Message-ID: <20190902135254.GC2431@bombadil.infradead.org>
Date:   Mon, 2 Sep 2019 06:52:54 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     syzbot <syzbot+03ee87124ee05af991bd@...kaller.appspotmail.com>,
        hughd@...gle.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: KASAN: use-after-free Read in shmem_fault (2)

On Sat, Aug 31, 2019 at 12:58:26PM +0800, Hillf Danton wrote:
> On Fri, 30 Aug 2019 12:40:06 -0700
> > syzbot found the following crash on:
> > 
> > HEAD commit:    a55aa89a Linux 5.3-rc6
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=12f4beb6600000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=2a6a2b9826fdadf9
> > dashboard link: https://syzkaller.appspot.com/bug?extid=03ee87124ee05af991bd
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > 
> > Unfortunately, I don't have any reproducer for this crash yet.
> > 
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+03ee87124ee05af991bd@...kaller.appspotmail.com
> > 
> > ==================================================================
> > BUG: KASAN: use-after-free in perf_trace_lock_acquire+0x401/0x530  
> > include/trace/events/lock.h:13
> > Read of size 8 at addr ffff8880a5cf2c50 by task syz-executor.0/26173
> > 
> > CPU: 0 PID: 26173 Comm: syz-executor.0 Not tainted 5.3.0-rc6 #146
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
> > Google 01/01/2011
> > Call Trace:
> >   __dump_stack lib/dump_stack.c:77 [inline]
> >   dump_stack+0x172/0x1f0 lib/dump_stack.c:113
> >   print_address_description.cold+0xd4/0x306 mm/kasan/report.c:351
> >   __kasan_report.cold+0x1b/0x36 mm/kasan/report.c:482
> >   kasan_report+0x12/0x17 mm/kasan/common.c:618
> >   __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:132
> >   perf_trace_lock_acquire+0x401/0x530 include/trace/events/lock.h:13
> >   trace_lock_acquire include/trace/events/lock.h:13 [inline]
> >   lock_acquire+0x2de/0x410 kernel/locking/lockdep.c:4411
> >   __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
> >   _raw_spin_lock+0x2f/0x40 kernel/locking/spinlock.c:151
> >   spin_lock include/linux/spinlock.h:338 [inline]
> >   shmem_fault+0x5ec/0x7b0 mm/shmem.c:2034
> >   __do_fault+0x111/0x540 mm/memory.c:3083
> >   do_shared_fault mm/memory.c:3535 [inline]
> >   do_fault mm/memory.c:3613 [inline]
> >   handle_pte_fault mm/memory.c:3840 [inline]
> >   __handle_mm_fault+0x2adf/0x3f20 mm/memory.c:3964
> >   handle_mm_fault+0x1b5/0x6b0 mm/memory.c:4001
> >   do_user_addr_fault arch/x86/mm/fault.c:1441 [inline]
> >   __do_page_fault+0x536/0xdd0 arch/x86/mm/fault.c:1506
> >   do_page_fault+0x38/0x590 arch/x86/mm/fault.c:1530
> >   page_fault+0x39/0x40 arch/x86/entry/entry_64.S:1202
> > RIP: 0010:copy_user_generic_unrolled+0x89/0xc0  
> > arch/x86/lib/copy_user_64.S:91
> > Code: 38 4c 89 47 20 4c 89 4f 28 4c 89 57 30 4c 89 5f 38 48 8d 76 40 48 8d  
> > 7f 40 ff c9 75 b6 89 d1 83 e2 07 c1 e9 03 74 12 4c 8b 06 <4c> 89 07 48 8d  
> > 76 08 48 8d 7f 08 ff c9 75 ee 21 d2 74 10 89 d1 8a
> > RSP: 0018:ffff88806b927e18 EFLAGS: 00010202
> > RAX: 0000000000000001 RBX: 0000000000000008 RCX: 0000000000000001
> > RDX: 0000000000000000 RSI: ffff88806b927e80 RDI: 0000000020000000
> > RBP: ffff88806b927e50 R08: 0000000500000004 R09: ffffed100d724fd1
> > R10: ffffed100d724fd0 R11: ffff88806b927e87 R12: 0000000020000000
> > R13: ffff88806b927e80 R14: 0000000020000008 R15: 00007ffffffff000
> >   copy_to_user include/linux/uaccess.h:152 [inline]
> >   do_pipe2+0xec/0x160 fs/pipe.c:857
> >   __do_sys_pipe fs/pipe.c:878 [inline]
> >   __se_sys_pipe fs/pipe.c:876 [inline]
> >   __x64_sys_pipe+0x33/0x40 fs/pipe.c:876
> >   do_syscall_64+0xfd/0x6a0 arch/x86/entry/common.c:296
> >   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > RIP: 0033:0x459879
> > Code: fd b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7  
> > 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff  
> > ff 0f 83 cb b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> > RSP: 002b:00007f833e81fc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000016
> > RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 0000000000459879
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000020000000
> > RBP: 000000000075c118 R08: 0000000000000000 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000246 R12: 00007f833e8206d4
> > R13: 00000000004f5b47 R14: 00000000004db7b8 R15: 00000000ffffffff
