tracing: Add length protection to histogram string copies

The string copies to the histogram storage has a max size of 256 bytes
(defined by MAX_FILTER_STR_VAL). Only the string size of the event field
needs to be copied to the event storage, but no more than what is in the
event storage. Although nothing should be bigger than 256 bytes, there's
no protection against overwriting of the storage if one day there is.

Copy no more than the destination size, and enforce it.

Also had to turn MAX_FILTER_STR_VAL into an unsigned int, to keep the
min() comparison of the string sizes of comparable types.

Link: https://lore.kernel.org/all/CAHk-=wjREUihCGrtRBwfX47y_KrLCGjiq3t6QtoNJpmVrAEb1w@mail.gmail.com/
Link: https://lkml.kernel.org/r/[email protected]

Cc: Ingo Molnar <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Tom Zanussi <[email protected]>
Reported-by: Linus Torvalds <[email protected]>
Reviewed-by: Masami Hiramatsu <[email protected]>
Fixes: 63f84ae ("tracing/histogram: Do not copy the fixed-size char array field over the field size")
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

include/linux/trace_events.h

@@ -673,7 +673,7 @@ struct trace_event_file {
 
 #define PERF_MAX_TRACE_SIZE	8192
 
-#define MAX_FILTER_STR_VAL	256	/* Should handle KSYM_SYMBOL_LEN */
+#define MAX_FILTER_STR_VAL	256U	/* Should handle KSYM_SYMBOL_LEN */
 
 enum event_trigger_type {
 	ETT_NONE		= (0),

kernel/trace/trace_events_hist.c

@@ -3026,8 +3026,10 @@ static inline void __update_field_vars(struct tracing_map_elt *elt,
 		if (val->flags & HIST_FIELD_FL_STRING) {
 			char *str = elt_data->field_var_str[j++];
 			char *val_str = (char *)(uintptr_t)var_val;
+			unsigned int size;
 
-			strscpy(str, val_str, val->size);
+			size = min(val->size, STR_VAR_LEN_MAX);
+			strscpy(str, val_str, size);
 			var_val = (u64)(uintptr_t)str;
 		}
 		tracing_map_set_var(elt, var_idx, var_val);
@@ -4914,6 +4916,7 @@ static void hist_trigger_elt_update(struct hist_trigger_data *hist_data,
 			if (hist_field->flags & HIST_FIELD_FL_STRING) {
 				unsigned int str_start, var_str_idx, idx;
 				char *str, *val_str;
+				unsigned int size;
 
 				str_start = hist_data->n_field_var_str +
 					hist_data->n_save_var_str;
@@ -4922,7 +4925,9 @@ static void hist_trigger_elt_update(struct hist_trigger_data *hist_data,
 
 				str = elt_data->field_var_str[idx];
 				val_str = (char *)(uintptr_t)hist_val;
-				strscpy(str, val_str, hist_field->size);
+
+				size = min(hist_field->size, STR_VAR_LEN_MAX);
+				strscpy(str, val_str, size);
 
 				hist_val = (u64)(uintptr_t)str;
 			}