mm: fault in __do_fault

DISCUSSION Dmitry Vyukov 3 participants 5 messages Feb 28, 2017 – Mar 3, 2017

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Dmitry Vyukov

author

2017-02-28 14:05 UTC #0 lore raw

Hello,

The following program triggers GPF in __do_fault:
https://gist.githubusercontent.com/dvyukov/27345737fca18d92ef761e7fa08aec9b/raw/d99d02511d0bf9a8d6f6bd9c79d373a26924e974/gistfile1.txt

general protection fault: 0000 [#1] SMP KASAN
Modules linked in:
CPU: 3 PID: 2955 Comm: a.out Not tainted 4.10.0+ #230
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff88006b4e2480 task.stack: ffff88006b0b0000
RIP: 0010:__read_once_size include/linux/compiler.h:247 [inline]
RIP: 0010:compound_head include/linux/page-flags.h:146 [inline]
RIP: 0010:trylock_page include/linux/pagemap.h:442 [inline]
RIP: 0010:lock_page include/linux/pagemap.h:452 [inline]
RIP: 0010:__do_fault+0x247/0x3e0 mm/memory.c:2898
RSP: 0000:ffff88006b0b7470 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 1ffff1000d616e91 RCX: 0000000000000000
RDX: 0000000000000004 RSI: dffffc0000000000 RDI: 0000000000000020
RBP: ffff88006b0b7550 R08: 0000000000000001 R09: 0000000000000001
R10: 0000000000000000 R11: 0000000000000004 R12: ffff88006b0b74e8
R13: 0000000000000000 R14: ffff88006b0b7528 R15: 0000000000000000
FS:  00007fef8bb55700(0000) GS:ffff88006d180000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020012fe0 CR3: 000000006a8dc000 CR4: 00000000001406e0
Call Trace:
 do_read_fault mm/memory.c:3266 [inline]
 do_fault+0x5ee/0x2080 mm/memory.c:3366
 handle_pte_fault mm/memory.c:3596 [inline]
 __handle_mm_fault+0x1062/0x2cb0 mm/memory.c:3710
 handle_mm_fault+0x1e2/0x480 mm/memory.c:3747
 __do_page_fault+0x4f6/0xb60 arch/x86/mm/fault.c:1396
 trace_do_page_fault+0x141/0x6c0 arch/x86/mm/fault.c:1489
 do_async_page_fault+0x72/0xc0 arch/x86/kernel/kvm.c:264
 async_page_fault+0x28/0x30 arch/x86/entry/entry_64.S:1014
RIP: 0033:0x43dda0
RSP: 002b:00007fef8bb54878 EFLAGS: 00010202
RAX: 00007fef8bb548b0 RBX: 0000000000000000 RCX: 000000000000000e
RDX: 0000000000000400 RSI: 0000000020012fe0 RDI: 00007fef8bb548b0
RBP: 00007fef8bb54cd0 R08: 0000000000000400 R09: 00007fef8bb54d10
R10: 00007fef8bb559d0 R11: 0000000000000202 R12: 0000000000000000
R13: 0000000000000000 R14: 00007fef8bb559c0 R15: 00007fef8bb55700
Code: 00 e8 0e 35 b8 ff e8 79 09 a2 02 4c 89 e2 49 8d 7f 20 48 b8 00
00 00 00 00 fc ff df 48 c1 ea 03 c6 04 02 00 48 89 fa 48 c1 ea 03 <80>
3c 02 00 0f 85 ec 00 00 00 4c 89 e2 48 b8 00 00 00 00 00 fc
RIP: __read_once_size include/linux/compiler.h:247 [inline] RSP:
ffff88006b0b7470
RIP: compound_head include/linux/page-flags.h:146 [inline] RSP: ffff88006b0b7470
RIP: trylock_page include/linux/pagemap.h:442 [inline] RSP: ffff88006b0b7470
RIP: lock_page include/linux/pagemap.h:452 [inline] RSP: ffff88006b0b7470
RIP: __do_fault+0x247/0x3e0 mm/memory.c:2898 RSP: ffff88006b0b7470
---[ end trace d24250eecdbb8a34 ]---