> > 
> > Allocated by task 25774:
> >   save_stack+0x23/0x90 mm/kasan/common.c:69
> >   set_track mm/kasan/common.c:77 [inline]
> >   __kasan_kmalloc mm/kasan/common.c:493 [inline]
> >   __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:466
> >   kasan_slab_alloc+0xf/0x20 mm/kasan/common.c:501
> >   slab_post_alloc_hook mm/slab.h:520 [inline]
> >   slab_alloc mm/slab.c:3319 [inline]
> >   kmem_cache_alloc+0x121/0x710 mm/slab.c:3483
> >   shmem_alloc_inode+0x1c/0x50 mm/shmem.c:3630
> >   alloc_inode+0x68/0x1e0 fs/inode.c:227
> >   new_inode_pseudo+0x19/0xf0 fs/inode.c:916
> >   new_inode+0x1f/0x40 fs/inode.c:945
> >   shmem_get_inode+0x84/0x7e0 mm/shmem.c:2228
> >   __shmem_file_setup.part.0+0x1e2/0x2b0 mm/shmem.c:3985
> >   __shmem_file_setup mm/shmem.c:3979 [inline]
> >   shmem_kernel_file_setup mm/shmem.c:4015 [inline]
> >   shmem_zero_setup+0xe1/0x4cc mm/shmem.c:4059
> >   mmap_region+0x13d5/0x1760 mm/mmap.c:1804
> >   do_mmap+0x82e/0x1090 mm/mmap.c:1561
> >   do_mmap_pgoff include/linux/mm.h:2374 [inline]
> >   vm_mmap_pgoff+0x1c5/0x230 mm/util.c:391
> >   ksys_mmap_pgoff+0xf7/0x630 mm/mmap.c:1611
> >   __do_sys_mmap arch/x86/kernel/sys_x86_64.c:100 [inline]
> >   __se_sys_mmap arch/x86/kernel/sys_x86_64.c:91 [inline]
> >   __x64_sys_mmap+0xe9/0x1b0 arch/x86/kernel/sys_x86_64.c:91
> >   do_syscall_64+0xfd/0x6a0 arch/x86/entry/common.c:296
> >   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > 
> > Freed by task 26359:
> >   save_stack+0x23/0x90 mm/kasan/common.c:69
> >   set_track mm/kasan/common.c:77 [inline]
> >   __kasan_slab_free+0x102/0x150 mm/kasan/common.c:455
> >   kasan_slab_free+0xe/0x10 mm/kasan/common.c:463
> >   __cache_free mm/slab.c:3425 [inline]
> >   kmem_cache_free+0x86/0x320 mm/slab.c:3693
> >   shmem_free_in_core_inode+0x63/0xb0 mm/shmem.c:3640
> >   i_callback+0x44/0x80 fs/inode.c:216
> >   __rcu_reclaim kernel/rcu/rcu.h:222 [inline]
> >   rcu_do_batch kernel/rcu/tree.c:2114 [inline]
> >   rcu_core+0x67f/0x1580 kernel/rcu/tree.c:2314
> >   rcu_core_si+0x9/0x10 kernel/rcu/tree.c:2323
> >   __do_softirq+0x262/0x98c kernel/softirq.c:292
> > 
> > The buggy address belongs to the object at ffff8880a5cf2a90
> >   which belongs to the cache shmem_inode_cache(17:syz0) of size 1192
> > The buggy address is located 448 bytes inside of
> >   1192-byte region [ffff8880a5cf2a90, ffff8880a5cf2f38)
> > The buggy address belongs to the page:
> > page:ffffea0002973c80 refcount:1 mapcount:0 mapping:ffff88808e1418c0  
> > index:0xffff8880a5cf2ffd
> > flags: 0x1fffc0000000200(slab)
> > raw: 01fffc0000000200 ffffea0002592ec8 ffffea0002492588 ffff88808e1418c0
> > raw: ffff8880a5cf2ffd ffff8880a5cf2040 0000000100000003 0000000000000000
> > page dumped because: kasan: bad access detected
> > 
> > Memory state around the buggy address:
> >   ffff8880a5cf2b00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >   ffff8880a5cf2b80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > > ffff8880a5cf2c00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >                                                   ^
> >   ffff8880a5cf2c80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >   ffff8880a5cf2d00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > ==================================================================
> 
> 
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2021,6 +2021,12 @@ static vm_fault_t shmem_fault(struct vm_
>  			shmem_falloc_waitq = shmem_falloc->waitq;
>  			prepare_to_wait(shmem_falloc_waitq, &shmem_fault_wait,
>  					TASK_UNINTERRUPTIBLE);
> +			/*
> +			 * it is not trivial to see what will take place after
> +			 * releasing i_lock and taking a nap, so hold inode to
> +			 * be on the safe side.

I think the comment could be improved.  How about:

			 * The file could be unmapped by another thread after
			 * releasing i_lock, and the inode then freed.  Hold
			 * a reference to the inode to prevent this.

> +			 */
> +			__iget(inode);
>  			spin_unlock(&inode->i_lock);
>  			schedule();
>  
> @@ -2034,6 +2040,7 @@ static vm_fault_t shmem_fault(struct vm_
>  			spin_lock(&inode->i_lock);
>  			finish_wait(shmem_falloc_waitq, &shmem_fault_wait);
>  			spin_unlock(&inode->i_lock);
> +			iput(inode);
>  			return ret;
>  		}
>  		spin_unlock(&inode->i_lock);
> 
> 

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