[PATCH] uprobes: Fix handle_swbp() vs. unregister() + register() race once more

[PATCH] uprobes: Fix handle_swbp() vs. unregister() + register() race once more

[Andrea Parri]

Commit 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() +
register() race") added the UPROBE_COPY_INSN flag, and corresponding
smp_wmb() and smp_rmb() memory barriers, to ensure that handle_swbp()
uses fully-initialized uprobes only.

However, the smp_rmb() is mis-placed: this barrier should be placed
after handle_swbp() has tested for the flag, thus guaranteeing that
(program-order) subsequent loads from the uprobe can see the initial
stores performed by prepare_uprobe().

Move the smp_rmb() accordingly.  Also amend the comments associated
to the two memory barriers to indicate their actual locations.

Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: stable@kernel.org
Fixes: 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() + register() race")
---
 kernel/events/uprobes.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 96d4bee83489b..322e97bbb4370 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -829,7 +829,7 @@ static int prepare_uprobe(struct uprobe *uprobe, struct file *file,
 	BUG_ON((uprobe->offset & ~PAGE_MASK) +
 			UPROBE_SWBP_INSN_SIZE > PAGE_SIZE);
 
-	smp_wmb(); /* pairs with rmb() in find_active_uprobe() */
+	smp_wmb(); /* pairs with the smp_rmb() in handle_swbp() */
 	set_bit(UPROBE_COPY_INSN, &uprobe->flags);
 
  out:
@@ -2178,10 +2178,18 @@ static void handle_swbp(struct pt_regs *regs)
 	 * After we hit the bp, _unregister + _register can install the
 	 * new and not-yet-analyzed uprobe at the same address, restart.
 	 */
-	smp_rmb(); /* pairs with wmb() in install_breakpoint() */
 	if (unlikely(!test_bit(UPROBE_COPY_INSN, &uprobe->flags)))
 		goto out;
 
+	/*
+	 * Pairs with the smp_wmb() in prepare_uprobe().
+	 *
+	 * Guarantees that if we see the UPROBE_COPY_INSN bit set, then
+	 * we must also see the stores to &uprobe->arch performed by the
+	 * prepare_uprobe() call.
+	 */
+	smp_rmb();
+
 	/* Tracing handlers use ->utask to communicate with fetch methods */
 	if (!get_utask())
 		goto out;
-- 
2.17.1

Re: [PATCH] uprobes: Fix handle_swbp() vs. unregister() + register() race once more

[Oleg Nesterov]

On 11/22, Andrea Parri wrote:
>
> Commit 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() +
> register() race") added the UPROBE_COPY_INSN flag, and corresponding
> smp_wmb() and smp_rmb() memory barriers, to ensure that handle_swbp()
> uses fully-initialized uprobes only.
> 
> However, the smp_rmb() is mis-placed: this barrier should be placed
> after handle_swbp() has tested for the flag, thus guaranteeing that
> (program-order) subsequent loads from the uprobe can see the initial
> stores performed by prepare_uprobe().
> 
> Move the smp_rmb() accordingly.  Also amend the comments associated
> to the two memory barriers to indicate their actual locations.
> 
> Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Jiri Olsa <jolsa@redhat.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: stable@kernel.org
> Fixes: 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() + register() race")

Thanks,

Acked-by: Oleg Nesterov <oleg@redhat.com>



> ---
>  kernel/events/uprobes.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 96d4bee83489b..322e97bbb4370 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -829,7 +829,7 @@ static int prepare_uprobe(struct uprobe *uprobe, struct file *file,
>  	BUG_ON((uprobe->offset & ~PAGE_MASK) +
>  			UPROBE_SWBP_INSN_SIZE > PAGE_SIZE);
>  
> -	smp_wmb(); /* pairs with rmb() in find_active_uprobe() */
> +	smp_wmb(); /* pairs with the smp_rmb() in handle_swbp() */
>  	set_bit(UPROBE_COPY_INSN, &uprobe->flags);
>  
>   out:
> @@ -2178,10 +2178,18 @@ static void handle_swbp(struct pt_regs *regs)
>  	 * After we hit the bp, _unregister + _register can install the
>  	 * new and not-yet-analyzed uprobe at the same address, restart.
>  	 */
> -	smp_rmb(); /* pairs with wmb() in install_breakpoint() */
>  	if (unlikely(!test_bit(UPROBE_COPY_INSN, &uprobe->flags)))
>  		goto out;
>  
> +	/*
> +	 * Pairs with the smp_wmb() in prepare_uprobe().
> +	 *
> +	 * Guarantees that if we see the UPROBE_COPY_INSN bit set, then
> +	 * we must also see the stores to &uprobe->arch performed by the
> +	 * prepare_uprobe() call.
> +	 */
> +	smp_rmb();
> +
>  	/* Tracing handlers use ->utask to communicate with fetch methods */
>  	if (!get_utask())
>  		goto out;
> -- 
> 2.17.1
> 

