Re: [PATCH] powerpc/kprobes: Fix null pointer reference in arch_prepare_kprobe()

From: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
To: jniethe5@gmail.com, Li Huafei <lihuafei1@huawei.com>, mpe@ellerman.id.au
Cc: peterz@infradead.org, linux-kernel@vger.kernel.org,
	npiggin@gmail.com, rostedt@goodmis.org,
	linuxppc-dev@lists.ozlabs.org, mhiramat@kernel.org
Subject: Re: [PATCH] powerpc/kprobes: Fix null pointer reference in arch_prepare_kprobe()
Date: Fri, 30 Sep 2022 15:17:30 +0530	[thread overview]
Message-ID: <1664530538.ke6dp49pwh.naveen@linux.ibm.com> (raw)
In-Reply-To: <20220923093253.177298-1-lihuafei1@huawei.com>

Li Huafei wrote:
> I found a null pointer reference in arch_prepare_kprobe():

Good find!

> 
>   # echo 'p cmdline_proc_show' > kprobe_events
>   # echo 'p cmdline_proc_show+16' >> kprobe_events

I think we should extend multiple_kprobes selftest to also place 
contiguous probes to catch such errors.

>   [   67.278533][  T122] Kernel attempted to read user page (0) - exploit attempt? (uid: 0)
>   [   67.279326][  T122] BUG: Kernel NULL pointer dereference on read at 0x00000000
>   [   67.279738][  T122] Faulting instruction address: 0xc000000000050bfc
>   [   67.280486][  T122] Oops: Kernel access of bad area, sig: 11 [#1]
>   [   67.280846][  T122] LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA PowerNV
>   [   67.281435][  T122] Modules linked in:
>   [   67.281903][  T122] CPU: 0 PID: 122 Comm: sh Not tainted 6.0.0-rc3-00007-gdcf8e5633e2e #10
>   [   67.282547][  T122] NIP:  c000000000050bfc LR: c000000000050bec CTR: 0000000000005bdc
>   [   67.282920][  T122] REGS: c0000000348475b0 TRAP: 0300   Not tainted  (6.0.0-rc3-00007-gdcf8e5633e2e)
>   [   67.283424][  T122] MSR:  9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 88002444  XER: 20040006
>   [   67.284023][  T122] CFAR: c00000000022d100 DAR: 0000000000000000 DSISR: 40000000 IRQMASK: 0
>   [   67.284023][  T122] GPR00: c000000000050bec c000000034847850 c0000000013f6100 c000000001fb7718
>   [   67.284023][  T122] GPR04: c000000000515c10 c000000000e5fe08 c00000000133da60 c000000004839300
>   [   67.284023][  T122] GPR08: c0000000014ffb98 0000000000000000 c000000000515c0c c000000000e18576
>   [   67.284023][  T122] GPR12: c000000000e60170 c0000000015a0000 00000001155e0460 0000000000000000
>   [   67.284023][  T122] GPR16: 0000000000000000 00007fffe8eeb3c8 0000000116320728 0000000000000000
>   [   67.284023][  T122] GPR20: 0000000116320720 0000000000000000 c0000000012fa918 0000000000000006
>   [   67.284023][  T122] GPR24: c0000000014ffb98 c0000000011ed360 0000000000000000 c000000001fb7928
>   [   67.284023][  T122] GPR28: 0000000000000000 0000000000000000 000000007c0802a6 c000000001fb7918
>   [   67.287799][  T122] NIP [c000000000050bfc] arch_prepare_kprobe+0x10c/0x2d0
>   [   67.288490][  T122] LR [c000000000050bec] arch_prepare_kprobe+0xfc/0x2d0
>   [   67.289025][  T122] Call Trace:
>   [   67.289268][  T122] [c000000034847850] [c0000000012f77a0] 0xc0000000012f77a0 (unreliable)
>   [   67.289999][  T122] [c0000000348478d0] [c000000000231320] register_kprobe+0x3c0/0x7a0
>   [   67.290439][  T122] [c000000034847940] [c0000000002938c0] __register_trace_kprobe+0x140/0x1a0
>   [   67.290898][  T122] [c0000000348479b0] [c0000000002944c4] __trace_kprobe_create+0x794/0x1040
>   [   67.291330][  T122] [c000000034847b60] [c0000000002a1614] trace_probe_create+0xc4/0xe0
>   [   67.291717][  T122] [c000000034847bb0] [c00000000029363c] create_or_delete_trace_kprobe+0x2c/0x80
>   [   67.292158][  T122] [c000000034847bd0] [c000000000264420] trace_parse_run_command+0xf0/0x210
>   [   67.292611][  T122] [c000000034847c70] [c0000000002934a0] probes_write+0x20/0x40
>   [   67.292996][  T122] [c000000034847c90] [c00000000045e98c] vfs_write+0xfc/0x450
>   [   67.293356][  T122] [c000000034847d50] [c00000000045eec4] ksys_write+0x84/0x140
>   [   67.293716][  T122] [c000000034847da0] [c00000000002e4fc] system_call_exception+0x17c/0x3a0
>   [   67.294186][  T122] [c000000034847e10] [c00000000000c0e8] system_call_vectored_common+0xe8/0x278
>   [   67.294680][  T122] --- interrupt: 3000 at 0x7fffa5682de0
>   [   67.294937][  T122] NIP:  00007fffa5682de0 LR: 0000000000000000 CTR: 0000000000000000
>   [   67.295313][  T122] REGS: c000000034847e80 TRAP: 3000   Not tainted  (6.0.0-rc3-00007-gdcf8e5633e2e)
>   [   67.295725][  T122] MSR:  900000000280f033 <SF,HV,VEC,VSX,EE,PR,FP,ME,IR,DR,RI,LE>  CR: 44002408  XER: 00000000
>   [   67.296291][  T122] IRQMASK: 0
>   [   67.296291][  T122] GPR00: 0000000000000004 00007fffe8eeaec0 00007fffa5757300 0000000000000001
>   [   67.296291][  T122] GPR04: 0000000116329c60 0000000000000017 0000000000116329 0000000000000000
>   [   67.296291][  T122] GPR08: 0000000000000006 0000000000000000 0000000000000000 0000000000000000
>   [   67.296291][  T122] GPR12: 0000000000000000 00007fffa580ac60 00000001155e0460 0000000000000000
>   [   67.296291][  T122] GPR16: 0000000000000000 00007fffe8eeb3c8 0000000116320728 0000000000000000
>   [   67.296291][  T122] GPR20: 0000000116320720 0000000000000000 0000000000000000 0000000000000002
>   [   67.296291][  T122] GPR24: 00000001163206f0 0000000000000020 00007fffe8eeafa0 0000000000000001
>   [   67.296291][  T122] GPR28: 0000000000000000 0000000000000017 0000000116329c60 0000000000000001
>   [   67.299570][  T122] NIP [00007fffa5682de0] 0x7fffa5682de0
>   [   67.299837][  T122] LR [0000000000000000] 0x0
>   [   67.300072][  T122] --- interrupt: 3000
>   [   67.300447][  T122] Instruction dump:
>   [   67.300736][  T122] 386319d8 481342f5 60000000 60000000 60000000 e87f0028 3863fffc 481dc4d1
>   [   67.301230][  T122] 60000000 2c230000 41820018 e9230058 <81290000> 552936be 2c090001 4182018c
>   [   67.302102][  T122] ---[ end trace 0000000000000000 ]---
>   [   67.302496][  T122]

