Patch "x86/get_user: Use pointer masking to limit speculation" has been added to the 4.14-stable tree

Patch "x86/get_user: Use pointer masking to limit speculation" has been added to the 4.14-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    x86/get_user: Use pointer masking to limit speculation

to the 4.14-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     x86get_user_Use_pointer_masking_to_limit_speculation.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


Subject: x86/get_user: Use pointer masking to limit speculation
From: Dan Williams dan.j.williams@intel.com
Date: Mon Jan 29 17:02:54 2018 -0800

From: Dan Williams dan.j.williams@intel.com

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 <torvalds@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>
Cc: kernel-hardening@lists.openwall.com
Cc: gregkh@linuxfoundation.org
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


---
 arch/x86/lib/getuser.S |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -40,6 +40,8 @@ ENTRY(__get_user_1)
 	mov PER_CPU_VAR(current_task), %_ASM_DX
 	cmp TASK_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
@@ -54,6 +56,8 @@ ENTRY(__get_user_2)
 	mov PER_CPU_VAR(current_task), %_ASM_DX
 	cmp TASK_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
@@ -68,6 +72,8 @@ ENTRY(__get_user_4)
 	mov PER_CPU_VAR(current_task), %_ASM_DX
 	cmp TASK_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
@@ -83,6 +89,8 @@ ENTRY(__get_user_8)
 	mov PER_CPU_VAR(current_task), %_ASM_DX
 	cmp TASK_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
@@ -94,6 +102,8 @@ ENTRY(__get_user_8)
 	mov PER_CPU_VAR(current_task), %_ASM_DX
 	cmp TASK_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


Patches currently in stable-queue which might be from torvalds@linux-foundation.org are

queue-4.14/objtool_Add_support_for_alternatives_at_the_end_of_a_section.patch
queue-4.14/x86pti_Do_not_enable_PTI_on_CPUs_which_are_not_vulnerable_to_Meltdown.patch
queue-4.14/x86_Introduce_barrier_nospec.patch
queue-4.14/x86speculation_Use_Indirect_Branch_Prediction_Barrier_in_context_switch.patch
queue-4.14/x86get_user_Use_pointer_masking_to_limit_speculation.patch
queue-4.14/x86_Introduce___uaccess_begin_nospec()_and_uaccess_try_nospec.patch
queue-4.14/x86cpufeature_Blacklist_SPEC_CTRLPRED_CMD_on_early_Spectre_v2_microcodes.patch
queue-4.14/x86cpufeatures_Add_Intel_feature_bits_for_Speculation_Control.patch
queue-4.14/x86paravirt_Remove_noreplace-paravirt_cmdline_option.patch
queue-4.14/KVM_VMX_Make_indirect_call_speculation_safe.patch
queue-4.14/x86msr_Add_definitions_for_new_speculation_control_MSRs.patch
queue-4.14/x86alternative_Print_unadorned_pointers.patch
queue-4.14/KVMVMX_Allow_direct_access_to_MSR_IA32_SPEC_CTRL.patch
queue-4.14/x86cpufeatures_Add_CPUID_7_EDX_CPUID_leaf.patch
queue-4.14/array_index_nospec_Sanitize_speculative_array_de-references.patch
queue-4.14/Documentation_Document_array_index_nospec.patch
queue-4.14/x86entry64_Remove_the_SYSCALL64_fast_path.patch
queue-4.14/x86retpoline_Remove_the_esprsp_thunk.patch
queue-4.14/x86bugs_Drop_one_mitigation_from_dmesg.patch
queue-4.14/x86cpufeatures_Add_AMD_feature_bits_for_Speculation_Control.patch
queue-4.14/scripts-faddr2line-fix-cross_compile-unset-error.patch
queue-4.14/KVMSVM_Allow_direct_access_to_MSR_IA32_SPEC_CTRL.patch
queue-4.14/x86asm_Move_status_from_thread_struct_to_thread_info.patch
queue-4.14/KVMx86_Add_IBPB_support.patch
queue-4.14/x86_Implement_array_index_mask_nospec.patch
queue-4.14/KVMVMX_Emulate_MSR_IA32_ARCH_CAPABILITIES.patch
queue-4.14/nl80211_Sanitize_array_index_in_parse_txq_params.patch
queue-4.14/moduleretpoline_Warn_about_missing_retpoline_in_module.patch
queue-4.14/x86speculation_Add_basic_IBPB_(Indirect_Branch_Prediction_Barrier)_support.patch
queue-4.14/x86speculation_Simplify_indirect_branch_prediction_barrier().patch
queue-4.14/x86nospec_Fix_header_guards_names.patch
queue-4.14/KVM_x86_Make_indirect_calls_in_emulator_speculation_safe.patch
queue-4.14/x86uaccess_Use___uaccess_begin_nospec()_and_uaccess_try_nospec.patch
queue-4.14/x86entry64_Push_extra_regs_right_away.patch
queue-4.14/x86usercopy_Replace_open_coded_stacclac_with___uaccess_begin_end.patch
queue-4.14/vfs_fdtable_Prevent_bounds-check_bypass_via_speculative_execution.patch
queue-4.14/x86retpoline_Simplify_vmexit_fill_RSB().patch
queue-4.14/objtool_Warn_on_stripped_section_symbol.patch
queue-4.14/x86spectre_Report_get_user_mitigation_for_spectre_v1.patch
queue-4.14/x86cpufeatures_Clean_up_Spectre_v2_related_CPUID_flags.patch
queue-4.14/x86syscall_Sanitize_syscall_table_de-references_under_speculation.patch
queue-4.14/objtool_Improve_retpoline_alternative_handling.patch