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    Subject[PATCH 5.2 075/143] ftrace: Fix NULL pointer dereference in t_probe_next()
    Date
    From: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>

    commit 7bd46644ea0f6021dc396a39a8bfd3a58f6f1f9f upstream.

    LTP testsuite on powerpc results in the below crash:

    Unable to handle kernel paging request for data at address 0x00000000
    Faulting instruction address: 0xc00000000029d800
    Oops: Kernel access of bad area, sig: 11 [#1]
    LE SMP NR_CPUS=2048 NUMA PowerNV
    ...
    CPU: 68 PID: 96584 Comm: cat Kdump: loaded Tainted: G W
    NIP: c00000000029d800 LR: c00000000029dac4 CTR: c0000000001e6ad0
    REGS: c0002017fae8ba10 TRAP: 0300 Tainted: G W
    MSR: 9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE> CR: 28022422 XER: 20040000
    CFAR: c00000000029d90c DAR: 0000000000000000 DSISR: 40000000 IRQMASK: 0
    ...
    NIP [c00000000029d800] t_probe_next+0x60/0x180
    LR [c00000000029dac4] t_mod_start+0x1a4/0x1f0
    Call Trace:
    [c0002017fae8bc90] [c000000000cdbc40] _cond_resched+0x10/0xb0 (unreliable)
    [c0002017fae8bce0] [c0000000002a15b0] t_start+0xf0/0x1c0
    [c0002017fae8bd30] [c0000000004ec2b4] seq_read+0x184/0x640
    [c0002017fae8bdd0] [c0000000004a57bc] sys_read+0x10c/0x300
    [c0002017fae8be30] [c00000000000b388] system_call+0x5c/0x70

    The test (ftrace_set_ftrace_filter.sh) is part of ftrace stress tests
    and the crash happens when the test does 'cat
    $TRACING_PATH/set_ftrace_filter'.

    The address points to the second line below, in t_probe_next(), where
    filter_hash is dereferenced:
    hash = iter->probe->ops.func_hash->filter_hash;
    size = 1 << hash->size_bits;

    This happens due to a race with register_ftrace_function_probe(). A new
    ftrace_func_probe is created and added into the func_probes list in
    trace_array under ftrace_lock. However, before initializing the filter,
    we drop ftrace_lock, and re-acquire it after acquiring regex_lock. If
    another process is trying to read set_ftrace_filter, it will be able to
    acquire ftrace_lock during this window and it will end up seeing a NULL
    filter_hash.

    Fix this by just checking for a NULL filter_hash in t_probe_next(). If
    the filter_hash is NULL, then this probe is just being added and we can
    simply return from here.

    Link: http://lkml.kernel.org/r/05e021f757625cbbb006fad41380323dbe4e3b43.1562249521.git.naveen.n.rao@linux.vnet.ibm.com

    Cc: stable@vger.kernel.org
    Fixes: 7b60f3d876156 ("ftrace: Dynamically create the probe ftrace_ops for the trace_array")
    Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/trace/ftrace.c | 4 ++++
    1 file changed, 4 insertions(+)

    --- a/kernel/trace/ftrace.c
    +++ b/kernel/trace/ftrace.c
    @@ -3095,6 +3095,10 @@ t_probe_next(struct seq_file *m, loff_t
    hnd = &iter->probe_entry->hlist;

    hash = iter->probe->ops.func_hash->filter_hash;
    +
    + if (!hash)
    + return NULL;
    +
    size = 1 << hash->size_bits;

    retry:

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