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    Subject[PATCH 5.10 03/43] tracing: Fix check for trace_percpu_buffer validity in get_trace_buf()
    Date
    From: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>

    commit 823e670f7ed616d0ce993075c8afe0217885f79d upstream.

    With the new osnoise tracer, we are seeing the below splat:
    Kernel attempted to read user page (c7d880000) - exploit attempt? (uid: 0)
    BUG: Unable to handle kernel data access on read at 0xc7d880000
    Faulting instruction address: 0xc0000000002ffa10
    Oops: Kernel access of bad area, sig: 11 [#1]
    LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA pSeries
    ...
    NIP [c0000000002ffa10] __trace_array_vprintk.part.0+0x70/0x2f0
    LR [c0000000002ff9fc] __trace_array_vprintk.part.0+0x5c/0x2f0
    Call Trace:
    [c0000008bdd73b80] [c0000000001c49cc] put_prev_task_fair+0x3c/0x60 (unreliable)
    [c0000008bdd73be0] [c000000000301430] trace_array_printk_buf+0x70/0x90
    [c0000008bdd73c00] [c0000000003178b0] trace_sched_switch_callback+0x250/0x290
    [c0000008bdd73c90] [c000000000e70d60] __schedule+0x410/0x710
    [c0000008bdd73d40] [c000000000e710c0] schedule+0x60/0x130
    [c0000008bdd73d70] [c000000000030614] interrupt_exit_user_prepare_main+0x264/0x270
    [c0000008bdd73de0] [c000000000030a70] syscall_exit_prepare+0x150/0x180
    [c0000008bdd73e10] [c00000000000c174] system_call_vectored_common+0xf4/0x278

    osnoise tracer on ppc64le is triggering osnoise_taint() for negative
    duration in get_int_safe_duration() called from
    trace_sched_switch_callback()->thread_exit().

    The problem though is that the check for a valid trace_percpu_buffer is
    incorrect in get_trace_buf(). The check is being done after calculating
    the pointer for the current cpu, rather than on the main percpu pointer.
    Fix the check to be against trace_percpu_buffer.

    Link: https://lkml.kernel.org/r/a920e4272e0b0635cf20c444707cbce1b2c8973d.1640255304.git.naveen.n.rao@linux.vnet.ibm.com

    Cc: stable@vger.kernel.org
    Fixes: e2ace001176dc9 ("tracing: Choose static tp_printk buffer by explicit nesting count")
    Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    kernel/trace/trace.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/kernel/trace/trace.c
    +++ b/kernel/trace/trace.c
    @@ -3144,7 +3144,7 @@ static char *get_trace_buf(void)
    {
    struct trace_buffer_struct *buffer = this_cpu_ptr(trace_percpu_buffer);

    - if (!buffer || buffer->nesting >= 4)
    + if (!trace_percpu_buffer || buffer->nesting >= 4)
    return NULL;

    buffer->nesting++;

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