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

Jisheng Zhang
2021-04-08T11:21:29Z

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.

> 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)