prog: fix syz_kfuzztest_run allocation strategy

Previously, the generated KFuzzTest programs were reusing the address of
the top-level input struct. A problem could arise when the encoded blob
is large and overflows into another allocated region - this certainly
happens in the case where the input struct points to some large char
buffer, for example.

While this wasn't directly a problem, it could lead to racy behavior
when running KFuzzTest targets concurrently.

To fix this, we now introduce an additional buffer parameter into
syz_kfuzztest_run that is as big as the maximum accepted input size in
the KFuzzTest kernel code. When this buffer is allocated, we ensure that
we have some allocated space in the program that can hold the entire
encoded input.

This works in practice, but has not been tested with concurrent
KFuzzTest executions yet.

1 files changed, 3 insertions, 0 deletions

diff --git a/prog/kfuzztest.go b/prog/kfuzztest.go
index 1ff65e29a..dacd54885 100644
--- a/prog/kfuzztest.go
+++ b/prog/kfuzztest.go
@@ -22,6 +22,9 @@ const (
 	// x86_64, so we hardcode it for now. A more robust solution would involve
 	// reading this from the debugfs entry at boot before fuzzing begins.
 	kFuzzTestMinalign uint64 = 8
+
+	// Maximum input size accepted by the KFuzzTest kernel module.
+	KFuzzTestMaxInputSize uint64 = 64 << 10
 )
 
 func kFuzzTestWritePrefix(buf *bytes.Buffer) {