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Subject: Re: [PATCH] riscv/kprobe: fix kernel panic when invoking sys_read
 traced by kprobe
To:     Jisheng Zhang <Jisheng.Zhang@synaptics.com>
CC:     <paul.walmsley@sifive.com>, <palmer@dabbelt.com>,
        <aou@eecs.berkeley.edu>, <guoren@linux.alibaba.com>,
        <mhiramat@kernel.org>, <penberg@kernel.org>, <lkp@intel.com>,
        <me@packi.ch>, <linux-riscv@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>
References: <20210330081848.14043-1-liaochang1@huawei.com>
 <20210408192040.746b0fcc@xhacker.debian>
From:   "liaochang (A)" <liaochang1@huawei.com>
Message-ID: <2aed1884-39d5-4573-71c7-89f805a8a92e@huawei.com>
Date:   Fri, 9 Apr 2021 12:16:26 +0800
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?? 2021/4/8 19:20, Jisheng Zhang д??:
> On Tue, 30 Mar 2021 16:18:48 +0800
> Liao Chang <liaochang1@huawei.com> wrote:
> 
> 
>>
>> The execution of sys_read end up hitting a BUG_ON() in __find_get_block
>> after installing kprobe at sys_read, the BUG message like the following:
>>
>> [   65.708663] ------------[ cut here ]------------
>> [   65.709987] kernel BUG at fs/buffer.c:1251!
>> [   65.711283] Kernel BUG [#1]
>> [   65.712032] Modules linked in:
>> [   65.712925] CPU: 0 PID: 51 Comm: sh Not tainted 5.12.0-rc4 #1
>> [   65.714407] Hardware name: riscv-virtio,qemu (DT)
>> [   65.715696] epc : __find_get_block+0x218/0x2c8
>> [   65.716835]  ra : __getblk_gfp+0x1c/0x4a
>> [   65.717831] epc : ffffffe00019f11e ra : ffffffe00019f56a sp : ffffffe002437930
>> [   65.719553]  gp : ffffffe000f06030 tp : ffffffe0015abc00 t0 : ffffffe00191e038
>> [   65.721290]  t1 : ffffffe00191e038 t2 : 000000000000000a s0 : ffffffe002437960
>> [   65.723051]  s1 : ffffffe00160ad00 a0 : ffffffe00160ad00 a1 : 000000000000012a
>> [   65.724772]  a2 : 0000000000000400 a3 : 0000000000000008 a4 : 0000000000000040
>> [   65.726545]  a5 : 0000000000000000 a6 : ffffffe00191e000 a7 : 0000000000000000
>> [   65.728308]  s2 : 000000000000012a s3 : 0000000000000400 s4 : 0000000000000008
>> [   65.730049]  s5 : 000000000000006c s6 : ffffffe00240f800 s7 : ffffffe000f080a8
>> [   65.731802]  s8 : 0000000000000001 s9 : 000000000000012a s10: 0000000000000008
>> [   65.733516]  s11: 0000000000000008 t3 : 00000000000003ff t4 : 000000000000000f
>> [   65.734434]  t5 : 00000000000003ff t6 : 0000000000040000
>> [   65.734613] status: 0000000000000100 badaddr: 0000000000000000 cause: 0000000000000003
>> [   65.734901] Call Trace:
>> [   65.735076] [<ffffffe00019f11e>] __find_get_block+0x218/0x2c8
>> [   65.735417] [<ffffffe00020017a>] __ext4_get_inode_loc+0xb2/0x2f6
>> [   65.735618] [<ffffffe000201b6c>] ext4_get_inode_loc+0x3a/0x8a
>> [   65.735802] [<ffffffe000203380>] ext4_reserve_inode_write+0x2e/0x8c
>> [   65.735999] [<ffffffe00020357a>] __ext4_mark_inode_dirty+0x4c/0x18e
>> [   65.736208] [<ffffffe000206bb0>] ext4_dirty_inode+0x46/0x66
>> [   65.736387] [<ffffffe000192914>] __mark_inode_dirty+0x12c/0x3da
>> [   65.736576] [<ffffffe000180dd2>] touch_atime+0x146/0x150
>> [   65.736748] [<ffffffe00010d762>] filemap_read+0x234/0x246
>> [   65.736920] [<ffffffe00010d834>] generic_file_read_iter+0xc0/0x114
>> [   65.737114] [<ffffffe0001f5d7a>] ext4_file_read_iter+0x42/0xea
>> [   65.737310] [<ffffffe000163f2c>] new_sync_read+0xe2/0x15a
>> [   65.737483] [<ffffffe000165814>] vfs_read+0xca/0xf2
>> [   65.737641] [<ffffffe000165bae>] ksys_read+0x5e/0xc8
>> [   65.737816] [<ffffffe000165c26>] sys_read+0xe/0x16
>> [   65.737973] [<ffffffe000003972>] ret_from_syscall+0x0/0x2
>> [   65.738858] ---[ end trace fe93f985456c935d ]---
>>
>> A simple reproducer looks like:
>>         echo 'p:myprobe sys_read fd=%a0 buf=%a1 count=%a2' > /sys/kernel/debug/tracing/kprobe_events
>>         echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
>>         cat /sys/kernel/debug/tracing/trace
>>
> 
> I can't reproduce the BUG_ON with the above step, I may miss something.
> 
My test platform versions
Kernel: 0d02ec6b3136 Linux 5.12-rc4
QEMU: fdd76fecdd Update version for v5.0.0 release

