x86, hw_breakpoints: Fix racy access to ptrace breakpoints

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

1 files changed, 26 insertions, 10 deletions

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 45892dc4b72a..f65e5b521dbd 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;
 }
 
 /*