Re: libpq compression

On Tue, Dec 22, 2020 at 07:15:23PM +0100, Tomas Vondra wrote:
>
>
> On 12/22/20 6:56 PM, Robert Haas wrote:
> >On Tue, Dec 22, 2020 at 6:24 AM Daniil Zakhlystov
> ><usernamedt(at)yandex-team(dot)ru> wrote:
> >>When using bidirectional compression, Postgres resource usage correlates with the selected compression level. For example, here is the Postgresql application memory usage:
> >>
> >>No compression - 1.2 GiB
> >>
> >>ZSTD
> >>zstd:1 - 1.4 GiB
> >>zstd:7 - 4.0 GiB
> >>zstd:13 - 17.7 GiB
> >>zstd:19 - 56.3 GiB
> >>zstd:20 - 109.8 GiB - did not succeed
> >>zstd:21, zstd:22 > 140 GiB
> >>Postgres process crashes (out of memory)
> >
> >Good grief. So, suppose we add compression and support zstd. Then, can
> >unprivileged user capable of connecting to the database can negotiate
> >for zstd level 1 and then choose to actually send data compressed at
> >zstd level 22, crashing the server if it doesn't have a crapton of
> >memory? Honestly, I wouldn't blame somebody for filing a CVE if we
> >allowed that sort of thing to happen. I'm not sure what the solution
> >is, but we can't leave a way for a malicious client to consume 140GB
> >of memory on the server *per connection*. I assumed decompression
> >memory was going to measured in kB or MB, not GB. Honestly, even at
> >say L7, if you've got max_connections=100 and a user who wants to make
> >trouble, you have a really big problem.
> >
> >Perhaps I'm being too pessimistic here, but man that's a lot of memory.
> >
>
> Maybe I'm just confused, but my assumption was this means there's a
> memory leak somewhere - that we're not resetting/freeing some piece
> of memory, or so. Why would zstd need so much memory? It seems like
> a pretty serious disadvantage, so how could it become so popular?
>
>
> regards
>

Hi,

It looks like the space needed for decompression is between 1kb and
3.75tb:

https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#window_descriptor

Sheesh! Looks like it would definitely need to be bounded to control
resource use.

Regards,
Ken