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

[Jisheng Zhang]

On Fri, 9 Apr 2021 12:16:26 +0800
"liaochang (A)" <liaochang1@xxxxxxxxxx> wrote:


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

Thanks for the information. Inspired by this patch, I think arm64 also
has the similar issue. I will send out a similar patch for arm64.

Thanks