Patch "mm/page_ext.c: check if page_ext is not prepared" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

mm/page_ext.c: check if page_ext is not prepared

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-page_ext.c-check-if-page_ext-is-not-prepared.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jaewon Kim <[email protected]>
Date: Wed, 15 Nov 2017 17:39:07 -0800
Subject: mm/page_ext.c: check if page_ext is not prepared

From: Jaewon Kim <[email protected]>

commit e492080e640c2d1235ddf3441cae634cfffef7e1 upstream.

online_page_ext() and page_ext_init() allocate page_ext for each section, but they do not allocate if the first PFN is !pfn_present(pfn) or !pfn_valid(pfn). Then section->page_ext remains as NULL. lookup_page_ext checks NULL only if CONFIG_DEBUG_VM is enabled. For a valid PFN, __set_page_owner will try to get page_ext through lookup_page_ext. Without CONFIG_DEBUG_VM lookup_page_ext will misuse NULL pointer as value 0. This incurrs invalid address access.

This is the panic example when PFN 0x100000 is not valid but PFN 0x13FC00 is being used for page_ext. section->page_ext is NULL, get_entry returned invalid page_ext address as 0x1DFA000 for a PFN 0x13FC00.

To avoid this panic, CONFIG_DEBUG_VM should be removed so that page_ext will be checked at all times.

  Unable to handle kernel paging request at virtual address 01dfa014
  ------------[ cut here ]------------
  Kernel BUG at ffffff80082371e0 [verbose debug info unavailable]
  Internal error: Oops: 96000045 [#1] PREEMPT SMP
  Modules linked in:
  PC is at __set_page_owner+0x48/0x78
  LR is at __set_page_owner+0x44/0x78
    __set_page_owner+0x48/0x78
    get_page_from_freelist+0x880/0x8e8
    __alloc_pages_nodemask+0x14c/0xc48
    __do_page_cache_readahead+0xdc/0x264
    filemap_fault+0x2ac/0x550
    ext4_filemap_fault+0x3c/0x58
    __do_fault+0x80/0x120
    handle_mm_fault+0x704/0xbb0
    do_page_fault+0x2e8/0x394
    do_mem_abort+0x88/0x124

Pre-4.7 kernels also need commit f86e4271978b ("mm: check the return value of lookup_page_ext for all call sites").

---
 mm/page_ext.c |    4 ----
 1 file changed, 4 deletions(-)

@@ -106,7 +106,6 @@ struct page_ext *lookup_page_ext(struct
 	struct page_ext *base;
 
 	base = NODE_DATA(page_to_nid(page))->node_page_ext;
-#ifdef CONFIG_DEBUG_VM
 	/*
 	 * The sanity checks the page allocator does upon freeing a
 	 * page can reach here before the page_ext arrays are

@@ -115,7 +114,6 @@ struct page_ext *lookup_page_ext(struct
 	 */
 	if (unlikely(!base))
 		return NULL;
-#endif
 	offset = pfn - round_down(node_start_pfn(page_to_nid(page)),
 					MAX_ORDER_NR_PAGES);
 	return base + offset;

@@ -180,7 +178,6 @@ struct page_ext *lookup_page_ext(struct
 {
 	unsigned long pfn = page_to_pfn(page);
 	struct mem_section *section = __pfn_to_section(pfn);
-#ifdef CONFIG_DEBUG_VM
 	/*
 	 * The sanity checks the page allocator does upon freeing a
 	 * page can reach here before the page_ext arrays are

@@ -189,7 +186,6 @@ struct page_ext *lookup_page_ext(struct
 	 */
 	if (!section->page_ext)
 		return NULL;
-#endif
 	return section->page_ext + pfn;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/mm-page_ext.c-check-if-page_ext-is-not-prepared.patch