[PATCH] x86/mm: fix a crash with kmemleak_scan()

[Qian Cai <cai@lca.pw>]

The first kmemleak_scan() after boot would trigger a crash below because

kernel_init
  free_initmem
    mem_encrypt_free_decrypted_mem
      free_init_pages

unmapped some memory inside the .bss with DEBUG_PAGEALLOC=y. Since
kmemleak_init() will register the .data/.bss sections (only register
.data..ro_after_init if not within .data) and then kmemleak_scan() will
scan those address and dereference them looking for pointer referencing.
If free_init_pages() free and unmap pages in those sections,
kmemleak_scan() will trigger a crash if referencing one of those
addresses.

BUG: unable to handle kernel paging request at ffffffffbd402000
CPU: 12 PID: 325 Comm: kmemleak Not tainted 5.1.0-rc4+ #4
RIP: 0010:scan_block+0x58/0x160
Call Trace:
 scan_gray_list+0x1d9/0x280
 kmemleak_scan+0x485/0xad0
 kmemleak_scan_thread+0x9f/0xc4
 kthread+0x1d2/0x1f0
 ret_from_fork+0x35/0x40

Since kmemleak_free_part() is tolerant to unknown objects (not tracked by
kmemleak), it is fine to call it from free_init_pages() even if not all
address ranges passed to this function are known to kmemleak.

Signed-off-by: Qian Cai <cai@lca.pw>
---
 arch/x86/mm/init.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index f905a2371080..8dacdb96899e 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -5,6 +5,7 @@
 #include <linux/memblock.h>
 #include <linux/swapfile.h>
 #include <linux/swapops.h>
+#include <linux/kmemleak.h>
 
 #include <asm/set_memory.h>
 #include <asm/e820/api.h>
@@ -766,6 +767,11 @@ void free_init_pages(const char *what, unsigned long begin, unsigned long end)
 	if (debug_pagealloc_enabled()) {
 		pr_info("debug: unmapping init [mem %#010lx-%#010lx]\n",
 			begin, end - 1);
+		/*
+		 * Inform kmemleak about the hole in the memory since the
+		 * corresponding pages will be unmapped.
+		 */
+		kmemleak_free_part((void *)begin, end - begin);
 		set_memory_np(begin, (end - begin) >> PAGE_SHIFT);
 	} else {
 		/*
-- 
2.20.1 (Apple Git-117)