linux-kernel - [PATCH 4.4 177/193] x86/get_user: Use pointer masking to limit speculation

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Message-Id: <20180223170353.836877903@linuxfoundation.org>
Date:   Fri, 23 Feb 2018 19:26:50 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arch@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        torvalds@...ux-foundation.org, alan@...ux.intel.com,
        David Woodhouse <dwmw@...zon.co.uk>,
        Jack Wang <jinpu.wang@...fitbricks.com>
Subject: [PATCH 4.4 177/193] x86/get_user: Use pointer masking to limit speculation

4.4-stable review patch.  If anyone has any objections, please let me know.

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

From: Dan Williams <dan.j.williams@...el.com>

(cherry picked from 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@...ux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-arch@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>
Cc: kernel-hardening@...ts.openwall.com
Cc: gregkh@...uxfoundation.org
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: torvalds@...ux-foundation.org
Cc: alan@...ux.intel.com
Link: https://lkml.kernel.org/r/151727417469.33451.11804043010080838495.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
[jwang: port to 4.4]
Signed-off-by: Jack Wang <jinpu.wang@...fitbricks.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/x86/lib/getuser.S |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -38,6 +38,8 @@ ENTRY(__get_user_1)
 	GET_THREAD_INFO(%_ASM_DX)
 	cmp TI_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
@@ -51,6 +53,8 @@ ENTRY(__get_user_2)
 	GET_THREAD_INFO(%_ASM_DX)
 	cmp TI_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
@@ -64,6 +68,8 @@ ENTRY(__get_user_4)
 	GET_THREAD_INFO(%_ASM_DX)
 	cmp TI_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
@@ -78,6 +84,8 @@ ENTRY(__get_user_8)
 	GET_THREAD_INFO(%_ASM_DX)
 	cmp TI_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
@@ -89,6 +97,8 @@ ENTRY(__get_user_8)
 	GET_THREAD_INFO(%_ASM_DX)
 	cmp TI_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