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    Subject[PATCH 5.4 005/149] tracing: Do not reference char * as a string in histograms
    Date
    From: Steven Rostedt (VMware) <rostedt@goodmis.org>

    commit 704adfb5a9978462cd861f170201ae2b5e3d3a80 upstream.

    The histogram logic was allowing events with char * pointers to be used as
    normal strings. But it was easy to crash the kernel with:

    # echo 'hist:keys=filename' > events/syscalls/sys_enter_openat/trigger

    And open some files, and boom!

    BUG: unable to handle page fault for address: 00007f2ced0c3280
    #PF: supervisor read access in kernel mode
    #PF: error_code(0x0000) - not-present page
    PGD 1173fa067 P4D 1173fa067 PUD 1171b6067 PMD 1171dd067 PTE 0
    Oops: 0000 [#1] PREEMPT SMP
    CPU: 6 PID: 1810 Comm: cat Not tainted 5.13.0-rc5-test+ #61
    Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01
    v03.03 07/14/2016
    RIP: 0010:strlen+0x0/0x20
    Code: f6 82 80 2a 0b a9 20 74 11 0f b6 50 01 48 83 c0 01 f6 82 80 2a 0b
    a9 20 75 ef c3 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 <80> 3f 00 74
    10 48 89 f8 48 83 c0 01 80 38 00 75 f7 48 29 f8 c3

    RSP: 0018:ffffbdbf81567b50 EFLAGS: 00010246
    RAX: 0000000000000003 RBX: ffff93815cdb3800 RCX: ffff9382401a22d0
    RDX: 0000000000000100 RSI: 0000000000000000 RDI: 00007f2ced0c3280
    RBP: 0000000000000100 R08: ffff9382409ff074 R09: ffffbdbf81567c98
    R10: ffff9382409ff074 R11: 0000000000000000 R12: ffff9382409ff074
    R13: 0000000000000001 R14: ffff93815a744f00 R15: 00007f2ced0c3280
    FS: 00007f2ced0f8580(0000) GS:ffff93825a800000(0000)
    knlGS:0000000000000000
    CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 00007f2ced0c3280 CR3: 0000000107069005 CR4: 00000000001706e0
    Call Trace:
    event_hist_trigger+0x463/0x5f0
    ? find_held_lock+0x32/0x90
    ? sched_clock_cpu+0xe/0xd0
    ? lock_release+0x155/0x440
    ? kernel_init_free_pages+0x6d/0x90
    ? preempt_count_sub+0x9b/0xd0
    ? kernel_init_free_pages+0x6d/0x90
    ? get_page_from_freelist+0x12c4/0x1680
    ? __rb_reserve_next+0xe5/0x460
    ? ring_buffer_lock_reserve+0x12a/0x3f0
    event_triggers_call+0x52/0xe0
    ftrace_syscall_enter+0x264/0x2c0
    syscall_trace_enter.constprop.0+0x1ee/0x210
    do_syscall_64+0x1c/0x80
    entry_SYSCALL_64_after_hwframe+0x44/0xae

    Where it triggered a fault on strlen(key) where key was the filename.

    The reason is that filename is a char * to user space, and the histogram
    code just blindly dereferenced it, with obvious bad results.

    I originally tried to use strncpy_from_user/kernel_nofault() but found
    that there's other places that its dereferenced and not worth the effort.

    Just do not allow "char *" to act like strings.

    Link: https://lkml.kernel.org/r/20210715000206.025df9d2@rorschach.local.home

    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Masami Hiramatsu <mhiramat@kernel.org>
    Cc: Tzvetomir Stoyanov <tz.stoyanov@gmail.com>
    Cc: stable@vger.kernel.org
    Acked-by: Namhyung Kim <namhyung@kernel.org>
    Acked-by: Tom Zanussi <zanussi@kernel.org>
    Fixes: 79e577cbce4c4 ("tracing: Support string type key properly")
    Fixes: 5967bd5c4239 ("tracing: Let filter_assign_type() detect FILTER_PTR_STRING")
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    kernel/trace/trace_events_hist.c | 6 +++---
    1 file changed, 3 insertions(+), 3 deletions(-)

    --- a/kernel/trace/trace_events_hist.c
    +++ b/kernel/trace/trace_events_hist.c
    @@ -2571,7 +2571,9 @@ static struct hist_field *create_hist_fi
    if (WARN_ON_ONCE(!field))
    goto out;

    - if (is_string_field(field)) {
    + /* Pointers to strings are just pointers and dangerous to dereference */
    + if (is_string_field(field) &&
    + (field->filter_type != FILTER_PTR_STRING)) {
    flags |= HIST_FIELD_FL_STRING;

    hist_field->size = MAX_FILTER_STR_VAL;
    @@ -5326,8 +5328,6 @@ static inline void add_to_key(char *comp
    field = key_field->field;
    if (field->filter_type == FILTER_DYN_STRING)
    size = *(u32 *)(rec + field->offset) >> 16;
    - else if (field->filter_type == FILTER_PTR_STRING)
    - size = strlen(key);
    else if (field->filter_type == FILTER_STATIC_STRING)
    size = field->size;


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