Re: [syzbot] KASAN: use-after-free Read in get_wchan

[Palmer Dabbelt]

On Tue, 13 Apr 2021 22:56:13 PDT (-0700), dvyukov@xxxxxxxxxx wrote:
> On Wed, Apr 14, 2021 at 7:52 AM syzbot
> <syzbot+0806291048161061627c@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    b2b3d18f riscv: Make NUMA depend on MMU
> > git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git fixes
> > console output: https://syzkaller.appspot.com/x/log.txt?x=12b59d16d00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=81b3e7c68dad6e
> > dashboard link: https://syzkaller.appspot.com/bug?extid=0806291048161061627c
> > userspace arch: riscv64
> >
> > 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+0806291048161061627c@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > ==================================================================
> > BUG: KASAN: use-after-free in walk_stackframe arch/riscv/kernel/stacktrace.c:60 [inline]
> > BUG: KASAN: use-after-free in get_wchan+0x156/0x196 arch/riscv/kernel/stacktrace.c:136
> > Read of size 8 at addr ffffffe0058e3d90 by task syz-executor.0/4667
> >
> > CPU: 1 PID: 4667 Comm: syz-executor.0 Not tainted 5.12.0-rc5-syzkaller-00721-gb2b3d18fc20e #0
> > Hardware name: riscv-virtio,qemu (DT)
> > Call Trace:
> > [<ffffffe000009706>] walk_stackframe+0x0/0x23c arch/riscv/kernel/traps.c:201
> > [<ffffffe002a5f182>] dump_backtrace+0x40/0x4e arch/riscv/kernel/stacktrace.c:113
> > [<ffffffe002a5f1b2>] show_stack+0x22/0x2e arch/riscv/kernel/stacktrace.c:118
> > [<ffffffe002a68a3e>] __dump_stack lib/dump_stack.c:79 [inline]
> > [<ffffffe002a68a3e>] dump_stack+0x148/0x1d8 lib/dump_stack.c:120
> > [<ffffffe0003bc802>] print_address_description.constprop.0+0x52/0x31e mm/kasan/report.c:232
> > [<ffffffe0003bcd24>] __kasan_report mm/kasan/report.c:399 [inline]
> > [<ffffffe0003bcd24>] kasan_report+0x16e/0x18c mm/kasan/report.c:416
> > [<ffffffe0003bd588>] check_region_inline mm/kasan/generic.c:180 [inline]
> > [<ffffffe0003bd588>] __asan_load8+0x6e/0x80 mm/kasan/generic.c:253
> > [<ffffffe000009a98>] walk_stackframe arch/riscv/kernel/stacktrace.c:60 [inline]
>
> If it's walking the stack of another task, then it needs to use
> READ_ONCE_NOCHECK. See x86/arm64/s390 for examples:
> https://elixir.bootlin.com/linux/v5.12-rc7/A/ident/READ_ONCE_NOCHECK

Thanks, I just sent out a fix -- or at least hopefully one, I haven't 
actually tested it yet.

> > [<ffffffe000009a98>] get_wchan+0x156/0x196 arch/riscv/kernel/stacktrace.c:136
> > [<ffffffe000553364>] proc_pid_wchan+0x48/0xa4 fs/proc/base.c:390
> > [<ffffffe000554458>] proc_single_show+0x9c/0x13c fs/proc/base.c:774
> > [<ffffffe00045000c>] seq_read_iter+0x2e0/0x8f2 fs/seq_file.c:227
> > [<ffffffe00045081e>] seq_read+0x200/0x298 fs/seq_file.c:159
> > [<ffffffe0003f9210>] vfs_read+0x108/0x2ac fs/read_write.c:494
> > [<ffffffe0003f9724>] ksys_read+0xb4/0x1b8 fs/read_write.c:634
> > [<ffffffe0003f9850>] __do_sys_read fs/read_write.c:644 [inline]
> > [<ffffffe0003f9850>] sys_read+0x28/0x36 fs/read_write.c:642
> > [<ffffffe000005572>] ret_from_syscall+0x0/0x2
> >
> > The buggy address belongs to the page:
> > page:ffffffcf0216b8c0 refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x85ae3
> > flags: 0xffe000000000000()
> > raw: 0ffe000000000000 ffffffcf0216b8c8 ffffffcf0216b8c8 0000000000000000
> > raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
> > page dumped because: kasan: bad access detected
> >
> > Memory state around the buggy address:
> >  ffffffe0058e3c80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> >  ffffffe0058e3d00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > >ffffffe0058e3d80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> >                          ^
> >  ffffffe0058e3e00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> >  ffffffe0058e3e80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > ==================================================================
> >
> >
> > ---
> > 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@xxxxxxxxxxxxxxxx.
> >
> > syzbot will keep track of this issue. See:
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> >
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