Re: [PATCH] uprobes: Fix handle_swbp() vs. unregister() + register() race once more

[Oleg Nesterov]

On 11/22, Oleg Nesterov wrote:
> On 11/22, Andrea Parri wrote:
> >
> > Commit 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() +
> > register() race") added the UPROBE_COPY_INSN flag, and corresponding
> > smp_wmb() and smp_rmb() memory barriers, to ensure that handle_swbp()
> > uses fully-initialized uprobes only.
> > 
> > However, the smp_rmb() is mis-placed: this barrier should be placed
> > after handle_swbp() has tested for the flag, thus guaranteeing that
> > (program-order) subsequent loads from the uprobe can see the initial
> > stores performed by prepare_uprobe().
> > 
> > Move the smp_rmb() accordingly.  Also amend the comments associated
> > to the two memory barriers to indicate their actual locations.
> > 
> > Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> > Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> > Cc: Jiri Olsa <jolsa@redhat.com>
> > Cc: Namhyung Kim <namhyung@kernel.org>
> > Cc: Oleg Nesterov <oleg@redhat.com>
> > Cc: stable@kernel.org
> > Fixes: 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() + register() race")
> 
> Thanks,
> 
> Acked-by: Oleg Nesterov <oleg@redhat.com>

Yes, but I am not sure this is the -stable material...

Oleg.

Re: [PATCH] uprobes: Fix handle_swbp() vs. unregister() + register() race once more

[Andrea Parri]

On Thu, Nov 22, 2018 at 05:27:09PM +0100, Oleg Nesterov wrote:
> On 11/22, Oleg Nesterov wrote:
> > On 11/22, Andrea Parri wrote:
> > >
> > > Commit 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() +
> > > register() race") added the UPROBE_COPY_INSN flag, and corresponding
> > > smp_wmb() and smp_rmb() memory barriers, to ensure that handle_swbp()
> > > uses fully-initialized uprobes only.
> > > 
> > > However, the smp_rmb() is mis-placed: this barrier should be placed
> > > after handle_swbp() has tested for the flag, thus guaranteeing that
> > > (program-order) subsequent loads from the uprobe can see the initial
> > > stores performed by prepare_uprobe().
> > > 
> > > Move the smp_rmb() accordingly.  Also amend the comments associated
> > > to the two memory barriers to indicate their actual locations.
> > > 
> > > Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Ingo Molnar <mingo@redhat.com>
> > > Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> > > Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> > > Cc: Jiri Olsa <jolsa@redhat.com>
> > > Cc: Namhyung Kim <namhyung@kernel.org>
> > > Cc: Oleg Nesterov <oleg@redhat.com>
> > > Cc: stable@kernel.org
> > > Fixes: 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() + register() race")
> > 
> > Thanks,
> > 
> > Acked-by: Oleg Nesterov <oleg@redhat.com>

Thanks, Oleg.


> 
> Yes, but I am not sure this is the -stable material...

I'm inclined to leave it to you/the maintainers in question to decide
about this, ;-)  but please let me know if a resend or another action
is needed from me.

  Andrea


> 
> Oleg.
> 

Re: [PATCH] uprobes: Fix handle_swbp() vs. unregister() + register() race once more

[Ingo Molnar]

* Oleg Nesterov <oleg@redhat.com> wrote:

