Messages in this thread Patch in this message | | | Subject | [PATCH v4 06/10] x86, get_user: use pointer masking to limit speculation | From | Dan Williams <> | Date | Thu, 18 Jan 2018 16:02:15 -0800 |
| |
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. 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> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Kees Cook <keescook@chromium.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: x86@kernel.org Signed-off-by: Dan Williams <dan.j.williams@intel.com> --- arch/x86/include/asm/smap.h | 17 +++++++++++++++++ arch/x86/lib/getuser.S | 5 +++++ 2 files changed, 22 insertions(+)
diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h index db333300bd4b..2b4ad4c6a226 100644 --- a/arch/x86/include/asm/smap.h +++ b/arch/x86/include/asm/smap.h @@ -25,6 +25,23 @@ #include <asm/alternative-asm.h> +/* + * MASK_NOSPEC - sanitize the value of a user controlled value with + * respect to speculation + * + * In the get_user path once we have determined that the pointer is + * below the current address limit sanitize its value with respect to + * speculation. In the case when the pointer is above the address limit + * this directs the cpu to speculate with a NULL ptr rather than + * something targeting kernel memory. + * + * assumes CF is set from a previous 'cmp TASK_addr_limit, %ptr' + */ +.macro MASK_NOSPEC mask val + sbb \mask, \mask + and \mask, \val +.endm + #ifdef CONFIG_X86_SMAP #define ASM_CLAC \ diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S index c97d935a29e8..07d0e8a28b17 100644 --- a/arch/x86/lib/getuser.S +++ b/arch/x86/lib/getuser.S @@ -40,6 +40,7 @@ ENTRY(__get_user_1) mov PER_CPU_VAR(current_task), %_ASM_DX cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + MASK_NOSPEC %_ASM_DX, %_ASM_AX ASM_STAC 1: movzbl (%_ASM_AX),%edx xor %eax,%eax @@ -54,6 +55,7 @@ ENTRY(__get_user_2) mov PER_CPU_VAR(current_task), %_ASM_DX cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + MASK_NOSPEC %_ASM_DX, %_ASM_AX ASM_STAC 2: movzwl -1(%_ASM_AX),%edx xor %eax,%eax @@ -68,6 +70,7 @@ ENTRY(__get_user_4) mov PER_CPU_VAR(current_task), %_ASM_DX cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + MASK_NOSPEC %_ASM_DX, %_ASM_AX ASM_STAC 3: movl -3(%_ASM_AX),%edx xor %eax,%eax @@ -83,6 +86,7 @@ ENTRY(__get_user_8) mov PER_CPU_VAR(current_task), %_ASM_DX cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX jae bad_get_user + MASK_NOSPEC %_ASM_DX, %_ASM_AX ASM_STAC 4: movq -7(%_ASM_AX),%rdx xor %eax,%eax @@ -94,6 +98,7 @@ 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 + MASK_NOSPEC %_ASM_DX, %_ASM_AX ASM_STAC 4: movl -7(%_ASM_AX),%edx 5: movl -3(%_ASM_AX),%ecx
| |