[PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_SG

[Mikulas Patocka <mpatocka@redhat.com>]

On Thu, 19 Apr 2018, Michael S. Tsirkin wrote:

> Maybe make it conditional on CONFIG_DEBUG_SG too?
> Otherwise I think you just trigger a hard to debug memory corruption.

OK, here I resend the patch with CONFIG_DEBUG_SG. With CONFIG_DEBUG_SG, 
the DMA API will print a stacktrace where the misuse happened, so it's 
much easier to debug than with CONFIG_DEBUG_VM.

Fedora doesn't use CONFIG_DEBUG_SG in its default kernel (it only uses it 
in the debugging kernel), so users won't be hurt by this.



From: Mikulas Patocka <mpatocka@redhat.com>
Subject: [PATCH] kvmalloc: always use vmalloc if CONFIG_DEBUG_SG

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 vmalloc falls back to kmalloc
only if memory is fragmented.

In order to detect these bugs reliably I submit this patch that changes
kvmalloc to always use vmalloc if CONFIG_DEBUG_SG is turned on.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 mm/util.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-2.6/mm/util.c
===================================================================
--- linux-2.6.orig/mm/util.c	2018-04-18 15:46:23.000000000 +0200
+++ linux-2.6/mm/util.c	2018-04-19 23:14:14.000000000 +0200
@@ -395,6 +395,7 @@ EXPORT_SYMBOL(vm_mmap);
  */
 void *kvmalloc_node(size_t size, gfp_t flags, int node)
 {
+#ifndef CONFIG_DEBUG_SG
 	gfp_t kmalloc_flags = flags;
 	void *ret;
 
@@ -426,6 +427,7 @@ void *kvmalloc_node(size_t size, gfp_t f
 	 */
 	if (ret || size <= PAGE_SIZE)
 		return ret;
+#endif
 
 	return __vmalloc_node_flags_caller(size, node, flags,
 			__builtin_return_address(0));