Re: [PATCH] riscv/kprobe: fix kernel panic when invoking sys_read traced by kprobe

liaochang (A)
2021-04-09T04:18:23Z

在 2021/4/8 19:20, Jisheng Zhang 写道:
> 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