linux-kernel - [PATCH 3.2 084/104] x86: Implement array_index_mask

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Message-ID: <lsq.1520823814.879983504@decadent.org.uk>
Date:   Mon, 12 Mar 2018 03:03:34 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, alan@...ux.intel.com,
        kernel-hardening@...ts.openwall.com,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Dan Williams" <dan.j.williams@...el.com>,
        "Thomas Gleixner" <tglx@...utronix.de>, gregkh@...uxfoundation.org,
        linux-arch@...r.kernel.org
Subject: [PATCH 3.2 084/104] x86: Implement array_index_mask_nospec

3.2.101-rc1 review patch.  If anyone has any objections, please let me know.

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

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

commit babdde2698d482b6c0de1eab4f697cf5856c5859 upstream.

array_index_nospec() uses a mask to sanitize user controllable array
indexes, i.e. generate a 0 mask if 'index' >= 'size', and a ~0 mask
otherwise. While the default array_index_mask_nospec() handles the
carry-bit from the (index - size) result in software.

The x86 array_index_mask_nospec() does the same, but the carry-bit is
handled in the processor CF flag without conditional instructions in the
control flow.

Suggested-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: kernel-hardening@...ts.openwall.com
Cc: gregkh@...uxfoundation.org
Cc: alan@...ux.intel.com
Link: https://lkml.kernel.org/r/151727414808.33451.1873237130672785331.stgit@dwillia2-desk3.amr.corp.intel.com
[bwh: Backported to 3.2: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/include/asm/system.h | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

--- a/arch/x86/include/asm/system.h
+++ b/arch/x86/include/asm/system.h
@@ -455,6 +455,30 @@ void stop_this_cpu(void *dummy);
 #endif
 
 /**
+ * array_index_mask_nospec() - generate a mask that is ~0UL when the
+ * 	bounds check succeeds and 0 otherwise
+ * @index: array element index
+ * @size: number of elements in array
+ *
+ * Returns:
+ *     0 - (index < size)
+ */
+static inline unsigned long array_index_mask_nospec(unsigned long index,
+		unsigned long size)
+{
+	unsigned long mask;
+
+	asm ("cmp %1,%2; sbb %0,%0;"
+			:"=r" (mask)
+			:"r"(size),"r" (index)
+			:"cc");
+	return mask;
+}
+
+/* Override the default implementation from linux/nospec.h. */
+#define array_index_mask_nospec array_index_mask_nospec
+
+/**
  * read_barrier_depends - Flush all pending reads that subsequents reads
  * depend on.
  *