[PATCH 1/1] ksm: prevent crash after write_protect_page fails

[PATCH 1/1] ksm: prevent crash after write_protect_page fails

[Andrea Arcangeli]

"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@redhat.com>
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 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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Re: [PATCH 1/1] ksm: prevent crash after write_protect_page fails

[Kirill A. Shutemov]

On Sat, May 13, 2017 at 03:10:40PM +0200, Andrea Arcangeli wrote:
> "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@redhat.com>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov

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