4.9-stable review patch. If anyone has any objections, please let me know.
------------------ From: Andrea Arcangeli <[email protected]> commit a7306c3436e9c8e584a4b9fad5f3dc91be2a6076 upstream. "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 f765f540598a ("ksm: prepare to new THP semantics") introduced 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! CPU: 1 PID: 76 Comm: ksmd Not tainted 4.9.3-200.fc25.x86_64 #1 RIP: do_page_add_anon_rmap+0x1c4/0x240 Call Trace: page_add_anon_rmap+0x18/0x20 try_to_merge_with_ksm_page+0x50b/0x780 ksm_scan_thread+0x1211/0x1410 ? prepare_to_wait_event+0x100/0x100 ? try_to_merge_with_ksm_page+0x780/0x780 kthread+0xd9/0xf0 ? kthread_park+0x60/0x60 ret_from_fork+0x25/0x30 Fixes: f765f54059 ("ksm: prepare to new THP semantics") Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Andrea Arcangeli <[email protected]> Reported-by: Federico Simoncelli <[email protected]> Acked-by: Kirill A. Shutemov <[email protected]> Cc: Hugh Dickins <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- mm/ksm.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) --- a/mm/ksm.c +++ b/mm/ksm.c @@ -1002,8 +1002,7 @@ static int try_to_merge_one_page(struct goto out; if (PageTransCompound(page)) { - err = split_huge_page(page); - if (err) + if (split_huge_page(page)) goto out_unlock; }
