Messages in this thread Patch in this message | | | Date | Thu, 19 May 2011 11:30:36 -0700 | From | Greg KH <> | Subject | [04/24] ptrace: Prepare to fix racy accesses on task breakpoints |
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2.6.33-longterm review patch. If anyone has any objections, please let us know.
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From: Frederic Weisbecker <fweisbec@gmail.com>
commit bf26c018490c2fce7fe9b629083b96ce0e6ad019 upstream.
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> Link: http://lkml.kernel.org/r/1302284067-7860-2-git-send-email-fweisbec@gmail.com Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
--- 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(-)
--- a/include/linux/ptrace.h +++ b/include/linux/ptrace.h @@ -168,6 +168,10 @@ static inline void ptrace_init_task(stru child->ptrace = current->ptrace; __ptrace_link(child, current->parent); } + +#ifdef CONFIG_HAVE_HW_BREAKPOINT + atomic_set(&child->ptrace_bp_refcnt, 1); +#endif } /** @@ -336,6 +340,13 @@ extern int task_current_syscall(struct t 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 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1568,6 +1568,9 @@ struct task_struct { unsigned long memsw_bytes; /* 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. */ --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1001,7 +1001,7 @@ 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); /* * Flush inherited counters to the parent - before the parent * gets woken up by child-exit notifications. --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -22,6 +22,7 @@ #include <linux/pid_namespace.h> #include <linux/syscalls.h> #include <linux/uaccess.h> +#include <linux/hw_breakpoint.h> /* @@ -762,3 +763,19 @@ asmlinkage long compat_sys_ptrace(compat 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 */
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