[PATCH v3] kvmalloc: always use vmalloc if CONFIG_DEBUG_SG

[Mikulas Patocka]

The kvmalloc function tries to use kmalloc and falls back to vmalloc if
kmalloc fails.

Unfortunatelly, some kernel code has bugs - it uses kvmalloc and then
uses DMA-API on the returned memory or frees it with kfree. Such bugs were
found in the virtio-net driver, dm-integrity or RHEL7 powerpc-specific
code.

These bugs are hard to reproduce because kvmalloc falls back to vmalloc
only if memory is fragmented.

In order to detect these bugs reliably I submit this patch that changes
kvmalloc to fall back to vmalloc with 1/2 probability if CONFIG_DEBUG_SG
is turned on. CONFIG_DEBUG_SG is used, because it makes the DMA API layer
verify the addresses passed to it, and so the user will get a reliable
stacktrace.

Some bugs (such as buffer overflows) are better detected
with kmalloc code, so we must test the kmalloc path too.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

---
 mm/util.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

Index: linux-2.6/mm/util.c
===================================================================
--- linux-2.6.orig/mm/util.c	2018-04-23 00:12:05.000000000 +0200
+++ linux-2.6/mm/util.c	2018-04-23 17:57:02.000000000 +0200
@@ -14,6 +14,7 @@
 #include <linux/hugetlb.h>
 #include <linux/vmalloc.h>
 #include <linux/userfaultfd_k.h>
+#include <linux/random.h>
 
 #include <asm/sections.h>
 #include <linux/uaccess.h>
@@ -404,6 +405,12 @@ void *kvmalloc_node(size_t size, gfp_t f
 	 */
 	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
 
+#ifdef CONFIG_DEBUG_SG
+	/* Catch bugs when the caller uses DMA API on the result of kvmalloc. */
+	if (!(prandom_u32_max(2) & 1))
+		goto do_vmalloc;
+#endif
+
 	/*
 	 * We want to attempt a large physically contiguous block first because
 	 * it is less likely to fragment multiple larger blocks and therefore
@@ -427,6 +434,9 @@ void *kvmalloc_node(size_t size, gfp_t f
 	if (ret || size <= PAGE_SIZE)
 		return ret;
 
+#ifdef CONFIG_DEBUG_SG
+do_vmalloc:
+#endif
 	return __vmalloc_node_flags_caller(size, node, flags,
 			__builtin_return_address(0));
 }