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Date: Fri, 30 Sep 2022 15:17:30 +0530
From: "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To: jniethe5@...il.com, Li Huafei <lihuafei1@...wei.com>,
mpe@...erman.id.au
Cc: christophe.leroy@...roup.eu, linux-kernel@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, mhiramat@...nel.org,
npiggin@...il.com, peterz@...radead.org, rostedt@...dmis.org
Subject: Re: [PATCH] powerpc/kprobes: Fix null pointer reference in
arch_prepare_kprobe()
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@...wei.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@...ux.vnet.ibm.com>
- Naveen
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