Re: [PATCH] tracing: Do no reference char * as a string in histograms

[Tom Zanussi]

Hi Steve,

On Thu, 2021-07-15 at 00:02 -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> 
> 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.

The original commit introducing FILTER_PTR_STR only intended char *
string dereferences when it was known to be safe:

commit 87a342f5db69d53ea70493bb1ec69c9047677038
Author: Li Zefan <lizf@xxxxxxxxxxxxxx>
Date:   Fri Aug 7 10:33:43 2009 +0800

    tracing/filters: Support filtering for char * strings

    ...

    The filtering will not dereference "char *" unless the developer
    explicitly sets FILTER_PTR_STR in __field_ext.


But it looks like a later patch opened it up for trace_define_field()
to get around that for any char * field:  

commit 5967bd5c4239be449744a1471daf60c866486c24
Author: Cong Wang <xiyou.wangcong@xxxxxxxxx>
Date:   Sat May 25 09:58:00 2019 -0700

    tracing: Let filter_assign_type() detect FILTER_PTR_STRING
    
    filter_assign_type() could detect dynamic string and static
    string, but not string pointers. Teach filter_assign_type()
    to detect string pointers, and this will be needed by trace
    event injection code.
    
    BTW, trace event hist uses FILTER_PTR_STRING too.
    Link: http://lkml.kernel.org/r/20190525165802.25944-3-xiyou.wangcong@xxxxxxxxx


Not sure what the correct fix is at this point, but it would seem that
if the original intent was honored, the histogram code should still be
allowed to use them.

Tom



> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 79e577cbce4c4 ("tracing: Support string type key properly")
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> ---
>  kernel/trace/trace_events_hist.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_hist.c
> b/kernel/trace/trace_events_hist.c
> index 0207aeed31e6..16a9dfc9fffc 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -1689,7 +1689,9 @@ static struct hist_field
> *create_hist_field(struct hist_trigger_data *hist_data,
>  	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;
> @@ -4495,8 +4497,6 @@ static inline void add_to_key(char
> *compound_key, void *key,
>  		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;
>