Re: KCSAN: data-race in __rb_rotate_set_parents / vm_area_dup

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Oct 24, 2019 at 08:59:50PM +0200, Marco Elver wrote:
> On Thu, 24 Oct 2019 at 18:25, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Thu, Oct 24, 2019 at 09:07:08AM -0700, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following crash on:
> > >
> > > HEAD commit:    05f22368 x86, kcsan: Enable KCSAN for x86
> > > git tree:       https://github.com/google/ktsan.git kcsan
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1060c47b600000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=87d111955f40591f
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=c034966b0b02f94f7f34
> > > 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+c034966b0b02f94f7f34@syzkaller.appspotmail.com
> > >
> > > ==================================================================
> > > BUG: KCSAN: data-race in __rb_rotate_set_parents / vm_area_dup
> > >
> > > read to 0xffff88811eef53e8 of 200 bytes by task 7738 on cpu 0:
> > >  vm_area_dup+0x70/0xf0 kernel/fork.c:359
> > >  __split_vma+0x88/0x350 mm/mmap.c:2678
> > >  __do_munmap+0xb02/0xb60 mm/mmap.c:2803
> > >  do_munmap mm/mmap.c:2856 [inline]
> > >  mmap_region+0x165/0xd50 mm/mmap.c:1749
> > >  do_mmap+0x6d4/0xba0 mm/mmap.c:1577
> > >  do_mmap_pgoff include/linux/mm.h:2353 [inline]
> > >  vm_mmap_pgoff+0x12d/0x190 mm/util.c:496
> > >  ksys_mmap_pgoff+0x2d8/0x420 mm/mmap.c:1629
> > >  __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+0x91/0xc0 arch/x86/kernel/sys_x86_64.c:91
> > >  do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > >
> > > write to 0xffff88811eef5440 of 8 bytes by task 7737 on cpu 1:
> > >  __rb_rotate_set_parents+0x4d/0xf0 lib/rbtree.c:79
> > >  __rb_insert lib/rbtree.c:215 [inline]
> > >  __rb_insert_augmented+0x109/0x370 lib/rbtree.c:459
> > >  rb_insert_augmented include/linux/rbtree_augmented.h:50 [inline]
> > >  rb_insert_augmented_cached include/linux/rbtree_augmented.h:60 [inline]
> > >  vma_interval_tree_insert+0x196/0x230 mm/interval_tree.c:23
> > >  __vma_link_file+0xd9/0x110 mm/mmap.c:634
> > >  __vma_adjust+0x1ac/0x12a0 mm/mmap.c:842
> > >  vma_adjust include/linux/mm.h:2276 [inline]
> > >  __split_vma+0x208/0x350 mm/mmap.c:2707
> > >  split_vma+0x73/0xa0 mm/mmap.c:2736
> > >  mprotect_fixup+0x43f/0x510 mm/mprotect.c:413
> > >  do_mprotect_pkey+0x3eb/0x660 mm/mprotect.c:553
> > >  __do_sys_mprotect mm/mprotect.c:578 [inline]
> > >  __se_sys_mprotect mm/mprotect.c:575 [inline]
> > >  __x64_sys_mprotect+0x51/0x70 mm/mprotect.c:575
> > >  do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >
> > What is this thing trying to tell me? That the copy on alloc is racy,
> > because at that point the object isn't exposed yet.
> 
> In vm_area_dup, *orig is being concurrently modified while being
> copied into *new.

I'm not sure how it thinks there is concurrency here though;
__split_vma() *should* be holding mmap_sem for writing.

But yes, at least all the rb-tree and list crud should be re-initialized
for the object after copy.