[Xen-changelog] [xen-unstable] hvm: Handle extreme wallclock offsets safely.

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1274857192 -3600
# Node ID 459f35d8cac4f19e6eae0a7396f6c97e20ae955c
# Parent  145c822dd629160219676a39fda4d7dc26763508
hvm: Handle extreme wallclock offsets safely.

When a VM's wallclock offset is negative enough, gmtime() can be called
with an underflowed uint64, which it then tries to divide into years
by subtraction.  Handle the input as a 40-bit signed integer instead.

Signed-off-by: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
---
 xen/common/time.c |   13 ++++++++++++-
 1 files changed, 12 insertions(+), 1 deletion(-)

diff -r 145c822dd629 -r 459f35d8cac4 xen/common/time.c
--- a/xen/common/time.c Wed May 26 07:48:09 2010 +0100
+++ b/xen/common/time.c Wed May 26 07:59:52 2010 +0100
@@ -42,6 +42,18 @@ struct tm gmtime(unsigned long t)
     int y;
     const unsigned short int *ip;
 
+    y = 1970;
+#ifdef __x86_64__
+    /* Allow the concept of time before 1970.  64-bit only; for 32-bit
+     * time after 2038 seems more important than time before 1970. */
+    while ( t & (1UL<<39) )
+    {
+        y -= 400;
+        t += ((unsigned long)(365 * 303 + 366 * 97)) * SECS_PER_DAY;
+    }
+    t &= (1UL << 40) - 1;
+#endif
+
     days = t / SECS_PER_DAY;
     rem = t % SECS_PER_DAY;
 
@@ -53,7 +65,6 @@ struct tm gmtime(unsigned long t)
     tbuf.tm_wday = (4 + days) % 7;
     if ( tbuf.tm_wday < 0 )
         tbuf.tm_wday += 7;
-    y = 1970;
     while ( days >= (rem = __isleap(y) ? 366 : 365) )
     {
         ++y;

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