dwillia2-desk3.amr.corp.intel.com Signed-off-by: David Woodhouse [jwang: port to 4.4] Signed-off-by: Jack Wang Signed-off-by: Greg Kroah-Hartman --- arch/x86/lib/getuser.S | 10 ++++++++++ 1 file changed, 10 insertions(+) --- a/arch/x86/lib/getuser.S +++ b/arch/x86/lib/getuser.S @@ -38,6 +38,8 @@ ENTRY(__get_user_1) GET_THREAD_INFO(%_ASM_DX) cmp TI_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */ + and %_ASM_DX, %_ASM_AX ASM_STAC 1: movzbl (%_ASM_AX),%edx xor %eax,%eax @@ -51,6 +53,8 @@ ENTRY(__get_user_2) GET_THREAD_INFO(%_ASM_DX) cmp TI_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */ + and %_ASM_DX, %_ASM_AX ASM_STAC 2: movzwl -1(%_ASM_AX),%edx xor %eax,%eax @@ -64,6 +68,8 @@ ENTRY(__get_user_4) GET_THREAD_INFO(%_ASM_DX) cmp TI_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */ + and %_ASM_DX, %_ASM_AX ASM_STAC 3: movl -3(%_ASM_AX),%edx xor %eax,%eax @@ -78,6 +84,8 @@ ENTRY(__get_user_8) GET_THREAD_INFO(%_ASM_DX) cmp TI_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */ + and %_ASM_DX, %_ASM_AX ASM_STAC 4: movq -7(%_ASM_AX),%rdx xor %eax,%eax @@ -89,6 +97,8 @@ ENTRY(__get_user_8) GET_THREAD_INFO(%_ASM_DX) cmp TI_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user_8 + sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */ + and %_ASM_DX, %_ASM_AX ASM_STAC 4: movl -7(%_ASM_AX),%edx 5: movl -3(%_ASM_AX),%ecx From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.linuxfoundation.org ([140.211.169.12]:40394 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753710AbeBWSlE (ORCPT ); Fri, 23 Feb 2018 13:41:04 -0500 From: Greg Kroah-Hartman Subject: [PATCH 4.4 177/193] x86/get_user: Use pointer masking to limit speculation Date: Fri, 23 Feb 2018 19:26:50 +0100 Message-ID: <20180223170353.836877903@linuxfoundation.org> In-Reply-To: <20180223170325.997716448@linuxfoundation.org> References: <20180223170325.997716448@linuxfoundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-arch-owner@vger.kernel.org List-ID: To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Dan Williams , Thomas Gleixner , linux-arch@vger.kernel.org, Kees Cook , kernel-hardening@lists.openwall.com, Al Viro , Andy Lutomirski , torvalds@linux-foundation.org, alan@linux.intel.com, David Woodhouse , Jack Wang Message-ID: <20180223182650.Pq6_D0S8O1vK1_u1tAFwemjHXvXfNSTelmp4EWFfesQ@z> 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Dan Williams (cherry picked from commit c7f631cb07e7da06ac1d231ca178452339e32a94) Quoting Linus: I do think that it would be a good idea to very expressly document the fact that it's not that the user access itself is unsafe. I do agree that things like "get_user()" want to be protected, but not because of any direct bugs or problems with get_user() and friends, but simply because get_user() is an excellent source of a pointer that is obviously controlled from a potentially attacking user space. So it's a prime candidate for then finding _subsequent_ accesses that can then be used to perturb the cache. Unlike the __get_user() case get_user() includes the address limit check near the pointer de-reference. With that locality the speculation can be mitigated with pointer narrowing rather than a barrier, i.e. array_index_nospec(). Where the narrowing is performed by: cmp %limit, %ptr sbb %mask, %mask and %mask, %ptr With respect to speculation the value of %ptr is either less than %limit or NULL. Co-developed-by: Linus Torvalds Signed-off-by: Dan Williams Signed-off-by: Thomas Gleixner Cc: linux-arch@vger.kernel.org Cc: Kees Cook Cc: kernel-hardening@lists.openwall.com Cc: gregkh@linuxfoundation.org Cc: Al Viro Cc: Andy Lutomirski Cc: torvalds@linux-foundation.org Cc: alan@linux.intel.com Link: https://lkml.kernel.org/r/151727417469.33451.11804043010080838495.stgit@dwillia2-desk3.amr.corp.intel.com Signed-off-by: David Woodhouse [jwang: port to 4.4] Signed-off-by: Jack Wang Signed-off-by: Greg Kroah-Hartman --- arch/x86/lib/getuser.S | 10 ++++++++++ 1 file changed, 10 insertions(+) --- a/arch/x86/lib/getuser.S +++ b/arch/x86/lib/getuser.S @@ -38,6 +38,8 @@ ENTRY(__get_user_1) GET_THREAD_INFO(%_ASM_DX) cmp TI_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */ + and %_ASM_DX, %_ASM_AX ASM_STAC 1: movzbl (%_ASM_AX),%edx xor %eax,%eax @@ -51,6 +53,8 @@ ENTRY(__get_user_2) GET_THREAD_INFO(%_ASM_DX) cmp TI_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */ + and %_ASM_DX, %_ASM_AX ASM_STAC 2: movzwl -1(%_ASM_AX),%edx xor %eax,%eax @@ -64,6 +68,8 @@ ENTRY(__get_user_4) GET_THREAD_INFO(%_ASM_DX) cmp TI_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */ + and %_ASM_DX, %_ASM_AX ASM_STAC 3: movl -3(%_ASM_AX),%edx xor %eax,%eax @@ -78,6 +84,8 @@ ENTRY(__get_user_8) GET_THREAD_INFO(%_ASM_DX) cmp TI_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */ + and %_ASM_DX, %_ASM_AX ASM_STAC 4: movq -7(%_ASM_AX),%rdx xor %eax,%eax @@ -89,6 +97,8 @@ ENTRY(__get_user_8) GET_THREAD_INFO(%_ASM_DX) cmp TI_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user_8 + sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */ + and %_ASM_DX, %_ASM_AX ASM_STAC 4: movl -7(%_ASM_AX),%edx 5: movl -3(%_ASM_AX),%ecx