| From foo@baz Mon Feb 5 10:12:24 PST 2018 |
| 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 |