>> Here's what happens to hit that BUG_ON():
>>
>> 1) After installing kprobe at entry of sys_read, the first instruction
>>    is replaced by 'ebreak' instruction on riscv64 platform.
>>
>> 2) Once kernel reach the 'ebreak' instruction at the entry of sys_read,
>>    it trap into the riscv breakpoint handler, where it do something to
>>    setup for coming single-step of origin instruction, including backup
>>    the 'sstatus' in pt_regs, followed by disable interrupt during single
>>    stepping via clear 'SIE' bit of 'sstatus' in pt_regs.
>>
>> 3) Then kernel restore to the instruction slot contains two instructions,
>>    one is original instruction at entry of sys_read, the other is 'ebreak'.
>>    Here it trigger a 'Instruction page fault' exception (value at 'scause'
>>    is '0xc'), if PF is not filled into PageTabe for that slot yet.
>>
>> 4) Again kernel trap into page fault exception handler, where it choose
>>    different policy according to the state of running kprobe. Because
>>    afte 2) the state is KPROBE_HIT_SS, so kernel reset the current kprobe
>>    and 'pc' points back to the probe address.
>>
>> 5) Because 'epc' point back to 'ebreak' instrution at sys_read probe,
>>    kernel trap into breakpoint handler again, and repeat the operations
>>    at 2), however 'sstatus' without 'SIE' is keep at 4), it cause the
>>    real 'sstatus' saved at 2) is overwritten by the one withou 'SIE'.
> 
> Is kprobe_single_step_handler() handled firstly this time? thus we won't
> enter kprobe_breakpoint_handler().
> 
No,because this time kcb->ss.ctx.match_addr points to the single-step slot,but
instruction_pointer(regs) points to the first instruction of sys_read('ebreak')
so the condition is not token eventually, then we enter kprobe_breakpoint_handler().

bool __kprobes
kprobe_single_step_handler(struct pt_regs *regs)
{
        struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();

        if ((kcb->ss_ctx.ss_pending)
            && (kcb->ss_ctx.match_addr == instruction_pointer(regs))) {

>>
>> 6) When kernel cross the probe the 'sstatus' CSR restore with value
>>    without 'SIE', and reach __find_get_block where it requires the
>>    interrupt must be enabled.
>>
>> Fix this is very trivial, just restore the value of 'sstatus' in pt_regs
>> with backup one at 2) when the instruction being single stepped cause a
>> page fault.
> 
> 
> PS: I'd suggest improve the commit msg. To be honest, I can't exactly
> get all the meanings of above 1) ~ 6)
> .
I will reorganize commit msg, Thanks for review.
> 
BR,
Liao Chang