[External] : Re: BIO_read() crash

[Tomas Mraz]

On Mon, 2022-12-05 at 16:14 -0800, Benjamin Kaduk via openssl-users
wrote:
> On Mon, Dec 05, 2022 at 11:31:18AM -0800, Thomas Dwyer III wrote:
> > Why does EVP_get_digestbyname("md4") return non-NULL if the legacy
> > provider
> > isn't loaded? Similarly, why does it return non-NULL for "md5"
> > after doing
> > EVP_set_default_properties(NULL, "fips=yes")? This seems
> > unintuitive. Legacy
> > code that does not know about EVP_MD_fetch() checks the return
> > value of
> > EVP_get_digestbyname(). Isn't that where the error should be
> > detected? Why
> > let it get all the way to BIO_set_md() (or EVP_DigestInit() or
> > whatever)
> > before the error is detected?
> 
> To do so would introduce a time-of-check/time-of-use race, as the set
> of
> providers available may change in the intervening period.

Not just that. IMO the primary motivation was to keep the digests (and
similarly ciphers) returned by the EVP_md4() (and similar) or
EVP_get_digestbyname() calls still constant. I.e., they would not
change based on the providers involved, etc. This implied the implicit
fetching done inside the EVP_DigestInit() and similar init routines.

As any correct code required checking result of EVP_DigestInit() and
thus also the result of BIO_set_md(), it was the least of all evils in
how to implement these legacy EVP_MD functions returning constant
EVP_MDs.
-- 
Tomáš Mráz, OpenSSL