CREATE TEXT SEARCH DICTIONARY segfaulting on 9.6+

I spent a bit of time investigating this, and it seems the new code is
somewhat too trusting when it comes to data from the affix/dict files.
In this particular case, it boils down to this code in NISortDictionary:

if (Conf->useFlagAliases)
{
for (i = 0; i < Conf->nspell; i++)
{
char *end;

if (*Conf->Spell[i]->p.flag != '\0')
{
curaffix = strtol(Conf->Spell[i]->p.flag, &end, 10);
if (Conf->Spell[i]->p.flag == end || errno == ERANGE)
ereport(ERROR,
(errcode(ERRCODE_CONFIG_FILE_ERROR),
errmsg("invalid affix alias \"%s\"",
Conf->Spell[i]->p.flag)));
}
...
Conf->Spell[i]->p.d.affix = curaffix;
...
}
...
}

So it simply grabs whatever it finds in the dict file, parses it and
then (later) we use it as index to access the AffixData array, even if
the value is way out of bounds.

For example in the example, hunspell_sample_long.affix contains about
10 affixes, but then we parse the hunspell_sample_num.dict file, and we
stumble upon

book/302,301,202,303

and we parse the flags as integers, and interpret them as indexes in the
AffixData array. Clearly, 303 is waaaay out of bounds, triggering the
segfault crash.

So I think we need some sort of cross-check here. We certainly need to
make NISortDictionary() check the affix value is within AffixData
bounds, and error out when the index is non-sensical (maybe negative
and/or exceeding nAffixData). Maybe there's a simple way to check if the
affix/dict files match. The failing affix has

FLAG num

while with

FLAG long

it works just fine. But I'm not sure that's actually possible, because I
don't see anything in hunspell_sample_num.dict that would allow us to
decide that it expects "FLAG num" and not "FLAG long". Furthermore, we
certainly can't rely on this - we still need to check the range.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Hello Tomas,

On 2019/10/13 10:26, Tomas Vondra wrote:

over in pgsql-bugs [1] we got a report about CREATE TEXT SEARCH
DICTIONARY causing segfaults on 12.0. Simply running

   CREATE TEXT SEARCH DICTIONARY hunspell_num (Template=ispell,
   DictFile=hunspell_sample_num, AffFile=hunspell_sample_long);

does trigger a crash, 100% of the time. The crash was reported on 12.0,
but it's in fact present since 9.6.

On 9.5 the example does not work, because that version does not (a)
include the hunspell dictionaries used in the example, and (b) it does
not support long flags. So even after copying the dictionaries and
tweaking them a bit it still passes without a crash.

This crash is not because of long flags, but because of aliases (more
thoughts below).

Looking at the commit history of spell.c, there seems to be a bunch of
commits in 2016 (e.g. f4ceed6ceba3) touching exactly this part of the
code (hunspell), and it also correlates quite nicely with the affected
branches (9.6+). So my best guess is it's a bug in those changes.

Yeah, there was a lot changes.

So it simply grabs whatever it finds in the dict file, parses it and
then (later) we use it as index to access the AffixData array, even if
the value is way out of bounds.

Yes, we enter this code if an affix file defines aliases (AF parameter).
AffixData array is used to store those aliases.

More about hunspell format you can find here:
https://linux.die.net/man/4/hunspell

In the example we have the following aliases:
AF 11
AF cZ #1
AF cL #2
...
AF sB #11

And in the dictionary file we should use their indexes (from 1 to 11).
These aliases defines set of flags and in the dict file we can use only
single index:
book/3
book/11

but not:
book/3,4
book/2,11

I added checking of this last case in the attached patch. PostgreSQL
will raise an error if it sees non-numeric and non-whitespace character
after the index.

Aliases can be used with all flag types: 'default' (i.e. FM_CHAR),
'long', and if I'm not mistaken 'num'.

So I think we need some sort of cross-check here. We certainly need to
make NISortDictionary() check the affix value is within AffixData
bounds, and error out when the index is non-sensical (maybe negative
and/or exceeding nAffixData).

I agree, I attached the patch which do this. I also added couple
asserts, tests and fixed condition in getAffixFlagSet():

-		if (curaffix > 0 && curaffix <= Conf->nAffixData)
+		if (curaffix > 0 && curaffix < Conf->nAffixData)

I think it could be a bug, because curaffix can't be equal to
Conf->nAffixData.

Maybe there's a simple way to check if the affix/dict files match.

I'm not sure how to properly fix this either. The only thing we could
check is commas in affix flags in a dict file:

book/302,301,202,303

FM_CHAR and FM_LONG dictionaries can't have commas. They should have the
following affix flags:

book/sGsJpUsS # 4 affixes for FM_LONG
book/GJUS # 4 affixes for FM_CHAR

But I guess they could have numbers in flags (as help says "Set flag
type. Default type is the extended ASCII (8-bit) character.") and other
non alphanumeric characters (as some language dictionaries have):

book/s1s2s3s4 # 4 affixes for FM_LONG

--
Artur

Arthur Zakirov <zaartur@gmail.com> writes:

On 2019/10/13 10:26, Tomas Vondra wrote:

So I think we need some sort of cross-check here. We certainly need to
make NISortDictionary() check the affix value is within AffixData
bounds, and error out when the index is non-sensical (maybe negative
and/or exceeding nAffixData).

I agree, I attached the patch which do this. I also added couple
asserts, tests and fixed condition in getAffixFlagSet():

-		if (curaffix > 0 && curaffix <= Conf->nAffixData)
+		if (curaffix > 0 && curaffix < Conf->nAffixData)

Looks reasonable to me, and we need to get something done before
the upcoming releases, so I pushed this. Perhaps there's more
that could be done later.

regards, tom lane

On Sun, Nov 3, 2019 at 5:48 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:

Arthur Zakirov <zaartur@gmail.com> writes:

On 2019/10/13 10:26, Tomas Vondra wrote:

So I think we need some sort of cross-check here. We certainly need to
make NISortDictionary() check the affix value is within AffixData
bounds, and error out when the index is non-sensical (maybe negative
and/or exceeding nAffixData).

I agree, I attached the patch which do this. I also added couple
asserts, tests and fixed condition in getAffixFlagSet():

-             if (curaffix > 0 && curaffix <= Conf->nAffixData)
+             if (curaffix > 0 && curaffix < Conf->nAffixData)

Looks reasonable to me, and we need to get something done before
the upcoming releases, so I pushed this. Perhaps there's more
that could be done later.

Great, thank you!

--
Artur