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

[Jisheng Zhang]

On Tue, 30 Mar 2021 16:18:48 +0800
Liao Chang <liaochang1@xxxxxxxxxx> 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.

> 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().

> 
> 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)