kexec: fix segmentation fault in kimage_add_entry

A segmentation fault can occur in kimage_add_entry in kexec.c when loading
a kernel image into memory.  The fault occurs because a page is requested
by calling kimage_alloc_page with gfp_mask GFP_KERNEL and the function may
actually return a page with gfp_mask GFP_HIGHUSER.  The high mem page is
returned because it was swapped with the kernel page due to the kernel
page being a page that will shortly be copied to.

This patch ensures that kimage_alloc_page returns a page that was created
with the correct gfp flags.

I have verified the change and fixed the whitespace damage of the original
patch.  Jonathan did a great job of tracking this down after he hit the
problem.  -- Eric
Signed-off-by: NJonathan Steel <jon.steel@esentire.com>
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>
Acked-by: NSimon Horman <horms@verge.net.au>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/kexec.c

@@ -753,8 +753,14 @@ static struct page *kimage_alloc_page(struct kimage *image,
 			*old = addr | (*old & ~PAGE_MASK);
 
 			/* The old page I have found cannot be a
-			 * destination page, so return it.
+			 * destination page, so return it if it's
+			 * gfp_flags honor the ones passed in.
 			 */
+			if (!(gfp_mask & __GFP_HIGHMEM) &&
+			    PageHighMem(old_page)) {
+				kimage_free_pages(old_page);
+				continue;
+			}
 			addr = old_addr;
 			page = old_page;
 			break;