linux-kernel - Re: [PATCH 04/29] x86/livepatch: Validate __fentry_

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Date:   Wed, 23 Feb 2022 11:09:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, Miroslav Benes <mbenes@...e.cz>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 04/29] x86/livepatch: Validate __fentry__ location

On Fri, Feb 18, 2022 at 01:08:31PM -0800, Josh Poimboeuf wrote:
> On Fri, Feb 18, 2022 at 05:49:06PM +0100, Peter Zijlstra wrote:
> > Currently livepatch assumes __fentry__ lives at func+0, which is most
> > likely untrue with IBT on. Override the weak klp_get_ftrace_location()
> > function with an arch specific version that's IBT aware.
> > 
> > Also make the weak fallback verify the location is an actual ftrace
> > location as a sanity check.
> > 
> > Suggested-by: Miroslav Benes <mbenes@...e.cz>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  arch/x86/include/asm/livepatch.h |    9 +++++++++
> >  kernel/livepatch/patch.c         |    2 +-
> >  2 files changed, 10 insertions(+), 1 deletion(-)
> > 
> > --- a/arch/x86/include/asm/livepatch.h
> > +++ b/arch/x86/include/asm/livepatch.h
> > @@ -17,4 +17,13 @@ static inline void klp_arch_set_pc(struc
> >  	ftrace_instruction_pointer_set(fregs, ip);
> >  }
> >  
> > +#define klp_get_ftrace_location klp_get_ftrace_location
> > +static inline unsigned long klp_get_ftrace_location(unsigned long faddr)
> > +{
> > +	unsigned long addr = ftrace_location(faddr);
> > +	if (!addr && IS_ENABLED(CONFIG_X86_IBT))
> > +		addr = ftrace_location(faddr + 4);
> > +	return addr;
> 
> I'm kind of surprised this logic doesn't exist in ftrace itself.  Is
> livepatch really the only user that needs to find the fentry for a given
> function?
> 
> I had to do a double take for the ftrace_location() semantics, as I
> originally assumed that's what it did, based on its name and signature.
> 
> Instead it apparently functions like a bool but returns its argument on
> success.
> 
> Though the function comment tells a different story:
> 
> /**
>  * ftrace_location - return true if the ip giving is a traced location
> 
> So it's all kinds of confusing...

Yes.. so yesterday, when making function-graph tracing not explode, I
ran into a similar issue. Steve suggested something along the lines of
.... this.

(modified from his actual suggestion to also cover this case)

Let me go try this...

--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1578,7 +1578,23 @@ unsigned long ftrace_location_range(unsi
  */
 unsigned long ftrace_location(unsigned long ip)
 {
-	return ftrace_location_range(ip, ip);
+	struct dyn_ftrace *rec;
+	unsigned long offset;
+	unsigned long size;
+
+	rec = lookup_rec(ip, ip);
+	if (!rec) {
+		if (!kallsyms_lookup(ip, &size, &offset, NULL, NULL))
+			goto out;
+
+		rec = lookup_rec(ip - offset, (ip - offset) + size);
+	}
+
+	if (rec)
+		return rec->ip;
+
+out:
+	return 0;
 }
 
 /**
@@ -5110,11 +5126,16 @@ int register_ftrace_direct(unsigned long
 	struct ftrace_func_entry *entry;
 	struct ftrace_hash *free_hash = NULL;
 	struct dyn_ftrace *rec;
-	int ret = -EBUSY;
+	int ret = -ENODEV;
 
 	mutex_lock(&direct_mutex);
 
+	ip = ftrace_location(ip);
+	if (!ip)
+		goto out_unlock;
+
 	/* See if there's a direct function at @ip already */
+	ret = -EBUSY;
 	if (ftrace_find_rec_direct(ip))
 		goto out_unlock;
 
@@ -5222,6 +5243,10 @@ int unregister_ftrace_direct(unsigned lo
 
 	mutex_lock(&direct_mutex);
 
+	ip = ftrace_location(ip);
+	if (!ip)
+		goto out_unlock;
+
 	entry = find_direct_entry(&ip, NULL);
 	if (!entry)
 		goto out_unlock;