[GIT PULL] hw_breakpoint fixes

[GIT PULL] hw_breakpoint fixes

[Frederic Weisbecker]

Ingo,

Please pull the perf/urgent branch that can be found at:

git://git.kernel.org/pub/scm/linux/kernel/git/frederic/random-tracing.git
	perf/urgent

I couldn't get Oleg's review/ack for the generic ptrace part
but I think he was fine with the general solution when it was
a simple draft. Few things have changed since then, I essentially
addressed his review.

Thanks,
	Frederic
---

Frederic Weisbecker (5):
      ptrace: Prepare to fix racy accesses on task breakpoints
      x86, hw_breakpoints: Fix racy access to ptrace breakpoints
      powerpc, hw_breakpoints: Fix racy access to ptrace breakpoints
      arm, hw_breakpoints: Fix racy access to ptrace breakpoints
      sh, hw_breakpoints: Fix racy access to ptrace breakpoints


 arch/arm/kernel/ptrace.c     |    8 ++++++++
 arch/powerpc/kernel/ptrace.c |    3 +++
 arch/sh/kernel/ptrace_32.c   |    4 ++++
 arch/x86/kernel/ptrace.c     |   36 ++++++++++++++++++++++++++----------
 include/linux/ptrace.h       |   13 ++++++++++++-
 include/linux/sched.h        |    3 +++
 kernel/exit.c                |    2 +-
 kernel/ptrace.c              |   17 +++++++++++++++++
 8 files changed, 74 insertions(+), 12 deletions(-)

[PATCH 1/5] ptrace: Prepare to fix racy accesses on task breakpoints

[Frederic Weisbecker]

When a task is traced and is in a stopped state, the tracer
may execute a ptrace request to examine the tracee state and
get its task struct. Right after, the tracee can be killed
and thus its breakpoints released.
This can happen concurrently when the tracer is in the middle
of reading or modifying these breakpoints, leading to dereferencing
a freed pointer.

Hence, to prepare the fix, create a generic breakpoint reference
holding API. When a reference on the breakpoints of a task is
held, the breakpoints won't be released until the last reference
is dropped. After that, no more ptrace request on the task's
breakpoints can be serviced for the tracer.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Prasad <prasad@linux.vnet.ibm.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: v2.6.33.. <stable@kernel.org>
Link: http://lkml.kernel.org/r/1302284067-7860-2-git-send-email-fweisbec@gmail.com
---
 include/linux/ptrace.h |   13 ++++++++++++-
 include/linux/sched.h  |    3 +++
 kernel/exit.c          |    2 +-
 kernel/ptrace.c        |   17 +++++++++++++++++
 4 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index a1147e5..9178d5c 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -189,6 +189,10 @@ static inline void ptrace_init_task(struct task_struct *child, bool ptrace)
 		child->ptrace = current->ptrace;
 		__ptrace_link(child, current->parent);
 	}
+
+#ifdef CONFIG_HAVE_HW_BREAKPOINT
+	atomic_set(&child->ptrace_bp_refcnt, 1);
+#endif
 }
 
 /**
@@ -350,6 +354,13 @@ extern int task_current_syscall(struct task_struct *target, long *callno,
 				unsigned long args[6], unsigned int maxargs,
 				unsigned long *sp, unsigned long *pc);
 
-#endif
+#ifdef CONFIG_HAVE_HW_BREAKPOINT
+extern int ptrace_get_breakpoints(struct task_struct *tsk);
+extern void ptrace_put_breakpoints(struct task_struct *tsk);
+#else
+static inline void ptrace_put_breakpoints(struct task_struct *tsk) { }
+#endif /* CONFIG_HAVE_HW_BREAKPOINT */
+
+#endif /* __KERNEL */
 
 #endif
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 18d63ce..781abd1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1537,6 +1537,9 @@ struct task_struct {
 		unsigned long memsw_nr_pages; /* uncharged mem+swap usage */
 	} memcg_batch;
 #endif
+#ifdef CONFIG_HAVE_HW_BREAKPOINT
+	atomic_t ptrace_bp_refcnt;
+#endif
 };
 
 /* Future-safe accessor for struct task_struct's cpus_allowed. */
diff --git a/kernel/exit.c b/kernel/exit.c
index f5d2f63..8dd8741 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1016,7 +1016,7 @@ NORET_TYPE void do_exit(long code)
 	/*
 	 * FIXME: do that only when needed, using sched_exit tracepoint
 	 */
-	flush_ptrace_hw_breakpoint(tsk);
+	ptrace_put_breakpoints(tsk);
 
 	exit_notify(tsk, group_dead);
 #ifdef CONFIG_NUMA
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 0fc1eed..dc7ab65 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -22,6 +22,7 @@
 #include <linux/syscalls.h>
 #include <linux/uaccess.h>
 #include <linux/regset.h>
+#include <linux/hw_breakpoint.h>
 
