"err" needs to be left set to -EFAULT if split_huge_page
succeeds. Otherwise if "err" gets clobbered with zero and
write_protect_page fails, try_to_merge_one_page() will succeed instead
of returning -EFAULT and then try_to_merge_with_ksm_page() will
continue thinking kpage is a PageKsm when in fact it's still an
anonymous page. Eventually it'll crash in page_add_anon_rmap.
This has been reproduced on Fedora25 kernel but I can reproduce with
upstream too.
The bug was introduced in commit
f765f540598a129dc4bb6f698fd4acc92f296c14 in v4.5.
page:fffff67546ce1cc0 count:4 mapcount:2 mapping:ffffa094551e36e1 index:0x7f0f46673
flags: 0x2ffffc0004007c(referenced|uptodate|dirty|lru|active|swapbacked)
page dumped because: VM_BUG_ON_PAGE(!PageLocked(page))
page->mem_cgroup:ffffa09674bf0000
------------[ cut here ]------------
kernel BUG at mm/rmap.c:1222!
invalid opcode: 0000 [#1] SMP
CPU: 1 PID: 76 Comm: ksmd Not tainted 4.9.3-200.fc25.x86_64 #1
task: ffffa0968be65b80 task.stack: ffffc0e941b3c000
RIP: 0010:[<ffffffff9a20ac94>] [<ffffffff9a20ac94>] do_page_add_anon_rmap+0x1c4/0x240
RSP: 0018:ffffc0e941b3fd48 EFLAGS: 00010282
RAX: 0000000000000021 RBX: fffff67546ce1cc0 RCX: 0000000000000006
RDX: 0000000000000000 RSI: 0000000000000246 RDI: ffffa0969ec4e0a0
RBP: ffffc0e941b3fd70 R08: 0000000000018a84 R09: 0000000000000005
R10: fffff6754f87de00 R11: 000000000000049f R12: 00007fe6f5f67000
R13: ffffa094990ecc00 R14: 0000000000000000 R15: ffffa093c369c480
FS: 0000000000000000(0000) GS:ffffa0969ec40000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f4c21c221f0 CR3: 0000000152e07000 CR4: 00000000000426e0
Stack:
fffff675470fd9c0 00007fe6f5f67000 fffff67540f8a900 ffffa094990ecc00
ffffa093c369c480 ffffc0e941b3fd80 ffffffff9a20ad28 ffffc0e941b3fe00
ffffffff9a228b4b 80000001c3f67807 0000000000000000 80000001c3f67805
Call Trace:
[<ffffffff9a20ad28>] page_add_anon_rmap+0x18/0x20
[<ffffffff9a228b4b>] try_to_merge_with_ksm_page+0x50b/0x780
[<ffffffff9a229fd1>] ksm_scan_thread+0x1211/0x1410
[<ffffffff9a0e7270>] ? prepare_to_wait_event+0x100/0x100
[<ffffffff9a228dc0>] ? try_to_merge_with_ksm_page+0x780/0x780
[<ffffffff9a0c2569>] kthread+0xd9/0xf0
[<ffffffff9a0c2490>] ? kthread_park+0x60/0x60
[<ffffffff9a81be55>] ret_from_fork+0x25/0x30
Reported-by: Federico Simoncelli <fsimonce@xxxxxxxxxx>
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
mm/ksm.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index b53fd58..fc0c73b 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1185,8 +1185,7 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
goto out;
if (PageTransCompound(page)) {
- err = split_huge_page(page);
- if (err)
+ if (split_huge_page(page))
goto out_unlock;
}
--
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