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    Subject[PATCH 4.15 39/60] x86/get_user: Use pointer masking to limit speculation
    Date
    4.15-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    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

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