Re: [PATCH v2] riscv/kprobe: Restore local irqflag if kprobe is cancelled

[Masami Hiramatsu]

On Fri, 16 Apr 2021 16:27:31 +0800
Liao Chang <liaochang1@xxxxxxxxxx> wrote:

> The execution of sys_read end up hitting a BUG_ON() in __find_get_block after
> installing probe at sys_read via kprobe, 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 trace
> 
> Here's what happens to hit that BUG_ON():
> 
> If instruction being single stepped caused page fault, the
> kprobe is cancelled to let the page fault handler continues
> as normal page fault. But the local irqflags are disabled,
> so CPU will restore 'sstatus' with 'SIE' masked. After page
> fault is serviced, the kprobe is triggered again, we overwrite
> the saved irqflag by calling kprobe_save_local_irqflag(). Note,
> 'SIE' is masked in this new saved irqflag. After kprobe is
> serviced, the CPU 'sstatus' is restored with 'SIE' masked.
> This overwritten 'sstatus' cause BUG_ON() in __find_get_block.
> 
> This bug is already fixed on arm64 by Jisheng Zhang.
> 
> Fixes: c22b0bcb1dd02 ("riscv: Add kprobes supported")
> Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx>

Looks good to me.

Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Thank you,

> ---
> 
> Changes in v2:
> - Reorganize commit message.
> 
>  arch/riscv/kernel/probes/kprobes.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
> index 7e2c78e2ca6b..d71f7c49a721 100644
> --- a/arch/riscv/kernel/probes/kprobes.c
> +++ b/arch/riscv/kernel/probes/kprobes.c
> @@ -260,8 +260,10 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int trapnr)
>  
>  		if (kcb->kprobe_status == KPROBE_REENTER)
>  			restore_previous_kprobe(kcb);
> -		else
> +		else {
> +			kprobes_restore_local_irqflag(kcb, regs);
>  			reset_current_kprobe();
> +		}
>  
>  		break;
>  	case KPROBE_HIT_ACTIVE:
> -- 
> 2.17.1
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>