 
 /*
@@ -879,3 +880,19 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
 	return ret;
 }
 #endif	/* CONFIG_COMPAT */
+
+#ifdef CONFIG_HAVE_HW_BREAKPOINT
+int ptrace_get_breakpoints(struct task_struct *tsk)
+{
+	if (atomic_inc_not_zero(&tsk->ptrace_bp_refcnt))
+		return 0;
+
+	return -1;
+}
+
+void ptrace_put_breakpoints(struct task_struct *tsk)
+{
+	if (atomic_dec_and_test(&tsk->ptrace_bp_refcnt))
+		flush_ptrace_hw_breakpoint(tsk);
+}
+#endif /* CONFIG_HAVE_HW_BREAKPOINT */
-- 
1.7.3.2

Re: [PATCH 1/5] ptrace: Prepare to fix racy accesses on task breakpoints

[Ingo Molnar]

(Linus and Andrew Cc:-ed as well)

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> When a task is traced and is in a stopped state, the tracer
> may execute a ptrace request to examine the tracee state and
> get its task struct. Right after, the tracee can be killed
> and thus its breakpoints released.
> This can happen concurrently when the tracer is in the middle
> of reading or modifying these breakpoints, leading to dereferencing
> a freed pointer.
> 
> Hence, to prepare the fix, create a generic breakpoint reference
> holding API. When a reference on the breakpoints of a task is
> held, the breakpoints won't be released until the last reference
> is dropped. After that, no more ptrace request on the task's
> breakpoints can be serviced for the tracer.
> 
> Reported-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

Ok, this series looks a bit scary - and this ptrace.h change does not have 
Oleg's Acked-by. (the arch bits all have maintaner Acked-by's)

The changes look a bit ugly as well: beyond the ugly ifdeffery, we have 
ptrace.h::ptrace_init_task(), which is only used in 
tracehook.h::tracehook_finish_clone() which is only used in 
kernel/fork.c::copy_process().

That's two levels of obfuscation to do something rather simple - i think we 
should get rid of the tracehook.h redirections, it did not work out in the end 
as a method of capturing events - ftrace TRACE_EVENT() seems better structured 
and more maintainable.

But i guess we could live with this fix for v2.6.39, if neither Oleg nor Linus 
and Andrew are hating this further complication of the ptrace mess enough to 
NAK it. Thoughts?

Plus, i'd really love it if you did some stress-testing of this change of a 
mixed ptrace breakpoints and perf breakpoints workload, on some sufficiently 
SMP box. gdb's hbreak is a very low freq way of testing thus such regressions 
take ages to be reported.

Thanks,

	Ingo

Re: [PATCH 1/5] ptrace: Prepare to fix racy accesses on task breakpoints

[Frederic Weisbecker]

On Wed, May 04, 2011 at 08:31:06AM +0200, Ingo Molnar wrote:
> 
> (Linus and Andrew Cc:-ed as well)
> 
> * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > When a task is traced and is in a stopped state, the tracer
> > may execute a ptrace request to examine the tracee state and
> > get its task struct. Right after, the tracee can be killed
> > and thus its breakpoints released.
> > This can happen concurrently when the tracer is in the middle
> > of reading or modifying these breakpoints, leading to dereferencing
> > a freed pointer.
> > 
> > Hence, to prepare the fix, create a generic breakpoint reference
> > holding API. When a reference on the breakpoints of a task is
> > held, the breakpoints won't be released until the last reference
> > is dropped. After that, no more ptrace request on the task's
> > breakpoints can be serviced for the tracer.
> > 
> > Reported-by: Oleg Nesterov <oleg@redhat.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> 
> Ok, this series looks a bit scary - and this ptrace.h change does not have 
> Oleg's Acked-by. (the arch bits all have maintaner Acked-by's)
> 
> The changes look a bit ugly as well: beyond the ugly ifdeffery, we have 
> ptrace.h::ptrace_init_task(), which is only used in 
> tracehook.h::tracehook_finish_clone() which is only used in 
> kernel/fork.c::copy_process().
> 
> That's two levels of obfuscation to do something rather simple - i think we 
> should get rid of the tracehook.h redirections, it did not work out in the end 
> as a method of capturing events - ftrace TRACE_EVENT() seems better structured 
> and more maintainable.
> 
> But i guess we could live with this fix for v2.6.39, if neither Oleg nor Linus 
> and Andrew are hating this further complication of the ptrace mess enough to 
> NAK it. Thoughts?
> 
> Plus, i'd really love it if you did some stress-testing of this change of a 
> mixed ptrace breakpoints and perf breakpoints workload, on some sufficiently 
> SMP box. gdb's hbreak is a very low freq way of testing thus such regressions 
> take ages to be reported.

