linux-kernel - Re: [RFC PATCH v3 2/6] uretprobes/x86: hijack return address

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Date:	Fri, 1 Mar 2013 12:00:43 +0100
From:	Anton Arapov <anton@...hat.com>
To:	Ananth N Mavinakayanahalli <ananth@...ibm.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Josh Stone <jistone@...hat.com>,
	Frank Eigler <fche@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC PATCH v3 2/6] uretprobes/x86: hijack return address

On Fri, Mar 01, 2013 at 11:15:36AM +0530, Ananth N Mavinakayanahalli wrote:
> On Thu, Feb 28, 2013 at 12:00:11PM +0100, Anton Arapov wrote:
> >   hijack the return address and replace it with a "trampoline"
> > 
> > v2:
> >   - remove ->doomed flag, kill task immediately
> > 
> > Signed-off-by: Anton Arapov <anton@...hat.com>
> > ---
> >  arch/x86/include/asm/uprobes.h |  1 +
> >  arch/x86/kernel/uprobes.c      | 29 +++++++++++++++++++++++++++++
> >  2 files changed, 30 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/uprobes.h b/arch/x86/include/asm/uprobes.h
> > index 8ff8be7..c353555 100644
> > --- a/arch/x86/include/asm/uprobes.h
> > +++ b/arch/x86/include/asm/uprobes.h
> > @@ -55,4 +55,5 @@ extern int  arch_uprobe_post_xol(struct arch_uprobe *aup, struct pt_regs *regs);
> >  extern bool arch_uprobe_xol_was_trapped(struct task_struct *tsk);
> >  extern int  arch_uprobe_exception_notify(struct notifier_block *self, unsigned long val, void *data);
> >  extern void arch_uprobe_abort_xol(struct arch_uprobe *aup, struct pt_regs *regs);
> > +extern unsigned long arch_uretprobe_hijack_return_addr(unsigned long rp_trampoline_vaddr, struct pt_regs *regs);
> >  #endif	/* _ASM_UPROBES_H */
> > diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> > index 0ba4cfb..85e2153 100644
> > --- a/arch/x86/kernel/uprobes.c
> > +++ b/arch/x86/kernel/uprobes.c
> > @@ -697,3 +697,32 @@ bool arch_uprobe_skip_sstep(struct arch_uprobe *auprobe, struct pt_regs *regs)
> >  		send_sig(SIGTRAP, current, 0);
> >  	return ret;
> >  }
> > +
> > +extern unsigned long arch_uretprobe_hijack_return_addr(unsigned long
> > +		rp_trampoline_vaddr, struct pt_regs *regs)
> > +{
> > +	int rasize, ncopied;
> > +	unsigned long orig_ret_vaddr = 0; /* clear high bits for 32-bit apps */
> > +
> > +	rasize = is_ia32_task() ? 4 : 8;
> > +	ncopied = copy_from_user(&orig_ret_vaddr, (void __user *)regs->sp, rasize);
> > +	if (unlikely(ncopied))
> 
> What if ncopied < rasize? Agreed that the upper order bits can be 0, but should
> you not validate ncopied == rasize?

  Function returns 0 in case copy_from_user() was not able to copy
return address entirely, and "if (ncopied)" makes sure of it. We 
can't continue if we have no correct return address.

  copy_from_user() returns number of bytes that were *not* copied,
thus "ncopied == rasize" means copy_from_user() was not able to copy
*all* bytes. I don't see the point of such check here.

  Or am I missing anything?

thank you!
Anton.

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