[PATCH] tracing/filters: Treat zero-length dynamic strings as empty

[Kyle Zeng <kylebot@openai.com>]

The regex callbacks use a length of zero for NUL-terminated strings of
unknown size, such as the trailing buffer in a trace_marker record.
However, filter_pred_strloc() and filter_pred_strrelloc() pass the length
from a data locator directly to those callbacks. A zero-length locator
has no data and need not point at a NUL byte.

In particular, syscall entry events can contain an empty captured array
or a failed user-string copy. Matching such a field with an equality,
prefix, or substring filter can read beyond the record. The per-CPU perf
trace buffer is reused, so an unprivileged counting syscall event can
disclose bytes left by a previous kernel tracepoint even when raw
sampling is denied and exclude_kernel prevents counting that tracepoint.

Handle both locator formats through a common helper. For a zero-length
field, pass an empty string and its actual size to the regex callback.
This gives all matching modes the empty-string semantics without reading
the locator's target. Keep nonzero fields and the legacy trace_marker
unknown-length convention unchanged.

Fixes: 10f20e9f9da9 ("tracing: Have zero size length in filter logic be full string")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-6-astra
Signed-off-by: Kyle Zeng <kylebot@openai.com>
---
 kernel/trace/trace_events_filter.c | 38 +++++++++++++++++++-----------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 2b46ca536045..02aaa92e0361 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -884,6 +884,25 @@ static int filter_pred_pchar_user(struct filter_pred *pred, void *event)
 	return filter_pchar(pred, str);
 }
 
+static __always_inline int filter_dyn_string(struct filter_pred *pred,
+					     char *str, int len)
+{
+	int cmp;
+
+	/*
+	 * The regex callbacks use zero for an unknown-length, NUL-terminated
+	 * string. A zero-length data_loc or rel_loc field has no bytes at all.
+	 * Match an empty string instead of reading past the end of the field.
+	 */
+	if (!len) {
+		str = "";
+		len = 1;
+	}
+
+	cmp = pred->regex->match(str, pred->regex, len);
+	return cmp ^ pred->not;
+}
+
 /*
  * Filter predicate for dynamic sized arrays of characters.
  * These are implemented through a list of strings at the end
@@ -900,13 +919,8 @@ static int filter_pred_strloc(struct filter_pred *pred, void *event)
 	int str_loc = str_item & 0xffff;
 	int str_len = str_item >> 16;
 	char *addr = (char *)(event + str_loc);
-	int cmp, match;
-
-	cmp = pred->regex->match(addr, pred->regex, str_len);
 
-	match = cmp ^ pred->not;
-
-	return match;
+	return filter_dyn_string(pred, addr, str_len);
 }
 
 /*
@@ -923,13 +937,8 @@ static int filter_pred_strrelloc(struct filter_pred *pred, void *event)
 	int str_loc = str_item & 0xffff;
 	int str_len = str_item >> 16;
 	char *addr = (char *)(&item[1]) + str_loc;
-	int cmp, match;
-
-	cmp = pred->regex->match(addr, pred->regex, str_len);
 
-	match = cmp ^ pred->not;
-
-	return match;
+	return filter_dyn_string(pred, addr, str_len);
 }
 
 /* Filter predicate for CPUs. */
@@ -1014,7 +1023,8 @@ static int filter_pred_function(struct filter_pred *pred, void *event)
  *
  * @str: the string to be searched
  * @r:   the regex structure containing the pattern string
- * @len: the length of the string to be searched (including '\0')
+ * @len: the length of the string to be searched (including '\0'), or zero
+ *       for a NUL-terminated string of unknown length
  *
  * Note:
  * - @str might not be NULL-terminated if it's of type DYN_STRING
@@ -1023,7 +1033,7 @@ static int filter_pred_function(struct filter_pred *pred, void *event)
 
 static int regex_match_full(char *str, struct regex *r, int len)
 {
-	/* len of zero means str is dynamic and ends with '\0' */
+	/* Zero is reserved for a NUL-terminated string of unknown length. */
 	if (!len)
 		return strcmp(str, r->pattern) == 0;