> On 11/22, Oleg Nesterov wrote:
> > On 11/22, Andrea Parri wrote:
> > >
> > > Commit 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() +
> > > register() race") added the UPROBE_COPY_INSN flag, and corresponding
> > > smp_wmb() and smp_rmb() memory barriers, to ensure that handle_swbp()
> > > uses fully-initialized uprobes only.
> > > 
> > > However, the smp_rmb() is mis-placed: this barrier should be placed
> > > after handle_swbp() has tested for the flag, thus guaranteeing that
> > > (program-order) subsequent loads from the uprobe can see the initial
> > > stores performed by prepare_uprobe().
> > > 
> > > Move the smp_rmb() accordingly.  Also amend the comments associated
> > > to the two memory barriers to indicate their actual locations.
> > > 
> > > Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
> > > Cc: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Ingo Molnar <mingo@redhat.com>
> > > Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> > > Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> > > Cc: Jiri Olsa <jolsa@redhat.com>
> > > Cc: Namhyung Kim <namhyung@kernel.org>
> > > Cc: Oleg Nesterov <oleg@redhat.com>
> > > Cc: stable@kernel.org
> > > Fixes: 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() + register() race")
> > 
> > Thanks,
> > 
> > Acked-by: Oleg Nesterov <oleg@redhat.com>
> 
> Yes, but I am not sure this is the -stable material...

So I left the Cc: stable tag intact, because this is a really low-risk 
fix (it just moves barriers around), and clearly fixes a bug that people 
might or might not have observed.

Even if they observed it the race is probably very hard to reproduce and 
almost impossible to report - so we are better off propagating this fix 
to -stable, as there's no realistic actionable way for users to actually 
complain about the bug if it affects them.

That's the general backporting policy for race fixes, unless they are 
really, really intrusive - which this one isn't really.

Thanks,

	Ingo

[tip:perf/urgent] uprobes: Fix handle_swbp() vs. unregister() + register() race once more

[tip-bot for Andrea Parri]

Commit-ID:  09d3f015d1e1b4fee7e9bbdcf54201d239393391
Gitweb:     https://git.kernel.org/tip/09d3f015d1e1b4fee7e9bbdcf54201d239393391
Author:     Andrea Parri <andrea.parri@amarulasolutions.com>
AuthorDate: Thu, 22 Nov 2018 17:10:31 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 23 Nov 2018 08:31:19 +0100

uprobes: Fix handle_swbp() vs. unregister() + register() race once more

Commit:

  142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() + register() race")

added the UPROBE_COPY_INSN flag, and corresponding smp_wmb() and smp_rmb()
memory barriers, to ensure that handle_swbp() uses fully-initialized
uprobes only.

However, the smp_rmb() is mis-placed: this barrier should be placed
after handle_swbp() has tested for the flag, thus guaranteeing that
(program-order) subsequent loads from the uprobe can see the initial
stores performed by prepare_uprobe().

Move the smp_rmb() accordingly.  Also amend the comments associated
to the two memory barriers to indicate their actual locations.

Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: stable@kernel.org
Fixes: 142b18ddc8143 ("uprobes: Fix handle_swbp() vs unregister() + register() race")
Link: http://lkml.kernel.org/r/20181122161031.15179-1-andrea.parri@amarulasolutions.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/events/uprobes.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 96d4bee83489..322e97bbb437 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -829,7 +829,7 @@ static int prepare_uprobe(struct uprobe *uprobe, struct file *file,
 	BUG_ON((uprobe->offset & ~PAGE_MASK) +
 			UPROBE_SWBP_INSN_SIZE > PAGE_SIZE);
 
-	smp_wmb(); /* pairs with rmb() in find_active_uprobe() */
+	smp_wmb(); /* pairs with the smp_rmb() in handle_swbp() */
 	set_bit(UPROBE_COPY_INSN, &uprobe->flags);
 
  out:
@@ -2178,10 +2178,18 @@ static void handle_swbp(struct pt_regs *regs)
 	 * After we hit the bp, _unregister + _register can install the
 	 * new and not-yet-analyzed uprobe at the same address, restart.
 	 */
-	smp_rmb(); /* pairs with wmb() in install_breakpoint() */
 	if (unlikely(!test_bit(UPROBE_COPY_INSN, &uprobe->flags)))
 		goto out;
 
+	/*
+	 * Pairs with the smp_wmb() in prepare_uprobe().
+	 *
+	 * Guarantees that if we see the UPROBE_COPY_INSN bit set, then
+	 * we must also see the stores to &uprobe->arch performed by the
+	 * prepare_uprobe() call.
+	 */
+	smp_rmb();
+
 	/* Tracing handlers use ->utask to communicate with fetch methods */
 	if (!get_utask())
 		goto out;