But the perf breakpoints (those created using perf syscall) are not touched
at all by this patch. Only the ptrace ones.

What I can stress test is trying some ptrace breakpoint request and at the
same time SIGKILL the child, which is the only way to reproduce the bug
supposed to be fixed. And run that in a loop for one night. I'll try that.

[PATCH 2/5] x86, hw_breakpoints: Fix racy access to ptrace breakpoints

[Frederic Weisbecker]

While the tracer accesses ptrace breakpoints, the child task may
concurrently exit due to a SIGKILL and thus release its breakpoints
at the same time. We can then dereference some freed pointers.

To fix this, hold a reference on the child breakpoints before
manipulating them.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Prasad <prasad@linux.vnet.ibm.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: v2.6.33.. <stable@kernel.org>
Link: http://lkml.kernel.org/r/1302284067-7860-3-git-send-email-fweisbec@gmail.com
---
 arch/x86/kernel/ptrace.c |   36 ++++++++++++++++++++++++++----------
 1 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 45892dc..f65e5b5 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -608,6 +608,9 @@ static int ptrace_write_dr7(struct task_struct *tsk, unsigned long data)
 	unsigned len, type;
 	struct perf_event *bp;
 
+	if (ptrace_get_breakpoints(tsk) < 0)
+		return -ESRCH;
+
 	data &= ~DR_CONTROL_RESERVED;
 	old_dr7 = ptrace_get_dr7(thread->ptrace_bps);
 restore:
@@ -655,6 +658,9 @@ restore:
 		}
 		goto restore;
 	}
+
+	ptrace_put_breakpoints(tsk);
+
 	return ((orig_ret < 0) ? orig_ret : rc);
 }
 
@@ -668,10 +674,17 @@ static unsigned long ptrace_get_debugreg(struct task_struct *tsk, int n)
 
 	if (n < HBP_NUM) {
 		struct perf_event *bp;
+
+		if (ptrace_get_breakpoints(tsk) < 0)
+			return -ESRCH;
+
 		bp = thread->ptrace_bps[n];
 		if (!bp)
-			return 0;
-		val = bp->hw.info.address;
+			val = 0;
+		else
+			val = bp->hw.info.address;
+
+		ptrace_put_breakpoints(tsk);
 	} else if (n == 6) {
 		val = thread->debugreg6;
 	 } else if (n == 7) {
@@ -686,6 +699,10 @@ static int ptrace_set_breakpoint_addr(struct task_struct *tsk, int nr,
 	struct perf_event *bp;
 	struct thread_struct *t = &tsk->thread;
 	struct perf_event_attr attr;
+	int err = 0;
+
+	if (ptrace_get_breakpoints(tsk) < 0)
+		return -ESRCH;
 
 	if (!t->ptrace_bps[nr]) {
 		ptrace_breakpoint_init(&attr);
@@ -709,24 +726,23 @@ static int ptrace_set_breakpoint_addr(struct task_struct *tsk, int nr,
 		 * writing for the user. And anyway this is the previous
 		 * behaviour.
 		 */
-		if (IS_ERR(bp))
-			return PTR_ERR(bp);
+		if (IS_ERR(bp)) {
+			err = PTR_ERR(bp);
+			goto put;
+		}
 
 		t->ptrace_bps[nr] = bp;
 	} else {
-		int err;
-
 		bp = t->ptrace_bps[nr];
 
 		attr = bp->attr;
 		attr.bp_addr = addr;
 		err = modify_user_hw_breakpoint(bp, &attr);
-		if (err)
-			return err;
 	}
 
-
-	return 0;
+put:
+	ptrace_put_breakpoints(tsk);
+	return err;
 }
 
 /*
-- 
1.7.3.2

[PATCH 3/5] powerpc, hw_breakpoints: Fix racy access to ptrace breakpoints

[Frederic Weisbecker]

While the tracer accesses ptrace breakpoints, the child task may
concurrently exit due to a SIGKILL and thus release its breakpoints
at the same time. We can then dereference some freed pointers.

