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 4.15-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-4.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
Subject: x86/get_user: Use pointer masking to limit speculation
From: Dan Williams dan.j.williams@xxxxxxxxx
Date: Mon Jan 29 17:02:54 2018 -0800
From: Dan Williams dan.j.williams@xxxxxxxxx
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@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-arch@xxxxxxxxxxxxxxx
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
Cc: gregkh@xxxxxxxxxxxxxxxxxxx
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx
Cc: alan@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/151727417469.33451.11804043010080838495.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxxxxxxxxxxxxxxx are
queue-4.15/objtool_Add_support_for_alternatives_at_the_end_of_a_section.patch
queue-4.15/x86pti_Do_not_enable_PTI_on_CPUs_which_are_not_vulnerable_to_Meltdown.patch
queue-4.15/x86_Introduce_barrier_nospec.patch
queue-4.15/x86speculation_Use_Indirect_Branch_Prediction_Barrier_in_context_switch.patch
queue-4.15/x86get_user_Use_pointer_masking_to_limit_speculation.patch
queue-4.15/x86_Introduce___uaccess_begin_nospec()_and_uaccess_try_nospec.patch
queue-4.15/x86cpufeature_Blacklist_SPEC_CTRLPRED_CMD_on_early_Spectre_v2_microcodes.patch
queue-4.15/x86cpufeatures_Add_Intel_feature_bits_for_Speculation_Control.patch
queue-4.15/x86paravirt_Remove_noreplace-paravirt_cmdline_option.patch
queue-4.15/KVM_VMX_Make_indirect_call_speculation_safe.patch
queue-4.15/x86msr_Add_definitions_for_new_speculation_control_MSRs.patch
queue-4.15/x86alternative_Print_unadorned_pointers.patch
queue-4.15/KVMVMX_Allow_direct_access_to_MSR_IA32_SPEC_CTRL.patch
queue-4.15/x86cpufeatures_Add_CPUID_7_EDX_CPUID_leaf.patch
queue-4.15/array_index_nospec_Sanitize_speculative_array_de-references.patch
queue-4.15/Documentation_Document_array_index_nospec.patch
queue-4.15/x86entry64_Remove_the_SYSCALL64_fast_path.patch
queue-4.15/x86bugs_Drop_one_mitigation_from_dmesg.patch
queue-4.15/x86cpufeatures_Add_AMD_feature_bits_for_Speculation_Control.patch
queue-4.15/KVMSVM_Allow_direct_access_to_MSR_IA32_SPEC_CTRL.patch
queue-4.15/x86asm_Move_status_from_thread_struct_to_thread_info.patch
queue-4.15/KVMx86_Add_IBPB_support.patch
queue-4.15/x86_Implement_array_index_mask_nospec.patch
queue-4.15/KVMVMX_Emulate_MSR_IA32_ARCH_CAPABILITIES.patch
queue-4.15/nl80211_Sanitize_array_index_in_parse_txq_params.patch
queue-4.15/moduleretpoline_Warn_about_missing_retpoline_in_module.patch
queue-4.15/x86speculation_Add_basic_IBPB_(Indirect_Branch_Prediction_Barrier)_support.patch
queue-4.15/x86speculation_Simplify_indirect_branch_prediction_barrier().patch
queue-4.15/x86nospec_Fix_header_guards_names.patch
queue-4.15/KVM_x86_Make_indirect_calls_in_emulator_speculation_safe.patch
queue-4.15/x86uaccess_Use___uaccess_begin_nospec()_and_uaccess_try_nospec.patch
queue-4.15/x86entry64_Push_extra_regs_right_away.patch
queue-4.15/x86usercopy_Replace_open_coded_stacclac_with___uaccess_begin_end.patch
queue-4.15/vfs_fdtable_Prevent_bounds-check_bypass_via_speculative_execution.patch
queue-4.15/x86retpoline_Simplify_vmexit_fill_RSB().patch
queue-4.15/objtool_Warn_on_stripped_section_symbol.patch
queue-4.15/x86spectre_Report_get_user_mitigation_for_spectre_v1.patch
queue-4.15/x86cpufeatures_Clean_up_Spectre_v2_related_CPUID_flags.patch
queue-4.15/x86syscall_Sanitize_syscall_table_de-references_under_speculation.patch
queue-4.15/objtool_Improve_retpoline_alternative_handling.patch