http2: fix out-of-bounds write in altsvc frame buffers - #65216
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Signed-off-by: nashit hayyat <nashit@bugqore.com>
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Http2Session::AltSvc sizes the origin and value byte buffers to the exact string length with MaybeStackBuffer, then fills them via WriteOneByteV2 with String::WriteFlags::kNullTerminate, which writes length + 1 bytes for the trailing NUL. Once origin or alt is longer than the 1024-byte inline stack storage the buffer is heap-allocated at exactly the string length, so that NUL lands one byte past the end, a heap-buffer-overflow that ASAN flags on session.altsvc('h2=":8000"; ma=' + '0'.repeat(2000), streamId). The terminator is never needed because the buffers reach nghttp2_submit_altsvc with explicit origin_len and value_len, so drop kNullTerminate and let each write match its allocation.