To fix this, hold a reference on the child breakpoints before
manipulating them.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Prasad <prasad@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: v2.6.33.. <stable@kernel.org>
Link: http://lkml.kernel.org/r/1302284067-7860-4-git-send-email-fweisbec@gmail.com
---
 arch/powerpc/kernel/ptrace.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
index 55613e3..4edeeb3 100644
--- a/arch/powerpc/kernel/ptrace.c
+++ b/arch/powerpc/kernel/ptrace.c
@@ -1591,7 +1591,10 @@ long arch_ptrace(struct task_struct *child, long request,
 	}
 
 	case PTRACE_SET_DEBUGREG:
+		if (ptrace_get_breakpoints(child) < 0)
+			return -ESRCH;
 		ret = ptrace_set_debugreg(child, addr, data);
+		ptrace_put_breakpoints(child);
 		break;
 
 #ifdef CONFIG_PPC64
-- 
1.7.3.2

[PATCH 4/5] arm, hw_breakpoints: Fix racy access to ptrace breakpoints

[Frederic Weisbecker]

While the tracer accesses ptrace breakpoints, the child task may
concurrently exit due to a SIGKILL and thus release its breakpoints
at the same time. We can then dereference some freed pointers.

To fix this, hold a reference on the child breakpoints before
manipulating them.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Prasad <prasad@linux.vnet.ibm.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Link: http://lkml.kernel.org/r/1302284067-7860-5-git-send-email-fweisbec@gmail.com
---
 arch/arm/kernel/ptrace.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index 2bf27f3..8182f45 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -767,12 +767,20 @@ long arch_ptrace(struct task_struct *child, long request,
 
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
 		case PTRACE_GETHBPREGS:
+			if (ptrace_get_breakpoints(child) < 0)
+				return -ESRCH;
+
 			ret = ptrace_gethbpregs(child, addr,
 						(unsigned long __user *)data);
+			ptrace_put_breakpoints(child);
 			break;
 		case PTRACE_SETHBPREGS:
+			if (ptrace_get_breakpoints(child) < 0)
+				return -ESRCH;
+
 			ret = ptrace_sethbpregs(child, addr,
 						(unsigned long __user *)data);
+			ptrace_put_breakpoints(child);
 			break;
 #endif
 
-- 
1.7.3.2

[PATCH 5/5] sh, hw_breakpoints: Fix racy access to ptrace breakpoints

[Frederic Weisbecker]

While the tracer accesses ptrace breakpoints, the child task may
concurrently exit due to a SIGKILL and thus release its breakpoints
at the same time. We can then dereference some freed pointers.

To fix this, hold a reference on the child breakpoints before
manipulating them.

Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Prasad <prasad@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1302284067-7860-6-git-send-email-fweisbec@gmail.com
---
 arch/sh/kernel/ptrace_32.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/arch/sh/kernel/ptrace_32.c b/arch/sh/kernel/ptrace_32.c
index 2130ca6..3d7b209 100644
--- a/arch/sh/kernel/ptrace_32.c
+++ b/arch/sh/kernel/ptrace_32.c
@@ -117,7 +117,11 @@ void user_enable_single_step(struct task_struct *child)
 
 	set_tsk_thread_flag(child, TIF_SINGLESTEP);
 
+	if (ptrace_get_breakpoints(child) < 0)
+		return;
+
 	set_single_step(child, pc);
+	ptrace_put_breakpoints(child);
 }
 
 void user_disable_single_step(struct task_struct *child)
-- 
1.7.3.2

[GIT PULL] hw_breakpoint fixes

[Frederic Weisbecker]

Ingo,

Please pull the perf/urgent branch that can be found at:

git://git.kernel.org/pub/scm/linux/kernel/git/frederic/random-tracing.git
	perf/urgent

Thanks,
	Frederic
---

Frederic Weisbecker (1):
      x86: Fix instruction breakpoint encoding

Matt Helsley (1):
      hw breakpoints: Fix pid namespace bug


 arch/x86/include/asm/hw_breakpoint.h |    2 +-
 arch/x86/kernel/hw_breakpoint.c      |   40 ++++++++++++++++-----------------
 kernel/hw_breakpoint.c               |    3 +-
 3 files changed, 22 insertions(+), 23 deletions(-)

Re: [GIT PULL] hw_breakpoint fixes

[Ingo Molnar]

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> Ingo,
> 
> Please pull the perf/urgent branch that can be found at:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/random-tracing.git
> 	perf/urgent
> 
> Thanks,
> 	Frederic
> ---
> 
> Frederic Weisbecker (1):
>       x86: Fix instruction breakpoint encoding
> 
> Matt Helsley (1):
>       hw breakpoints: Fix pid namespace bug
> 
> 
>  arch/x86/include/asm/hw_breakpoint.h |    2 +-
>  arch/x86/kernel/hw_breakpoint.c      |   40 ++++++++++++++++-----------------
>  kernel/hw_breakpoint.c               |    3 +-
>  3 files changed, 22 insertions(+), 23 deletions(-)

Pulled, thanks a lot Frederic!

	Ingo