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


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