From: Tom Lane <tgl@sss.pgh.pa.us>
To: Flavien GUEDEZ <flav.pg@oopacity.net>
cc: pgsql-bugs@lists.postgresql.org
Subject: Re: Insufficient memory access checks in pglz_decompress
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References: <01fc0593-e31e-463d-902c-dd43174acee2@oopacity.net>
Comments: In-reply-to Flavien GUEDEZ <flav.pg@oopacity.net>
	message dated "Wed, 18 Oct 2023 16:21:57 +0200"
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Date: Wed, 18 Oct 2023 11:14:27 -0400
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Flavien GUEDEZ <flav.pg@oopacity.net> writes:
> After some investigations about very corrupted toast data in one 
> postgres instance, I found that the pglz_decompress function (in 
> common/pg_lzcompress.c) does not check correctly where it copies data 
> from using memcpy(), which could result in segfault.
> In this function, there are other checks to ensure that we do not copy 
> after the destination end, but not if we copy data from "before the 
> beginning".

Hmm, would it not be better to add this check to the existing "Check for
corrupt data" a bit further up?  Then you'd only need one instance of
the test, and only need to do it once per tag (note the comment pointing
out that dp - off stays the same), and overall it'd be less surprising IMO.

			regards, tom lane