Patch "mm: fix devm_memremap_pages crash, use mem_hotplug_{begin, done}" has been added to the 4.9-stable tree

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

    mm: fix devm_memremap_pages crash, use mem_hotplug_{begin, done}

to the 4.9-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-fix-devm_memremap_pages-crash-use-mem_hotplug_-begin-done.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Dan Williams <[email protected]>
Date: Tue, 10 Jan 2017 16:57:36 -0800
Subject: mm: fix devm_memremap_pages crash, use mem_hotplug_{begin, done}

From: Dan Williams <[email protected]>

commit f931ab479dd24cf7a2c6e2df19778406892591fb upstream.

Both arch_add_memory() and arch_remove_memory() expect a single threaded context.

For example, arch/x86/mm/init_64.c::kernel_physical_mapping_init() does not hold any locks over this check and branch:

    if (pgd_val(*pgd)) {
    	pud = (pud_t *)pgd_page_vaddr(*pgd);
    	paddr_last = phys_pud_init(pud, __pa(vaddr),
    				   __pa(vaddr_end),
    				   page_size_mask);
    	continue;
    }

    pud = alloc_low_page();
    paddr_last = phys_pud_init(pud, __pa(vaddr), __pa(vaddr_end),
    			   page_size_mask);

The result is that two threads calling devm_memremap_pages() simultaneously can end up colliding on pgd initialization. This leads to crash signatures like the following where the loser of the race initializes the wrong pgd entry:

    BUG: unable to handle kernel paging request at ffff888ebfff0000
    IP: memcpy_erms+0x6/0x10
    PGD 2f8e8fc067 PUD 0 /* <---- Invalid PUD */
    Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
    CPU: 54 PID: 3818 Comm: systemd-udevd Not tainted 4.6.7+ #13
    task: ffff882fac290040 ti: ffff882f887a4000 task.ti: ffff882f887a4000
    RIP: memcpy_erms+0x6/0x10
    [..]
    Call Trace:
      ? pmem_do_bvec+0x205/0x370 [nd_pmem]
      ? blk_queue_enter+0x3a/0x280
      pmem_rw_page+0x38/0x80 [nd_pmem]
      bdev_read_page+0x84/0xb0

Hold the standard memory hotplug mutex over calls to arch_{add,remove}_memory().

---
 kernel/memremap.c |    4 ++++
 1 file changed, 4 insertions(+)

@@ -246,7 +246,9 @@ static void devm_memremap_pages_release(
 	/* pages are dead and unused, undo the arch mapping */
 	align_start = res->start & ~(SECTION_SIZE - 1);
 	align_size = ALIGN(resource_size(res), SECTION_SIZE);
+	mem_hotplug_begin();
 	arch_remove_memory(align_start, align_size);
+	mem_hotplug_done();
 	untrack_pfn(NULL, PHYS_PFN(align_start), align_size);
 	pgmap_radix_release(res);
 	dev_WARN_ONCE(dev, pgmap->altmap && pgmap->altmap->alloc,

@@ -358,7 +360,9 @@ void *devm_memremap_pages(struct device
 	if (error)
 		goto err_pfn_remap;
 
+	mem_hotplug_begin();
 	error = arch_add_memory(nid, align_start, align_size, true);
+	mem_hotplug_done();
 	if (error)
 		goto err_add_memory;
 

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

queue-4.9/mm-fix-devm_memremap_pages-crash-use-mem_hotplug_-begin-done.patch