[PATCH nf v2] lib/ts_bm: reset initial match offset for every block of text

[PATCH nf v2] lib/ts_bm: reset initial match offset for every block of text

[Jeremy Sowden]

The `shift` variable which indicates the offset in the string at which
to start matching the pattern is initialized to `bm->patlen - 1`, but it
is not reset when a new block is retrieved.  This means the implemen-
tation may start looking at later and later positions in each successive
block and miss occurrences of the pattern at the beginning.  E.g.,
consider a HTTP packet held in a non-linear skb, where the HTTP request
line occurs in the second block:

  [... 52 bytes of packet headers ...]
  GET /bmtest HTTP/1.1\r\nHost: www.example.com\r\n\r\n

and the pattern is "GET /bmtest".

Once the first block comprising the packet headers has been examined,
`shift` will be pointing to somewhere near the end of the block, and so
when the second block is examined the request line at the beginning will
be missed.

Reinitialize the variable for each new block.

Fixes: 8082e4ed0a61 ("[LIB]: Boyer-Moore extension for textsearch infrastructure strike #2")
Link: https://bugzilla.netfilter.org/show_bug.cgi?id=1390
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
---
Changes since v1

 The indentation and white-space fixes have been moved into a separate patch.

 lib/ts_bm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/ts_bm.c b/lib/ts_bm.c
index 1f2234221dd1..c8ecbf74ef29 100644
--- a/lib/ts_bm.c
+++ b/lib/ts_bm.c
@@ -60,10 +60,12 @@ static unsigned int bm_find(struct ts_config *conf, struct ts_state *state)
 	struct ts_bm *bm = ts_config_priv(conf);
 	unsigned int i, text_len, consumed = state->offset;
 	const u8 *text;
-	int shift = bm->patlen - 1, bs;
+	int bs;
 	const u8 icase = conf->flags & TS_IGNORECASE;
 
 	for (;;) {
+		int shift = bm->patlen - 1;
+
 		text_len = conf->get_next_block(consumed, &text, conf, state);
 
 		if (unlikely(text_len == 0))
-- 
2.39.2

Re: [PATCH nf v2] lib/ts_bm: reset initial match offset for every block of text

[Pablo Neira Ayuso]

On Mon, Jun 19, 2023 at 08:06:57PM +0100, Jeremy Sowden wrote:
> The `shift` variable which indicates the offset in the string at which
> to start matching the pattern is initialized to `bm->patlen - 1`, but it
> is not reset when a new block is retrieved.  This means the implemen-
> tation may start looking at later and later positions in each successive
> block and miss occurrences of the pattern at the beginning.  E.g.,
> consider a HTTP packet held in a non-linear skb, where the HTTP request
> line occurs in the second block:
> 
>   [... 52 bytes of packet headers ...]
>   GET /bmtest HTTP/1.1\r\nHost: www.example.com\r\n\r\n
> 
> and the pattern is "GET /bmtest".
> 
> Once the first block comprising the packet headers has been examined,
> `shift` will be pointing to somewhere near the end of the block, and so
> when the second block is examined the request line at the beginning will
> be missed.
> 
> Reinitialize the variable for each new block.

Applied to nf.git, thanks