On commit e5d56efc97f8240d0b5d66c03949382b6d7e5570

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Kirill A. Shutemov

2017-02-28 15:40 UTC #1 lore raw

Quoted 42 lines · Dmitry Vyukov

Hello,

The following program triggers GPF in __do_fault:
https://gist.githubusercontent.com/dvyukov/27345737fca18d92ef761e7fa08aec9b/raw/d99d02511d0bf9a8d6f6bd9c79d373a26924e974/gistfile1.txt

general protection fault: 0000 [#1] SMP KASAN
Modules linked in:
CPU: 3 PID: 2955 Comm: a.out Not tainted 4.10.0+ #230
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff88006b4e2480 task.stack: ffff88006b0b0000
RIP: 0010:__read_once_size include/linux/compiler.h:247 [inline]
RIP: 0010:compound_head include/linux/page-flags.h:146 [inline]
RIP: 0010:trylock_page include/linux/pagemap.h:442 [inline]
RIP: 0010:lock_page include/linux/pagemap.h:452 [inline]
RIP: 0010:__do_fault+0x247/0x3e0 mm/memory.c:2898
RSP: 0000:ffff88006b0b7470 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 1ffff1000d616e91 RCX: 0000000000000000
RDX: 0000000000000004 RSI: dffffc0000000000 RDI: 0000000000000020
RBP: ffff88006b0b7550 R08: 0000000000000001 R09: 0000000000000001
R10: 0000000000000000 R11: 0000000000000004 R12: ffff88006b0b74e8
R13: 0000000000000000 R14: ffff88006b0b7528 R15: 0000000000000000
FS:  00007fef8bb55700(0000) GS:ffff88006d180000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020012fe0 CR3: 000000006a8dc000 CR4: 00000000001406e0
Call Trace:
 do_read_fault mm/memory.c:3266 [inline]
 do_fault+0x5ee/0x2080 mm/memory.c:3366
 handle_pte_fault mm/memory.c:3596 [inline]
 __handle_mm_fault+0x1062/0x2cb0 mm/memory.c:3710
 handle_mm_fault+0x1e2/0x480 mm/memory.c:3747
 __do_page_fault+0x4f6/0xb60 arch/x86/mm/fault.c:1396
 trace_do_page_fault+0x141/0x6c0 arch/x86/mm/fault.c:1489
 do_async_page_fault+0x72/0xc0 arch/x86/kernel/kvm.c:264
 async_page_fault+0x28/0x30 arch/x86/entry/entry_64.S:1014
RIP: 0033:0x43dda0
RSP: 002b:00007fef8bb54878 EFLAGS: 00010202
RAX: 00007fef8bb548b0 RBX: 0000000000000000 RCX: 000000000000000e
RDX: 0000000000000400 RSI: 0000000020012fe0 RDI: 00007fef8bb548b0
RBP: 00007fef8bb54cd0 R08: 0000000000000400 R09: 00007fef8bb54d10
R10: 00007fef8bb559d0 R11: 0000000000000202 R12: 0000000000000000
R13: 0000000000000000 R14: 00007fef8bb559c0 R15: 00007fef8bb55700
Code: 00 e8 0e 35 b8 ff e8 79 09 a2 02 4c 89 e2 49 8d 7f 20 48 b8 00

Quoted 8 lines

00 00 00 00 fc ff df 48 c1 ea 03 c6 04 02 00 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 ec 00 00 00 4c 89 e2 48 b8 00 00 00 00 00 fc

RIP: __read_once_size include/linux/compiler.h:247 [inline] RSP:
ffff88006b0b7470
RIP: compound_head include/linux/page-flags.h:146 [inline] RSP: ffff88006b0b7470
RIP: trylock_page include/linux/pagemap.h:442 [inline] RSP: ffff88006b0b7470
RIP: lock_page include/linux/pagemap.h:452 [inline] RSP: ffff88006b0b7470
RIP: __do_fault+0x247/0x3e0 mm/memory.c:2898 RSP: ffff88006b0b7470

__do_fault() expects to see a page if ->fault() returns zero, but handle_userfault() mess this up, making shmem_fault() return 0 without setting vmf->page.

