Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail

From: Ben Dooks <ben.dooks@codethink.co.uk>
To: Dmitry Vyukov <dvyukov@google.com>,
	syzbot <syzbot+e74b94fe601ab9552d69@syzkaller.appspotmail.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	linux-riscv <linux-riscv@lists.infradead.org>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>,
	Benjamin Segall <bsegall@google.com>,
	dietmar.eggemann@arm.com, Juri Lelli <juri.lelli@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, Mel Gorman <mgorman@suse.de>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	Vincent Guittot <vincent.guittot@linaro.org>
Subject: Re: [syzbot] BUG: unable to handle kernel access to user memory in schedule_tail
Date: Wed, 10 Mar 2021 22:24:30 +0000	[thread overview]
Message-ID: <dbdca868-7ef2-47b3-ac26-12fe61f3156a@codethink.co.uk> (raw)
In-Reply-To: <CACT4Y+ZHMYijYAkeLMX=p9jx6pBivJz06h_1rGt-k9=AgfkWQg@mail.gmail.com>

On 10/03/2021 17:16, Dmitry Vyukov wrote:
> On Wed, Mar 10, 2021 at 5:46 PM syzbot
> <syzbot+e74b94fe601ab9552d69@syzkaller.appspotmail.com> wrote:
>>
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    0d7588ab riscv: process: Fix no prototype for arch_dup_tas..
>> 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=1212c6e6d00000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=e3c595255fb2d136
>> dashboard link: https://syzkaller.appspot.com/bug?extid=e74b94fe601ab9552d69
>> 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+e74b94fe601ab9552d69@syzkaller.appspotmail.com
> 
> +riscv maintainers
> 
> This is riscv64-specific.
> I've seen similar crashes in put_user in other places. It looks like
> put_user crashes in the user address is not mapped/protected (?).

The unmapped case should have been handled.

I think this issue is that the check for user-mode access added. From
what I read the code may be wrong in

+	if (!user_mode(regs) && addr < TASK_SIZE &&
+			unlikely(!(regs->status & SR_SUM)))
+		die_kernel_fault("access to user memory without uaccess routines",
+				addr, regs);

I think the SR_SUM check might be wrong, as I read the standard the
SR_SUM should be set to disable user-space access. So the check
should be unlikely(regs->status & SR_SUM) to say access without
having disabled the protection.

Without this, you can end up with an infinite loop in the fault handler.

> 
>> Unable to handle kernel access to user memory without uaccess routines at virtual address 000000002749f0d0
>> Oops [#1]
>> Modules linked in:
>> CPU: 1 PID: 4875 Comm: syz-executor.0 Not tainted 5.12.0-rc2-syzkaller-00467-g0d7588ab9ef9 #0
>> Hardware name: riscv-virtio,qemu (DT)
>> epc : schedule_tail+0x72/0xb2 kernel/sched/core.c:4264
>>   ra : task_pid_vnr include/linux/sched.h:1421 [inline]
>>   ra : schedule_tail+0x70/0xb2 kernel/sched/core.c:4264
>> epc : ffffffe00008c8b0 ra : ffffffe00008c8ae sp : ffffffe025d17ec0
>>   gp : ffffffe005d25378 tp : ffffffe00f0d0000 t0 : 0000000000000000
>>   t1 : 0000000000000001 t2 : 00000000000f4240 s0 : ffffffe025d17ee0
>>   s1 : 000000002749f0d0 a0 : 000000000000002a a1 : 0000000000000003
>>   a2 : 1ffffffc0cfac500 a3 : ffffffe0000c80cc a4 : 5ae9db91c19bbe00
>>   a5 : 0000000000000000 a6 : 0000000000f00000 a7 : ffffffe000082eba
>>   s2 : 0000000000040000 s3 : ffffffe00eef96c0 s4 : ffffffe022c77fe0
>>   s5 : 0000000000004000 s6 : ffffffe067d74e00 s7 : ffffffe067d74850
>>   s8 : ffffffe067d73e18 s9 : ffffffe067d74e00 s10: ffffffe00eef96e8
>>   s11: 000000ae6cdf8368 t3 : 5ae9db91c19bbe00 t4 : ffffffc4043cafb2
>>   t5 : ffffffc4043cafba t6 : 0000000000040000
>> status: 0000000000000120 badaddr: 000000002749f0d0 cause: 000000000000000f
>> Call Trace:
>> [<ffffffe00008c8b0>] schedule_tail+0x72/0xb2 kernel/sched/core.c:4264
>> [<ffffffe000005570>] ret_from_exception+0x0/0x14
>> Dumping ftrace buffer:
>>     (ftrace buffer empty)
>> ---[ end trace b5f8f9231dc87dda ]---
>>
>>
>> ---
>> 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.
>>
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> 
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-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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