Please consider trimming the backtrace to only capture the necessary 
information:
https://docs.kernel.org/process/submitting-patches.html#backtraces-in-commit-mesages

> 
> The address being probed has some special:
> 
>   cmdline_proc_show: Probe based on ftrace
>   cmdline_proc_show+16: Probe for the next instruction at the ftrace location
> 
> The ftrace-based kprobe does not generate kprobe::ainsn::insn, it gets
> set to NULL. In arch_prepare_kprobe() it will check for:
> 
>   ...
>   prev = get_kprobe(p->addr - 1);
>   preempt_enable_no_resched();
>   if (prev && ppc_inst_prefixed(ppc_inst_read(prev->ainsn.insn))) {
>   ...
> 
> If prev is based on ftrace, 'ppc_inst_read(prev->ainsn.insn)' will occur
> with a null pointer reference. At this point prev->addr will not be a
> prefixed instruction, so the check can be skipped.
> 
> Check if prev is ftrace-based kprobe before reading 'prev->ainsn.insn'
> to fix this problem.
> 
> Fixes: b4657f7650ba ("powerpc/kprobes: Don't allow breakpoints on suffixes")
> Signed-off-by: Li Huafei <lihuafei1@huawei.com>
> ---
>  arch/powerpc/kernel/kprobes.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> index 912d4f8a13be..9f6cbbd56809 100644
> --- a/arch/powerpc/kernel/kprobes.c
> +++ b/arch/powerpc/kernel/kprobes.c
> @@ -161,7 +161,12 @@ int arch_prepare_kprobe(struct kprobe *p)
>  	preempt_disable();
>  	prev = get_kprobe(p->addr - 1);
>  	preempt_enable_no_resched();
> -	if (prev && ppc_inst_prefixed(ppc_inst_read(prev->ainsn.insn))) {
> +	/*
> +	 * When prev is a ftrace-based kprobe, we don't have an insn, and it
> +	 * doesn't probe for prefixed instruction.
> +	 */
> +	if (prev && !kprobe_ftrace(prev) &&
> +	    ppc_inst_prefixed(ppc_inst_read(prev->ainsn.insn))) {
>  		printk("Cannot register a kprobe on the second word of prefixed instruction\n");
>  		ret = -EINVAL;
>  	}

It's fine to keep the if condition on a single line.

Other than that, thanks for the fix!
Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>

- Naveen