ZIO: Add overflow checks for linear buffers

Since we use a limited set of kmem caches, quite often we have unused
memory after the end of the buffer.  Put there up to a 512-byte canary
when built with debug to detect buffer overflows at the free time.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes openzfs#15553

lib/libspl/include/assert.h

@@ -64,6 +64,9 @@ libspl_assert(const char *buf, const char *file, const char *func, int line)
 #undef verify
 #endif
 
+#define	PANIC(fmt, a...)						\
+	libspl_assertf(__FILE__, __FUNCTION__, __LINE__, fmt, ## a)
+
 #define	VERIFY(cond)							\
 	(void) ((!(cond)) &&						\
 	    libspl_assert(#cond, __FILE__, __FUNCTION__, __LINE__))

module/zfs/zio.c

@@ -306,6 +306,53 @@ zio_fini(void)
  * ==========================================================================
  */
 
+#ifdef ZFS_DEBUG
+static const ulong_t zio_buf_canary = (ulong_t)0xdeadc0dedead210b;
+#endif
+
+/*
+ * Use empty space after the buffer to detect overflows.
+ *
+ * Since zio_init() creates kmem caches only for certain set of buffer sizes,
+ * allocations of different sizes may have some unused space after the data.
+ * Filling part of that space with a known pattern on allocation and checking
+ * it on free should allow us to detect some buffer overflows.
+ */
+static void
+zio_buf_put_canary(ulong_t *p, size_t size, kmem_cache_t **cache, size_t c)
+{
+#ifdef ZFS_DEBUG
+	size_t off = P2ROUNDUP(size, sizeof (ulong_t));
+	ulong_t *canary = p + off / sizeof (ulong_t);
+	size_t asize = (c + 1) << SPA_MINBLOCKSHIFT;
+	if (c + 1 < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT &&
+	    cache[c] == cache[c + 1])
+		asize = (c + 2) << SPA_MINBLOCKSHIFT;
+	for (; off < asize; canary++, off += sizeof (ulong_t))
+		*canary = zio_buf_canary;
+#endif
+}
+
+static void
+zio_buf_check_canary(ulong_t *p, size_t size, kmem_cache_t **cache, size_t c)
+{
+#ifdef ZFS_DEBUG
+	size_t off = P2ROUNDUP(size, sizeof (ulong_t));
+	ulong_t *canary = p + off / sizeof (ulong_t);
+	size_t asize = (c + 1) << SPA_MINBLOCKSHIFT;
+	if (c + 1 < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT &&
+	    cache[c] == cache[c + 1])
+		asize = (c + 2) << SPA_MINBLOCKSHIFT;
+	for (; off < asize; canary++, off += sizeof (ulong_t)) {
+		if (unlikely(*canary != zio_buf_canary)) {
+			PANIC("ZIO buffer overflow %p (%zu) + %zu %#lx != %#lx",
+			    p, size, (canary - p) * sizeof (ulong_t),
+			    *canary, zio_buf_canary);
+		}
+	}
+#endif
+}
+
 /*
  * Use zio_buf_alloc to allocate ZFS metadata.  This data will appear in a
  * crashdump if the kernel panics, so use it judiciously.  Obviously, it's
@@ -322,7 +369,9 @@ zio_buf_alloc(size_t size)
 	atomic_add_64(&zio_buf_cache_allocs[c], 1);
 #endif
 
-	return (kmem_cache_alloc(zio_buf_cache[c], KM_PUSHPAGE));
+	void *p = kmem_cache_alloc(zio_buf_cache[c], KM_PUSHPAGE);
+	zio_buf_put_canary(p, size, zio_buf_cache, c);
+	return (p);
 }
 
 /*
@@ -338,7 +387,9 @@ zio_data_buf_alloc(size_t size)
 
 	VERIFY3U(c, <, SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);
 
-	return (kmem_cache_alloc(zio_data_buf_cache[c], KM_PUSHPAGE));
+	void *p = kmem_cache_alloc(zio_data_buf_cache[c], KM_PUSHPAGE);
+	zio_buf_put_canary(p, size, zio_data_buf_cache, c);
+	return (p);
 }
 
 void
@@ -351,6 +402,7 @@ zio_buf_free(void *buf, size_t size)
 	atomic_add_64(&zio_buf_cache_frees[c], 1);
 #endif
 
+	zio_buf_check_canary(buf, size, zio_buf_cache, c);
 	kmem_cache_free(zio_buf_cache[c], buf);
 }
 
@@ -361,6 +413,7 @@ zio_data_buf_free(void *buf, size_t size)
 
 	VERIFY3U(c, <, SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);
 
+	zio_buf_check_canary(buf, size, zio_data_buf_cache, c);
 	kmem_cache_free(zio_data_buf_cache[c], buf);
 }