Re: BUG #19525: In `contrib/dict_int`, handling a token whose first byte is a null byte causes `pnstrdup()` .

The fix looks correct. Recomputing len from the copy via strlen(txt) after pnstrdup() in dict_int directly addresses the root cause I reported. The dict_xsyn fix is also a clean approach since skipping the intermediate copy avoids the length mismatch entirely. Thank you for the patch!










 王跃林
3020001251@tju.edu.cn











Original:
From：Ayush Tiwari <ayushtiwari.slg01@gmail.com>Date：2026-06-18 22:41:32(中国 (GMT+08:00))To：3020001251<3020001251@tju.edu.cn> , pgsql-bugs<pgsql-bugs@lists.postgresql.org>Cc：Subject：Re: BUG #19525: In `contrib/dict_int`, handling a token whose first byte is a null byte causes `pnstrdup()` .Hi,

On Thu, 18 Jun 2026 at 18:54, PG Bug reporting form <noreply@postgresql.org> wrote:

The following bug has been logged on the website:

 Bug reference:      19525
 Logged by:          Yuelin Wang
 Email address:      3020001251@tju.edu.cn
 PostgreSQL version: 19beta1
 Operating system:   Linux (Ubuntu 24.04, x86_64)
 Description:       

 **Component**: `contrib/dict_int/dict_int.c`, function `dintdict_lexize()`
 (line 109)

 Requires a `SQL_ASCII`-encoded database (to bypass null-byte encoding
 checks) and superuser to install the extension and create a helper function
 that passes a `bytea` token directly to the lexize callback. Once the
 dictionary is created, any role granted `EXECUTE` on the helper can trigger
 the crash.

 ```sql
 -- 1. Create SQL_ASCII database (null bytes are not rejected)
 CREATE DATABASE vuln_ascii ENCODING 'SQL_ASCII' TEMPLATE template0;
 \c vuln_ascii

 -- 2. Install extension and create an intdict dictionary with
 REJECTLONG=false
 CREATE EXTENSION dict_int;
 CREATE TEXT SEARCH DICTIONARY intdict_test (
     TEMPLATE = intdict_template,
     MAXLEN = 8192,
     REJECTLONG = false
 );

 -- 3. Create a C helper (raw_lexize.so) that invokes the lexize callback
 with
 --    a raw bytea token, bypassing the text encoding layer.
 CREATE FUNCTION raw_lexize(dict regdictionary, token bytea)
     RETURNS text[] AS 'raw_lexize', 'raw_lexize' LANGUAGE C STRICT;

 -- 4. Trigger: null byte at position 0 causes pnstrdup to allocate 1 byte,
 --    but txt[8192] = '\0' writes 8191 bytes past the end of the allocation.
 SELECT raw_lexize('intdict_test',
     decode('00' || repeat('78', 10000), 'hex'));
 -- Server closes connection; ASan reports heap-buffer-overflow WRITE of size
 1
 -- at dict_int.c:109 in dintdict_lexize.
 ```

 ASan confirmation (server killed the backend; connection dropped):
 ```
 ==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x525000052880
 WRITE of size 1 at 0x525000052880 thread T0
     #0 in dintdict_lexize
 /data/ylwang/Projects/postgres/contrib/dict_int/dict_int.c:109
     #1 in FunctionCall4Coll .../src/backend/utils/fmgr/fmgr.c:1215
     #2 in raw_lexize /tmp/raw_lexize.c:37
 SUMMARY: AddressSanitizer: heap-buffer-overflow
 .../contrib/dict_int/dict_int.c:109 in dintdict_lexize

Thanks for the report and repro! 


`pnstrdup(ptr, len)` uses `strnlen(ptr, len)` internally, so when the token
 begins with a null byte it allocates only 1 byte. The variable `len` is not
 updated to reflect this and retains the original token length, so the guard
 at line 98 (`if (len > d->maxlen)`) passes, and line 109 writes `'\0'` at
 offset `d->maxlen` (e.g., 8192) into a 1-byte allocation.

 The fix is to recompute the effective length from the allocated buffer after
 the `pnstrdup` call, for example by replacing the `if (len > d->maxlen)`
 check with `if (strlen(txt) > d->maxlen)`. This ensures the truncation
 offset is always within the bounds of what `pnstrdup` actually allocated.

Your analysis seems right to me.

While looking around I think dict_xsyn may have a related issue: in
dxsyn_lexize() the token is copied with pnstrdup() and the original
length is then handed to str_tolower(), which reads that many bytes and
so could read past the shorter copy.

Attaching a patch that fixes both the above issues.

Regards,
Ayush