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Subject[PATCH v2] mm: access to uninitialized struct page
Date
The following two bugs were reported by Fengguang Wu:

kernel reboot-without-warning in early-boot stage, last printk:
early console in setup code

http://lkml.kernel.org/r/20180418135300.inazvpxjxowogyge@wfg-t540p.sh.intel.com

And, also:
[per_cpu_ptr_to_phys] PANIC: early exception 0x0d
IP 10:ffffffffa892f15f error 0 cr2 0xffff88001fbff000

http://lkml.kernel.org/r/20180419013128.iurzouiqxvcnpbvz@wfg-t540p.sh.intel.com

Both of the problems are due to accessing uninitialized struct page from
trap_init(). We must first do mm_init() in order to initialize allocated
struct pages, and than we can access fields of any struct page that belongs
to memory that's been allocated.

Below is explanation of the root cause.

The issue arises in this stack:

start_kernel()
trap_init()
setup_cpu_entry_areas()
setup_cpu_entry_area(cpu)
get_cpu_gdt_paddr(cpu)
per_cpu_ptr_to_phys(addr)
pcpu_addr_to_page(addr)
virt_to_page(addr)
pfn_to_page(__pa(addr) >> PAGE_SHIFT)
The returned "struct page" is sometimes uninitialized, and thus
failing later when used. It turns out sometimes is because it depends
on KASLR.

When boot is failing we have this when pfn_to_page() is called:
kasrl: 0x000000000d600000
addr: ffffffff83e0d000
pa: 1040d000
pfn: 1040d
page: ffff88001f113340
page->flags ffffffffffffffff <- Uninitialized!

When boot is successful:
kaslr: 0x000000000a800000
addr: ffffffff83e0d000
pa: d60d000
pfn: d60d
page: ffff88001f05b340
page->flags 280000000000 <- Initialized!

Here are physical addresses that BIOS provided to us:
e820: BIOS-provided physical RAM map:
BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
BIOS-e820: [mem 0x0000000000100000-0x000000001ffdffff] usable
BIOS-e820: [mem 0x000000001ffe0000-0x000000001fffffff] reserved
BIOS-e820: [mem 0x00000000feffc000-0x00000000feffffff] reserved
BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved

In both cases, working and non-working the real physical address is
the same:

pa - kasrl = 0x2E0D000

The only thing that is different is PFN.

We initialize struct pages in four places:

1. Early in boot a small set of struct pages is initialized to fill
the first section, and lower zones.
2. During mm_init() we initialize "struct pages" for all the memory
that is allocated, i.e reserved in memblock.
3. Using on-demand logic when pages are allocated after mm_init call
4. After smp_init() when the rest free deferred pages are initialized.

The above path happens before deferred memory is initialized, and thus
it must be covered either by 1, 2 or 3.

So, lets check what PFNs are initialized after (1).

memmap_init_zone() is called for pfn ranges:
1 - 1000, and 1000 - 1ffe0, but it quits after reaching pfn 0x10000,
as it leaves the rest to be initialized as deferred pages.

In the working scenario pfn ended up being below 1000, but in the
failing scenario it is above. Hence, we must initialize this page in
(2). But trap_init() is called before mm_init().

The bug was introduced by "mm: initialize pages on demand during boot"
because we lowered amount of pages that is initialized in the step
(1). But, it still could happen, because the number of initialized
pages was a guessing.

The current fix moves trap_init() to be called after mm_init, but as
alternative, we could increase pgdat->static_init_pgcnt:
In free_area_init_node we can increase:
pgdat->static_init_pgcnt = min_t(unsigned long, PAGES_PER_SECTION,
pgdat->node_spanned_pages);
Instead of one PAGES_PER_SECTION, set several, so the text is
covered for all KASLR offsets. But, this would still be guessing.
Therefore, I prefer the current fix.

Fixes: c9e97a1997fb ("mm: initialize pages on demand during boot")

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
init/main.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/init/main.c b/init/main.c
index b795aa341a3a..870f75581cea 100644
--- a/init/main.c
+++ b/init/main.c
@@ -585,8 +585,8 @@ asmlinkage __visible void __init start_kernel(void)
setup_log_buf(0);
vfs_caches_init_early();
sort_main_extable();
- trap_init();
mm_init();
+ trap_init();

ftrace_init();

--
2.17.0
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