[PATCH 3.11 040/233] thp: fix copy_page_rep GPF by testing is_huge_zero_pmd once only

From: Luis Henriques
Date: Fri Feb 07 2014 - 08:16:28 EST -stable review patch. If anyone has any objections, please let me know.


From: Hugh Dickins <hughd@xxxxxxxxxx>

commit eecc1e426d681351a6026a7d3e7d225f38955b6c upstream.

We see General Protection Fault on RSI in copy_page_rep: that RSI is
what you get from a NULL struct page pointer.

RIP: 0010:[<ffffffff81154955>] [<ffffffff81154955>] copy_page_rep+0x5/0x10
RSP: 0000:ffff880136e15c00 EFLAGS: 00010286
RAX: ffff880000000000 RBX: ffff880136e14000 RCX: 0000000000000200
RDX: 6db6db6db6db6db7 RSI: db73880000000000 RDI: ffff880dd0c00000
RBP: ffff880136e15c18 R08: 0000000000000200 R09: 000000000005987c
R10: 000000000005987c R11: 0000000000000200 R12: 0000000000000001
R13: ffffea00305aa000 R14: 0000000000000000 R15: 0000000000000000
FS: 00007f195752f700(0000) GS:ffff880c7fc20000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000093010000 CR3: 00000001458e1000 CR4: 00000000000027e0
Call Trace:

do_huge_pmd_wp_page() tests is_huge_zero_pmd(orig_pmd) four times: but
since shrink_huge_zero_page() can free the huge_zero_page, and we have
no hold of our own on it here (except where the fourth test holds
page_table_lock and has checked pmd_same), it's possible for it to
answer yes the first time, but no to the second or third test. Change
all those last three to tests for NULL page.

(Note: this is not the same issue as trinity's DEBUG_PAGEALLOC BUG
in copy_page_rep with RSI: ffff88009c422000, reported by Sasha Levin
in https://lkml.org/lkml/2013/3/29/103. I believe that one is due
to the source page being split, and a tail page freed, while copy
is in progress; and not a problem without DEBUG_PAGEALLOC, since
the pmd_same check will prevent a miscopy from being made visible.)

Fixes: 97ae17497e99 ("thp: implement refcounting for huge zero page")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[ luis: backported to 3.11: based on hughd's backport to 3.10.27 ]
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
mm/huge_memory.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 31950d6..5e91a87 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1170,7 +1170,7 @@ alloc:

if (unlikely(!new_page)) {
- if (is_huge_zero_pmd(orig_pmd)) {
+ if (!page) {
ret = do_huge_pmd_wp_zero_page_fallback(mm, vma,
address, pmd, orig_pmd, haddr);
} else {
@@ -1194,7 +1194,7 @@ alloc:
goto out;

- if (is_huge_zero_pmd(orig_pmd))
+ if (!page)
clear_huge_page(new_page, haddr, HPAGE_PMD_NR);
copy_user_huge_page(new_page, page, haddr, vma, HPAGE_PMD_NR);
@@ -1219,7 +1219,7 @@ alloc:
page_add_new_anon_rmap(new_page, vma, haddr);
set_pmd_at(mm, haddr, pmd, entry);
update_mmu_cache_pmd(vma, address, pmd);
- if (is_huge_zero_pmd(orig_pmd)) {
+ if (!page) {
add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
} else {

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