Re: [PATCH v2 1/9] tracing/filters: Fix MATCH_END_ONLY regex matching for fixed-size strings

[Masami Hiramatsu (Google)]

On Sun, 25 Oct 2026 17:37:19 +0900, Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> wrote:
> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> 
> regex_match_end() calculated string length as "len - 1", assuming @len is
> the string length including trailing '\0'. However, for fixed-size string
> fields, filter_pred_string() passes field->size as @len, and for "comm",
> filter_pred_comm() passes TASK_COMM_LEN as @len.
> 
> Because @len in these cases represents the buffer capacity rather than the
> actual length of the string, regex_match_end() tests against the tail of
> the buffer (typically trailing '\0' padding) rather than the end of the
> string. Furthermore, if @len is 0 (denoting an empty dynamic array without
> trailing '\0'), "len - 1" underflows to -1.
> 
> First check "len < r->len" and return 0, which safely rejects negative
> lengths and cases where the buffer capacity is smaller than the pattern.
> Then compute the actual string length using strnlen(str, len), which
> bounds the scan by @len (stopping at the first '\0') without dereferencing
> past the buffer.
> 
> Fixes: a3291c14ecf0 ("tracing/filters: Fix MATCH_END_ONLY filter matching")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> ---
> Changes in v2:
>  - Check "len < r->len" to guard against negative lengths and avoid
>    calling strnlen() when the buffer capacity is smaller than the pattern.
>  - Use strnlen(str, len) directly instead of calling strlen(str) when
>    len is 0, avoiding an out-of-bounds read on 0-length dynamic arrays.
> ---
>  kernel/trace/trace_events_filter.c |   10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> index 5e74d2dae40a..fac51a2953dd 100644
> --- a/kernel/trace/trace_events_filter.c
> +++ b/kernel/trace/trace_events_filter.c
> @@ -1075,10 +1075,14 @@ static int regex_match_middle(char *str, struct regex *r, int len)
>  
>  static int regex_match_end(char *str, struct regex *r, int len)
>  {
> -	int strlen = len - 1;
> +	int slen;
>  
> -	if (strlen >= r->len &&
> -	    memcmp(str + strlen - r->len, r->pattern, r->len) == 0)
> +	if (len < r->len)
> +		return 0;
> +
> +	slen = strnlen(str, len);

Hmm, this fixes 2 different issues. One is out-of-bound access issue (len == 0 case)
and MATCH_END_ONLY for COMM/STATIC_STRING issue. For example, FILTER_COMM will pass
the TASK_COMM_LEN as @len, thus "str + (len - 1)" points the end of comm array.
Thus in most cases, the filter checks only the zero padding area.

OK, let me split this into 2 patches.

Thanks,

> +	if (slen >= r->len &&
> +	    memcmp(str + slen - r->len, r->pattern, r->len) == 0)
>  		return 1;
>  	return 0;
>  }
> 
>

-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>