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Date:   Fri, 1 Mar 2019 07:32:58 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        syzbot <syzbot+ca95b2b7aef9e7cbd6ab@...kaller.appspotmail.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/unwind: handle NULL pointer calls better in
 frame unwinder

On Fri, Mar 01, 2019 at 04:12:00AM +0100, Jann Horn wrote:
> When the frame unwinder is invoked for an oops caused by a call to NULL,
> it currently skips the parent function because BP still points to the
> parent's stack frame; the (nonexistent) current function only has the first
> half of a stack frame, and BP doesn't point to it yet.
> 
> Add a special case for IP==0 that calculates a fake BP from SP, then uses
> the real BP for the next frame.
> 
> Note that this handles first_frame specially: We return information about
> the parent function as long as the saved IP is >=first_frame, even if the
> fake BP points below it.
> 
> With an artificially-added NULL call in prctl_set_seccomp(), before this
> patch, the trace is:
> 
> Call Trace:
>  ? prctl_set_seccomp+0x3a/0x50
>  __x64_sys_prctl+0x457/0x6f0
>  ? __ia32_sys_prctl+0x750/0x750
>  do_syscall_64+0x72/0x160
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> After this patch, the trace is:
> 
> Call Trace:
>  prctl_set_seccomp+0x3a/0x50
>  __x64_sys_prctl+0x457/0x6f0
>  ? __ia32_sys_prctl+0x750/0x750
>  do_syscall_64+0x72/0x160
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
>  arch/x86/include/asm/unwind.h  |  6 ++++++
>  arch/x86/kernel/unwind_frame.c | 25 ++++++++++++++++++++++---
>  2 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/unwind.h b/arch/x86/include/asm/unwind.h
> index 1f86e1b0a5cd..499578f7e6d7 100644
> --- a/arch/x86/include/asm/unwind.h
> +++ b/arch/x86/include/asm/unwind.h
> @@ -23,6 +23,12 @@ struct unwind_state {
>  #elif defined(CONFIG_UNWINDER_FRAME_POINTER)
>  	bool got_irq;
>  	unsigned long *bp, *orig_sp, ip;
> +	/*
> +	 * If non-NULL: The current frame is incomplete and doesn't contain a
> +	 * valid BP. When looking for the next frame, use this instead of the
> +	 * non-existent saved BP.
> +	 */
> +	unsigned long *next_bp;
>  	struct pt_regs *regs;
>  #else
>  	unsigned long *sp;
> diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
> index 3dc26f95d46e..9b9fd4826e7a 100644
> --- a/arch/x86/kernel/unwind_frame.c
> +++ b/arch/x86/kernel/unwind_frame.c
> @@ -320,10 +320,14 @@ bool unwind_next_frame(struct unwind_state *state)
>  	}
>  
>  	/* Get the next frame pointer: */
> -	if (state->regs)
> +	if (state->next_bp) {
> +		next_bp = state->next_bp;
> +		state->next_bp = NULL;
> +	} else if (state->regs) {
>  		next_bp = (unsigned long *)state->regs->bp;
> -	else
> +	} else {
>  		next_bp = (unsigned long *)READ_ONCE_TASK_STACK(state->task, *state->bp);
> +	}
>  
>  	/* Move to the next frame if it's safe: */
>  	if (!update_stack_state(state, next_bp))
> @@ -398,6 +402,21 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
>  
>  	bp = get_frame_pointer(task, regs);
>  
> +	/*
> +	 * If we crash with IP==0, the last successfully executed instruction
> +	 * was probably an indirect function call with a NULL function pointer.
> +	 * That means that SP points into the middle of an incomplete frame:
> +	 * *SP is a return pointer, and *(SP-sizeof(unsigned long)) is where we
> +	 * would have written a frame pointer if we hadn't crashed.
> +	 * Pretend that the frame is complete and that BP points to it, but save
> +	 * the real BP so that we can use it when looking for the next frame.
> +	 */
> +	if (regs && regs->ip == 0 &&

Would it make sense to do 'regs->ip < PAGE_SIZE', a la show_fault_oops()?
E.g. to handle bugs where a function pointer gets loaded with NULL+offset.

> +	    (unsigned long *)kernel_stack_pointer(regs) >= first_frame) {
> +		state->next_bp = bp;
> +		bp = ((unsigned long *)kernel_stack_pointer(regs)) - 1;
> +	}
> +
>  	/* Initialize stack info and make sure the frame data is accessible: */
>  	get_stack_info(bp, state->task, &state->stack_info,
>  		       &state->stack_mask);
> @@ -410,7 +429,7 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
>  	 */
>  	while (!unwind_done(state) &&
>  	       (!on_stack(&state->stack_info, first_frame, sizeof(long)) ||
> -			state->bp < first_frame))
> +			(state->next_bp == NULL && state->bp < first_frame)))
>  		unwind_next_frame(state);
>  }
>  EXPORT_SYMBOL_GPL(__unwind_start);
> -- 
> 2.21.0.352.gf09ad66450-goog
> 

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