From mboxrd@z Thu Jan 1 00:00:00 1970 From: ji.zhang@mediatek.com (Ji.Zhang) Date: Thu, 12 Apr 2018 14:13:13 +0800 Subject: [PATCH] arm64: avoid race condition issue in dump_backtrace In-Reply-To: <20180411104656.4afhb4durpntxqxl@lakrids.cambridge.arm.com> References: <20180322055929.z25brvwlmdighz66@salmiak> <1521711329.26617.31.camel@mtksdccf07> <20180326113932.2i6qp3776jtmcqk4@lakrids.cambridge.arm.com> <1522229612.26617.47.camel@mtksdccf07> <20180328101240.moo44g5qd3qjuxgn@lakrids.cambridge.arm.com> <1522397292.26617.63.camel@mtksdccf07> <20180404090431.rqwtaqovipxa5gta@lakrids.cambridge.arm.com> <1523174328.26617.91.camel@mtksdccf07> <20180409112559.uh76jpiytznymw6w@lakrids.cambridge.arm.com> <1523428228.26617.100.camel@mtksdccf07> <20180411104656.4afhb4durpntxqxl@lakrids.cambridge.arm.com> Message-ID: <1523513593.26617.106.camel@mtksdccf07> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Wed, 2018-04-11 at 11:46 +0100, Mark Rutland wrote: > On Wed, Apr 11, 2018 at 02:30:28PM +0800, Ji.Zhang wrote: > > On Mon, 2018-04-09 at 12:26 +0100, Mark Rutland wrote: > > > On Sun, Apr 08, 2018 at 03:58:48PM +0800, Ji.Zhang wrote: > > > > Yes, I see where the loop is, I have missed that the loop may cross > > > > different stacks. > > > > Define a nesting order and check against is a good idea, and it can > > > > resolve the issue exactly, but as you mentioned before, we have no idea > > > > how to handle with overflow and sdei stack, and the nesting order is > > > > strongly related with the scenario of the stack, which means if someday > > > > we add another stack, we should consider the relationship of the new > > > > stack with other stacks. From the perspective of your experts, is that > > > > suitable for doing this in unwind? > > > > > > > > Or could we just find some way easier but not so accurate, eg. > > > > Proposal 1: > > > > When we do unwind and detect that the stack spans, record the last fp of > > > > previous stack and next time if we get into the same stack, compare it > > > > with that last fp, the new fp should still smaller than last fp, or > > > > there should be potential loop. > > > > For example, when we unwind from irq to task, we record the last fp in > > > > irq stack such as last_irq_fp, and if it unwind task stack back to irq > > > > stack, no matter if it is the same irq stack with previous, just let it > > > > go and compare the new irq fp with last_irq_fp, although the process may > > > > be wrong since from task stack it could not unwind to irq stack, but the > > > > whole process will eventually stop. > > > > > > I agree that saving the last fp per-stack could work. > > > > > > > Proposal 2: > > > > So far we have four types of stack: task, irq, overflow and sdei, could > > > > we just assume that the MAX number of stack spanning is just 3 > > > > times?(task->irq->overflow->sdei or task->irq->sdei->overflow), if yes, > > > > we can just check the number of stack spanning when we detect the stack > > > > spans. > > > > > > I also agree that counting the number of stack transitions will prevent > > > an inifinite loop, even if less accurately than proposal 1. > > > > > > I don't have a strong preference either way. > > Thank you for your comment. > > Compared with proposal 1 and 2, I decide to use proposal2 since > > proposal1 seems a little complicated and it is not as easy as proposal2 > > when new stack is added. > > The sample is as below: > > diff --git a/arch/arm64/include/asm/stacktrace.h > > b/arch/arm64/include/asm/stacktrace.h > > index 902f9ed..72d1f34 100644 > > --- a/arch/arm64/include/asm/stacktrace.h > > +++ b/arch/arm64/include/asm/stacktrace.h > > @@ -92,4 +92,22 @@ static inline bool on_accessible_stack(struct > > task_struct *tsk, unsigned long sp > > return false; > > } > > > > +#define MAX_STACK_SPAN 3 > > Depending on configuration we can have: > > * task > * irq > * overflow (optional with VMAP_STACK) > * sdei (optional with ARM_SDE_INTERFACE && VMAP_STACK) > > So 3 isn't always correct. > > Also, could we please call this something like MAX_NR_STACKS? > > > +DECLARE_PER_CPU(int, num_stack_span); > > I'm pretty sure we can call unwind_frame() in a preemptible context, so > this isn't safe. > > Put this counter into the struct stackframe, and call it something like > nr_stacks; > > [...] > > > +DEFINE_PER_CPU(int, num_stack_span); > > As above, this can go. > > > + > > /* > > * AArch64 PCS assigns the frame pointer to x29. > > * > > @@ -56,6 +58,20 @@ int notrace unwind_frame(struct task_struct *tsk, > > struct stackframe *frame) > > frame->fp = READ_ONCE_NOCHECK(*(unsigned long *)(fp)); > > frame->pc = READ_ONCE_NOCHECK(*(unsigned long *)(fp + 8)); > > > > + if (!on_same_stack(tsk, fp, frame->fp)) { > > + int num = (int)__this_cpu_read(num_stack_span); > > + > > + if (num >= MAX_STACK_SPAN) > > + return -EINVAL; > > + num++; > > + __this_cpu_write(num_stack_span, num); > > + fp = frame->fp + 0x8; > > + } > > + if (fp <= frame->fp) { > > + pr_notice("fp invalid, stop unwind\n"); > > + return -EINVAL; > > + } > > I think this can be simplified to something like: > > bool same_stack; > > same_stack = on_same_stack(tsk, fp, frame->fp); > > if (fp <= frame->fp && same_stack) > return -EINVAL; > if (!same_stack && ++frame->nr_stacks > MAX_NR_STACKS) > return -EINVAL; > > ... assuming we add nr_stacks to struct stackframe. Thank you very much for your advice, they are very valuable. According to your suggestion, the modified code as follows. I did a little change that define MAX_NR_STACKS as the number of stacks, instead of the number of stack spans. diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h index 902f9ed..f235b86 100644 --- a/arch/arm64/include/asm/stacktrace.h +++ b/arch/arm64/include/asm/stacktrace.h @@ -24,9 +24,18 @@ #include #include +#ifndef CONFIG_VMAP_STACK +#define MAX_NR_STACKS 2 +#elif !defined(CONFIG_ARM_SDE_INTERFACE) +#define MAX_NR_STACKS 3 +#else +#define MAX_NR_STACKS 4 +#endif + struct stackframe { unsigned long fp; unsigned long pc; + int nr_stacks; #ifdef CONFIG_FUNCTION_GRAPH_TRACER int graph; #endif @@ -92,4 +101,20 @@ static inline bool on_accessible_stack(struct task_struct *tsk, unsigned long sp return false; } + +static inline bool on_same_stack(struct task_struct *tsk, + unsigned long sp1, unsigned long sp2) +{ + if (on_task_stack(tsk, sp1) && on_task_stack(tsk, sp2)) + return true; + if (on_irq_stack(sp1) && on_irq_stack(sp2)) + return true; + if (on_overflow_stack(sp1) && on_overflow_stack(sp2)) + return true; + if (on_sdei_stack(sp1) && on_sdei_stack(sp2)) + return true; + + return false; +} + #endif /* __ASM_STACKTRACE_H */ diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c index d5718a0..a09e247 100644 --- a/arch/arm64/kernel/stacktrace.c +++ b/arch/arm64/kernel/stacktrace.c @@ -27,6 +27,7 @@ #include #include + /* * AArch64 PCS assigns the frame pointer to x29. * @@ -43,6 +44,7 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame) { unsigned long fp = frame->fp; + bool same_stack; if (fp & 0xf) return -EINVAL; @@ -56,6 +58,13 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame) frame->fp = READ_ONCE_NOCHECK(*(unsigned long *)(fp)); frame->pc = READ_ONCE_NOCHECK(*(unsigned long *)(fp + 8)); + same_stack = on_same_stack(tsk, fp, frame->fp); + + if (fp <= frame->fp && same_stack) + return -EINVAL; + if (!same_stack && ++frame->nr_stacks > MAX_NR_STACKS) + return -EINVAL; + #ifdef CONFIG_FUNCTION_GRAPH_TRACER if (tsk->ret_stack && (frame->pc == (unsigned long)return_to_handler)) { diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c index eb2d151..3b1c472 100644 --- a/arch/arm64/kernel/traps.c +++ b/arch/arm64/kernel/traps.c @@ -120,6 +120,7 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) frame.fp = thread_saved_fp(tsk); frame.pc = thread_saved_pc(tsk); } + frame.nr_stacks = 1; #ifdef CONFIG_FUNCTION_GRAPH_TRACER frame.graph = tsk->curr_ret_stack; #endif -- 1.9.1 Best Regards, Ji