xen: clamp bitmaps to correct number of bits

Valgrind running xl create reports:
 ==24777== Invalid read of size 4
 ==24777==    at 0x4072805: libxl__get_numa_candidate (libxl_numa.c:203)
 ==24777==    by 0x40680B6: libxl__build_pre (libxl_dom.c:166)
 ==24777==    by 0x405B82E: libxl__domain_build (libxl_create.c:323)
 ==24777==    by 0x405BB9C: domcreate_bootloader_done (libxl_create.c:747)
 ==24777==    by 0x407AD27: bootloader_local_detached_cb (libxl_bootloader.c:281)
 ==24777==    by 0x40508D8: local_device_detach_cb (libxl.c:2470)
 ==24777==    by 0x4052B10: libxl__device_disk_local_initiate_detach (libxl.c:2445)
 ==24777==    by 0x407AE9F: bootloader_callback (libxl_bootloader.c:265)
 ==24777==    by 0x407C69A: libxl__bootloader_run (libxl_bootloader.c:392)
 ==24777==    by 0x405CB24: do_domain_create (libxl_create.c:687)
 ==24777==    by 0x405CC5E: libxl_domain_create_new (libxl_create.c:1177)
 ==24777==    by 0x805BDE2: create_domain (xl_cmdimpl.c:1812)
 ==24777==  Address 0x42dbdd8 is 8 bytes after a block of size 48 alloc'd
 ==24777==    at 0x4023340: calloc (vg_replace_malloc.c:593)
 ==24777==    by 0x406D479: libxl__zalloc (libxl_internal.c:88)
 ==24777==    by 0x404EF38: libxl_get_cpu_topology (libxl.c:3707)
 ==24777==    by 0x4072232: libxl__get_numa_candidate (libxl_numa.c:314)
 ==24777==    by 0x40680B6: libxl__build_pre (libxl_dom.c:166)
 ==24777==    by 0x405B82E: libxl__domain_build (libxl_create.c:323)
 ==24777==    by 0x405BB9C: domcreate_bootloader_done (libxl_create.c:747)
 ==24777==    by 0x407AD27: bootloader_local_detached_cb (libxl_bootloader.c:281)
 ==24777==    by 0x40508D8: local_device_detach_cb (libxl.c:2470)
 ==24777==    by 0x4052B10: libxl__device_disk_local_initiate_detach (libxl.c:2445)
 ==24777==    by 0x407AE9F: bootloader_callback (libxl_bootloader.c:265)
 ==24777==    by 0x407C69A: libxl__bootloader_run (libxl_bootloader.c:392)

This is because with nr_cpus=4 the bitmask returned from Xen
contains 0xff rather than 0x0f bit our bitmap walking routines (e.g.
libxl_for_each_set_bit) round up to the next byte (so it iterates
e.g. 8 times not 4). This then causes us to access of the end of
whatever array we are walking through each set bit for.

The principal of least surprise suggests that these bits ought not to
be set and this is not a hot path so fix this at the hypervisor layer
by clamping the bits in the returned bitmap to the correct limit.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Committed-by: Jan Beulich <jbeulich@suse.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/xen/common/bitmap.c b/xen/common/bitmap.c
index a34f519359..67dfd001e6 100644
--- a/xen/common/bitmap.c
+++ b/xen/common/bitmap.c
@@ -38,6 +38,21 @@
  * for the best explanations of this ordering.
  */
 
+/*
+ * If a bitmap has a number of bits which is not a multiple of 8 then
+ * the last few bits of the last byte of the bitmap can be
+ * unexpectedly set which can confuse consumers (e.g. in the tools)
+ * who also round up their loops to 8 bits. Ensure we clear those left
+ * over bits so as to prevent surprises.
+ */
+static void clamp_last_byte(uint8_t *bp, unsigned int nbits)
+{
+	unsigned int remainder = nbits % 8;
+
+	if (remainder)
+		bp[nbits/8] &= (1U << remainder) - 1;
+}
+
 int __bitmap_empty(const unsigned long *bitmap, int bits)
 {
 	int k, lim = bits/BITS_PER_LONG;
@@ -485,6 +500,7 @@ void bitmap_long_to_byte(uint8_t *bp, const unsigned long *lp, int nbits)
 			nbits -= 8;
 		}
 	}
+	clamp_last_byte(bp, nbits);
 }
 
 void bitmap_byte_to_long(unsigned long *lp, const uint8_t *bp, int nbits)
@@ -507,6 +523,7 @@ void bitmap_byte_to_long(unsigned long *lp, const uint8_t *bp, int nbits)
 void bitmap_long_to_byte(uint8_t *bp, const unsigned long *lp, int nbits)
 {
 	memcpy(bp, lp, (nbits+7)/8);
+	clamp_last_byte(bp, nbits);
 }
 
 void bitmap_byte_to_long(unsigned long *lp, const uint8_t *bp, int nbits)