mm/vmalloc: don't allow VM_NO_GUARD on vmap()

The vmalloc guard pages are added on top of each allocation, thereby
isolating any two allocations from one another.  The top guard of the
lower allocation is the bottom guard guard of the higher allocation etc.

Therefore VM_NO_GUARD is dangerous; it breaks the basic premise of
isolating separate allocations.

There are only two in-tree users of this flag, neither of which use it
through the exported interface.  Ensure it stays this way.

Link: https://lkml.kernel.org/r/YUMfdA36fuyZ+/xt@hirez.programming.kicks-ass.netSigned-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/vmalloc.h

@@ -22,7 +22,7 @@ struct notifier_block;		/* in notifier.h */
 #define VM_USERMAP		0x00000008	/* suitable for remap_vmalloc_range */
 #define VM_DMA_COHERENT		0x00000010	/* dma_alloc_coherent */
 #define VM_UNINITIALIZED	0x00000020	/* vm_struct is not fully initialized */
-#define VM_NO_GUARD		0x00000040      /* don't add guard page */
+#define VM_NO_GUARD		0x00000040      /* ***DANGEROUS*** don't add guard page */
 #define VM_KASAN		0x00000080      /* has allocated kasan shadow memory */
 #define VM_FLUSH_RESET_PERMS	0x00000100	/* reset direct map and flush TLB on unmap, can't be freed in atomic context */
 #define VM_MAP_PUT_PAGES	0x00000200	/* put pages and free array in vfree */

mm/vmalloc.c

@@ -2743,6 +2743,13 @@ void *vmap(struct page **pages, unsigned int count,
 
 	might_sleep();
 
+	/*
+	 * Your top guard is someone else's bottom guard. Not having a top
+	 * guard compromises someone else's mappings too.
+	 */
+	if (WARN_ON_ONCE(flags & VM_NO_GUARD))
+		flags &= ~VM_NO_GUARD;
+
 	if (count > totalram_pages())
 		return NULL;