fix(core): libtropic overwrite bug workaround

What changed, and why it matters

This commit adds a small workaround in the Trezor hardware wallet's random number generator code. The change allocates a slightly larger temporary buffer and only uses the inner portion, leaving unused 'bumper' bytes at the end. The commit message calls this a 'libtropic overwrite bug workaround,' suggesting an external library (libtropic) may write past the requested amount of random data. The patch does not fix the underlying library bug; it only pads the local buffer so any overflow lands in harmless scratch space rather than corrupting adjacent memory. Because this affects the cryptographic random number generator used for keys and secrets, a real overwrite could have security implications, but the diff itself does not show an actual vulnerability in Trezor code or prove the bug is exploitable.

Security signals we found

Evidence from the diff

Changed components

diff --git a/core/embed/sec/rng/rng_common.c b/core/embed/sec/rng/rng_common.c
index dfec14fb..d370cc02 100644
--- a/core/embed/sec/rng/rng_common.c
+++ b/core/embed/sec/rng/rng_common.c
@@ -55,10 +55,12 @@ bool rng_fill_buffer_strong(void* buffer, size_t buffer_size) {
   uint8_t* dst = (uint8_t*)buffer;
   size_t remaining = buffer_size;
 
-  uint8_t block[32];
+  static const int bumper = 4;  // !@# workaround
+
+  uint8_t block[32 + bumper];
 
   while (remaining > 0) {
-    size_t block_size = MIN(remaining, sizeof(block));
+    size_t block_size = MIN(remaining, sizeof(block) - bumper);
 #ifdef USE_OPTIGA
     if (!optiga_random_buffer(block, block_size)) {
       return false;