Ignore too-large inputs in ressol fuzzer

OSS-Fuzz just gave us a gigantic input and then timed out.

In practice ressol is only called with specific primes of various
common ECC parameter sets, so limit to 768 bits max.

[ci skip]

1 files changed, 6 insertions, 1 deletions

diff --git a/src/extra_tests/fuzzers/jigs/ressol.cpp b/src/extra_tests/fuzzers/jigs/ressol.cpp
index 4c3f8df69..3f7f82502 100644
--- a/src/extra_tests/fuzzers/jigs/ressol.cpp
+++ b/src/extra_tests/fuzzers/jigs/ressol.cpp
@@ -9,7 +9,12 @@
 
 void fuzz(const uint8_t in[], size_t len)
    {
-   if(len % 2 != 0)
+   /*
+   * This allows two values (a,p) up to 768 bits in length, which is
+   * sufficient to test ressol (modular square root) for since it is
+   * mostly used for ECC.
+   */
+   if(len % 2 != 0 || len > 2 * (768 / 8))
       return;
 
    const BigInt a = BigInt::decode(in, len / 2);