[PATCH] mm: validate pmd after splitting

[Minchan Kim]

syzbot reported following.

general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
CPU: 1 PID: 6826 Comm: syz-executor142 Not tainted 5.9.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:__lock_acquire+0x84/0x2ae0 kernel/locking/lockdep.c:4296
Code: ff df 8a 04 30 84 c0 0f 85 e3 16 00 00 83 3d 56 58 35 08 00 0f 84 0e 17 00 00 83 3d 25 c7 f5 07 00 74 2c 4c 89 e8 48 c1 e8 03 <80> 3c 30 00 74 12 4c 89 ef e8 3e d1 5a 00 48 be 00 00 00 00 00 fc
RSP: 0018:ffffc90004b9f850 EFLAGS: 00010006
RAX: 0000000000000003 RBX: 0000000000000001 RCX: 0000000000000000
RDX: 0000000000000000 RSI: dffffc0000000000 RDI: 0000000000000018
RBP: ffffc90004b9f9a8 R08: 0000000000000001 R09: 0000000000000000
R10: fffffbfff131e2e6 R11: 0000000000000000 R12: ffff8880937161c0
R13: 0000000000000018 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000002638880(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000002100003f CR3: 00000000a49a2000 CR4: 00000000001506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 lock_acquire+0x140/0x6f0 kernel/locking/lockdep.c:5006
 __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
 _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
 spin_lock include/linux/spinlock.h:354 [inline]
 madvise_cold_or_pageout_pte_range+0x52f/0x25c0 mm/madvise.c:389
 walk_pmd_range mm/pagewalk.c:89 [inline]
 walk_pud_range mm/pagewalk.c:160 [inline]
 walk_p4d_range mm/pagewalk.c:193 [inline]
 walk_pgd_range mm/pagewalk.c:229 [inline]
 __walk_page_range+0xe7b/0x1da0 mm/pagewalk.c:331
 walk_page_range+0x2c3/0x5c0 mm/pagewalk.c:427
 madvise_pageout_page_range mm/madvise.c:521 [inline]
 madvise_pageout mm/madvise.c:557 [inline]
 madvise_vma mm/madvise.c:946 [inline]
 do_madvise+0x12d0/0x2090 mm/madvise.c:1145
 __do_sys_madvise mm/madvise.c:1171 [inline]
 __se_sys_madvise mm/madvise.c:1169 [inline]
 __x64_sys_madvise+0x76/0x80 mm/madvise.c:1169
 do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

In case of split page of file-backed THP, it zaps the pmd instead of
remapping of sub-pages so need to check pmd validity after split.

Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reported-by: syzbot+ecf80462cb7d5d552bc7@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 1a4e58cce84e ("mm: introduce MADV_PAGEOUT")
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/madvise.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index d4aa5f776543..0e0d61003fc6 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -381,9 +381,9 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 		return 0;
 	}
 
+regular_page:
 	if (pmd_trans_unstable(pmd))
 		return 0;
-regular_page:
 #endif
 	tlb_change_page_size(tlb, PAGE_SIZE);
 	orig_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
-- 
2.28.0.618.gf4bc123cb7-goog