[GIT PULL] tracing: Do not reference char * as a string in histograms

[Steven Rostedt]

Linus,

tracing: Fix the histogram logic from possibly crashing the kernel

Working on the histogram code, I found that if you dereference a char
pointer in a trace event that happens to point to user space, it can crash
the kernel, as it does no checks of that pointer. I have code coming that
will do this better, so just remove this ability to treat character
pointers in trace events as stings in the histogram.

This does not have the controversial __assign_str_len() patch.


Please pull the latest trace-v5.14-5 tree, which can be found at:


  git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
trace-v5.14-5

Tag SHA1: da4cba2556dcb19abbc7f4d2fe67d71bdd04d757
Head SHA1: 704adfb5a9978462cd861f170201ae2b5e3d3a80


Steven Rostedt (VMware) (1):
      tracing: Do not reference char * as a string in histograms

----
 kernel/trace/trace_events_hist.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)
---------------------------
commit 704adfb5a9978462cd861f170201ae2b5e3d3a80
Author: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Date:   Thu Jul 15 00:02:06 2021 -0400

    tracing: Do not reference char * as a string in histograms
    
    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@xxxxxxxxxxxxxxxxxxxx
    
    Cc: Ingo Molnar <mingo@xxxxxxxxxx>
    Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
    Cc: Tzvetomir Stoyanov <tz.stoyanov@xxxxxxxxx>
    Cc: stable@xxxxxxxxxxxxxxx
    Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>
    Acked-by: Tom Zanussi <zanussi@xxxxxxxxxx>
    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@xxxxxxxxxxx>

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;