usercopy: Avoid HIGHMEM pfn warning

When running on a system with >512MB RAM with a 32-bit kernel built with:

	CONFIG_DEBUG_VIRTUAL=y
	CONFIG_HIGHMEM=y
	CONFIG_HARDENED_USERCOPY=y

all execve()s will fail due to argv copying into kmap()ed pages, and on
usercopy checking the calls ultimately of virt_to_page() will be looking
for "bad" kmap (highmem) pointers due to CONFIG_DEBUG_VIRTUAL=y:

 ------------[ cut here ]------------
 kernel BUG at ../arch/x86/mm/physaddr.c:83!
 invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
 CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.3.0-rc8 #6
 Hardware name: Dell Inc. Inspiron 1318/0C236D, BIOS A04 01/15/2009
 EIP: __phys_addr+0xaf/0x100
 ...
 Call Trace:
  __check_object_size+0xaf/0x3c0
  ? __might_sleep+0x80/0xa0
  copy_strings+0x1c2/0x370
  copy_strings_kernel+0x2b/0x40
  __do_execve_file+0x4ca/0x810
  ? kmem_cache_alloc+0x1c7/0x370
  do_execve+0x1b/0x20
  ...

The check is from arch/x86/mm/physaddr.c:

	VIRTUAL_BUG_ON((phys_addr >> PAGE_SHIFT) > max_low_pfn);

Due to the kmap() in fs/exec.c:

		kaddr = kmap(kmapped_page);
	...
	if (copy_from_user(kaddr+offset, str, bytes_to_copy)) ...

Now we can fetch the correct page to avoid the pfn check. In both cases,
hardened usercopy will need to walk the page-span checker (if enabled)
to do sanity checking.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: f5509cc1 ("mm: Hardened usercopy")
Cc: Matthew Wilcox <willy@infradead.org>
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/201909171056.7F2FFD17@keescook

mm/usercopy.c

@@ -11,6 +11,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/mm.h>
+#include <linux/highmem.h>
 #include <linux/slab.h>
 #include <linux/sched.h>
 #include <linux/sched/task.h>
@@ -227,7 +228,12 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
 	if (!virt_addr_valid(ptr))
 		return;
 
-	page = virt_to_head_page(ptr);
+	/*
+	 * When CONFIG_HIGHMEM=y, kmap_to_page() will give either the
+	 * highmem page or fallback to virt_to_page(). The following
+	 * is effectively a highmem-aware virt_to_head_page().
+	 */
+	page = compound_head(kmap_to_page((void *)ptr));
 
 	if (PageSlab(page)) {
 		/* Check slab allocator for flags and size. */