I'm not entirely sure it's fully correct, but the patch below fixes the crash for me.

Andrea, does it look okay for you?

fs/userfaultfd.c +1 -1

index 625b7285a37b..56f61f1a1dc1 100644
@@ -489,7 +489,7 @@ int handle_userfault(struct vm_fault *vmf, unsigned long reason)
                         * in such case.
                         */
                        down_read(&mm->mmap_sem);
-                       ret = 0;
+                       ret = VM_FAULT_NOPAGE;
                }
        }

Signature · 7 lines

 Kirill A. Shutemov

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Andrea Arcangeli

2017-02-28 15:55 UTC #2 lore raw

Quoted 7 lines · Kirill A. Shutemov

Andrea, does it look okay for you?
fs/userfaultfd.c +0 -0
index 625b7285a37b..56f61f1a1dc1 100644

fs/userfaultfd.c int handle_userfault(struct vm_fault *vmf, unsigned long reason) · line 494

Quoted 7 lines

fs/userfaultfd.c +1 -1

@@ -489,7 +489,7 @@ int handle_userfault(struct vm_fault *vmf, unsigned long reason)
                         * in such case.
                         */
                        down_read(&mm->mmap_sem);
-                       ret = 0;
+                       ret = VM_FAULT_NOPAGE;
                }
        }

Yes, I did the same fix.

Andrea Arcangeli

2017-02-28 15:42 UTC #3 lore raw

In reply to Dmitry Vyukov

Hello Dmitry,

Quoted 4 lines · Dmitry Vyukov

Hello,
The following program triggers GPF in __do_fault:
https://gist.githubusercontent.com/dvyukov/27345737fca18d92ef761e7fa08aec9b/raw/d99d02511d0bf9a8d6f6bd9c79d373a26924e974/gistfile1.txt

Can you verify this fix:

Dmitry Vyukov

author

2017-03-03 07:42 UTC #4 lore raw

Quoted 9 lines · Andrea Arcangeli

Hello Dmitry,
Quoted 4 lines · Dmitry Vyukov
Hello,
The following program triggers GPF in __do_fault:
https://gist.githubusercontent.com/dvyukov/27345737fca18d92ef761e7fa08aec9b/raw/d99d02511d0bf9a8d6f6bd9c79d373a26924e974/gistfile1.txt
Can you verify this fix:

Applied the patch on bots.

Quoted 37 lines

From a65381bc86d2963713b6a9c4a73cded7dd184282 Mon Sep 17 00:00:00 2001
From: Andrea Arcangeli <[email protected]>
Date: Tue, 28 Feb 2017 16:36:59 +0100
Subject: [PATCH 1/1] userfaultfd: shmem: __do_fault requires VM_FAULT_NOPAGE
__do_fault assumes vmf->page has been initialized and is valid if VM_FAULT_NOPAGE is not returned by vma->vm_ops->fault(vma, vmf).
handle_userfault() in turn should return VM_FAULT_NOPAGE if it doesn't return VM_FAULT_SIGBUS or VM_FAULT_RETRY (the other two possibilities).
This VM_FAULT_NOPAGE case is only invoked when signal are pending and it didn't matter for anonymous memory before. It only started to matter since shmem was introduced. hugetlbfs also takes a different path and doesn't exercise __do_fault.
Signed-off-by Andrea Arcangeli <[email protected]>
---
 fs/userfaultfd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
fs/userfaultfd.c +1 -2
index fb6d02b..de28f43 100644
@@ -500,7 +500,7 @@ int handle_userfault(struct vm_fault *vmf, unsigned long reason)
                         * in such case.
                         */
                        down_read(&mm->mmap_sem);
-                       ret = 0;
+                       ret = VM_FAULT_NOPAGE;
                }
        }
 
 
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Quoted 3 lines

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