linux-kernel - [ 61/72] ARM: 7768/1: prevent risks of out-of-bound access in ASID allocator

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Date:	Thu, 18 Jul 2013 22:26:18 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <marc.zyngier@....com>,
	Russell King <rmk+kernel@....linux.org.uk>
Subject: [ 61/72] ARM: 7768/1: prevent risks of out-of-bound access in ASID allocator

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

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

From: Marc Zyngier <Marc.Zyngier@....com>

commit b8e4a4740fa2b17c0a447b3ab783b3dc10702e27 upstream.

On a CPU that never ran anything, both the active and reserved ASID
fields are set to zero. In this case the ASID_TO_IDX() macro will
return -1, which is not a very useful value to index a bitmap.

Instead of trying to offset the ASID so that ASID #1 is actually
bit 0 in the asid_map bitmap, just always ignore bit 0 and start
the search from bit 1. This makes the code a bit more readable,
and without risk of OoB access.

Acked-by: Will Deacon <will.deacon@....com>
Acked-by: Catalin Marinas <catalin.marinas@....com>
Reported-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Marc Zyngier <marc.zyngier@....com>
Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/arm/mm/context.c |   17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

--- a/arch/arm/mm/context.c
+++ b/arch/arm/mm/context.c
@@ -39,10 +39,7 @@
  * non 64-bit operations.
  */
 #define ASID_FIRST_VERSION	(1ULL << ASID_BITS)
-#define NUM_USER_ASIDS		(ASID_FIRST_VERSION - 1)
-
-#define ASID_TO_IDX(asid)	((asid & ~ASID_MASK) - 1)
-#define IDX_TO_ASID(idx)	((idx + 1) & ~ASID_MASK)
+#define NUM_USER_ASIDS		ASID_FIRST_VERSION
 
 static DEFINE_RAW_SPINLOCK(cpu_asid_lock);
 static atomic64_t asid_generation = ATOMIC64_INIT(ASID_FIRST_VERSION);
@@ -137,7 +134,7 @@ static void flush_context(unsigned int c
 			 */
 			if (asid == 0)
 				asid = per_cpu(reserved_asids, i);
-			__set_bit(ASID_TO_IDX(asid), asid_map);
+			__set_bit(asid & ~ASID_MASK, asid_map);
 		}
 		per_cpu(reserved_asids, i) = asid;
 	}
@@ -176,17 +173,19 @@ static u64 new_context(struct mm_struct
 		/*
 		 * Allocate a free ASID. If we can't find one, take a
 		 * note of the currently active ASIDs and mark the TLBs
-		 * as requiring flushes.
+		 * as requiring flushes. We always count from ASID #1,
+		 * as we reserve ASID #0 to switch via TTBR0 and indicate
+		 * rollover events.
 		 */
-		asid = find_first_zero_bit(asid_map, NUM_USER_ASIDS);
+		asid = find_next_zero_bit(asid_map, NUM_USER_ASIDS, 1);
 		if (asid == NUM_USER_ASIDS) {
 			generation = atomic64_add_return(ASID_FIRST_VERSION,
 							 &asid_generation);
 			flush_context(cpu);
-			asid = find_first_zero_bit(asid_map, NUM_USER_ASIDS);
+			asid = find_next_zero_bit(asid_map, NUM_USER_ASIDS, 1);
 		}
 		__set_bit(asid, asid_map);
-		asid = generation | IDX_TO_ASID(asid);
+		asid |= generation;
 		cpumask_clear(mm_cpumask(mm));
 	}
 


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