[UNIVERSAL 0] is reserved by X.680 for the encoding to use. BER uses
this to encode indefinite-length EOCs, but it is possible to encode it
in a definite-length element or in a non-EOC form (non-zero length, or
constructed).
Whether we accept such encodings is normally moot: parsers will reject
the tag as unsuitable for the type. However, the ANY type matches all
tags. Previously, we would allow this, but crypto/asn1 has some ad-hoc
checks for unexpected EOCs, in some contexts, but not others.
Generalize this check to simply rejecting [UNIVERSAL 0] in all forms.
This avoids a weird hole in the abstraction where tags are sometimes
representable in BER and sometimes not. It also means we'll preserve
this check when migrating parsers from crypto/asn1.
Update-Note: There are two kinds of impacts I might expect from this
change. The first is BER parsers might be relying on the CBS DER/BER
element parser to pick up EOCs, as our ber.c does. This should be caught
by the most basic unit test and can be fixed by detecting EOCs
externally.
The second is code might be trying to parse "actual" elements with tag
[UNIVERSAL 0]. No actual types use this tag, so any non-ANY field is
already rejecting such inputs. However, it is possible some input has
this tag in a field with type ANY. This CL will cause us to reject that
input. Note, however, that crypto/asn1 already rejects unexpected EOCs
inside sequences, so many cases were already rejected anyway. Such
inputs are also invalid as the ANY should match some actual, unknown
ASN.1 type, and that type cannot use the reserved tag.
Fixed: 455
Change-Id: If42cacc01840439059baa0e67179d0f198234fc4
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/52245
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
David Benjamin
edbdc240ec