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Date:   Sat, 18 Jun 2022 01:14:08 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     syzbot <syzbot+4d875b4d2e2b60bae9b4@syzkaller.appspotmail.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        syzkaller-bugs@googlegroups.com, Yang Shi <shy828301@gmail.com>
Subject: Re: [syzbot] KASAN: null-ptr-deref Read in hugepage_vma_check
Message-ID: <Yq0Y0B1XGNIbCveO@casper.infradead.org>
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 <20220617165256.b5b9b257b30bc1dbb21fc8c6@linux-foundation.org>
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On Fri, Jun 17, 2022 at 04:52:56PM -0700, Andrew Morton wrote:
> On Fri, 17 Jun 2022 00:04:23 -0700 syzbot <syzbot+4d875b4d2e2b60bae9b4@syzkaller.appspotmail.com> wrote:
> 
> > Hello,
> > 
> > syzbot found the following issue on:
> > 
> > HEAD commit:    c6d7e3b385f1 Add linux-next specific files for 20220616
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=10d9fb1bf00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=9d495405e4a98620
> > dashboard link: https://syzkaller.appspot.com/bug?extid=4d875b4d2e2b60bae9b4
> > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > 
> > Unfortunately, I don't have any reproducer for this issue yet.
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+4d875b4d2e2b60bae9b4@syzkaller.appspotmail.com
> > 
> > ==================================================================
> > BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:71 [inline]
> > BUG: KASAN: null-ptr-deref in test_bit include/asm-generic/bitops/instrumented-non-atomic.h:134 [inline]
> > BUG: KASAN: null-ptr-deref in hugepage_vma_check+0x8e/0x750 mm/huge_memory.c:82
> > Read of size 8 at addr 00000000000005a8 by task syz-executor.5/21978
> 
> vma->vm_mm is NULL in show_smap->hugepage_vma_check().
> 
> Perhaps m_start() should be checking the mm after taking
> mmap_read_lock() instead of before?
> 
> Matthew, you mucked with it last ;) Can you please take a look?

I think this is Yang Shi's fault.  The last patch in that series which
touches this area does:

        seq_printf(m, "THPeligible:    %d\n",
-                  hugepage_vma_check(vma, vma->vm_flags, true));
+                  hugepage_vma_check(vma, vma->vm_flags, true, false));

and hugepage_vma_check() does:

        if ((vm_flags & VM_NOHUGEPAGE) ||
            test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
                return false;

Before those patches, it calls transparent_hugepage_active() which
does not (afaict) touch vma->vm_mm.

I haven't been paying attention to that patch series, so I don't have
a constructive suggestion for how to fix.

> > CPU: 0 PID: 21978 Comm: syz-executor.5 Not tainted 5.19.0-rc2-next-20220616-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Call Trace:
> >  <TASK>
> >  __dump_stack lib/dump_stack.c:88 [inline]
> >  dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
> >  kasan_report+0xbe/0x1f0 mm/kasan/report.c:495
> >  check_region_inline mm/kasan/generic.c:183 [inline]
> >  kasan_check_range+0x13d/0x180 mm/kasan/generic.c:189
> >  instrument_atomic_read include/linux/instrumented.h:71 [inline]
> >  test_bit include/asm-generic/bitops/instrumented-non-atomic.h:134 [inline]
> >  hugepage_vma_check+0x8e/0x750 mm/huge_memory.c:82
> >  show_smap+0x1c6/0x470 fs/proc/task_mmu.c:866
> >  traverse.part.0+0xcf/0x5f0 fs/seq_file.c:111
> >  traverse fs/seq_file.c:101 [inline]
> >  seq_read_iter+0x90f/0x1280 fs/seq_file.c:195
> >  seq_read+0x337/0x4b0 fs/seq_file.c:162
> >  do_loop_readv_writev fs/read_write.c:763 [inline]
> >  do_loop_readv_writev fs/read_write.c:750 [inline]
> >  do_iter_read+0x4f8/0x750 fs/read_write.c:805
> >  vfs_readv+0xe5/0x150 fs/read_write.c:923
> >  do_preadv fs/read_write.c:1015 [inline]
> >  __do_sys_preadv fs/read_write.c:1065 [inline]
> >  __se_sys_preadv fs/read_write.c:1060 [inline]
> >  __x64_sys_preadv+0x22b/0x310 fs/read_write.c:1060
> >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> >  entry_SYSCALL_64_after_hwframe+0x46/0xb0
> > RIP: 0033:0x7f5c1d889109
> > Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 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 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> > RSP: 002b:00007f5c1ea69168 EFLAGS: 00000246 ORIG_RAX: 0000000000000127
> > RAX: ffffffffffffffda RBX: 00007f5c1d99bf60 RCX: 00007f5c1d889109
> > RDX: 0000000000000001 RSI: 00000000200006c0 RDI: 0000000000000005
> > RBP: 00007f5c1d8e305d R08: 0000000000000000 R09: 0000000000000000
> > R10: 00000000fffffffe R11: 0000000000000246 R12: 0000000000000000
> > R13: 00007ffd8514b43f R14: 00007f5c1ea69300 R15: 0000000000022000
> >  </TASK>
> > ==================================================================
> > 
> > 
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@googlegroups.com.
> > 
> > syzbot will keep track of this issue. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.