mok: Avoid underflow in maximum variable size calculation

The code that mirrors MOK database to EFI variables gets the remaining
variable storage size from the firmware and subtracts the size needed
for any overhead to see if there is enough space to create a new entry.

However these calculations are on unsigned integer types, they can
underflow and result in huge values when the firmware is about to run
out of usable variable space. Explicitly check against this.

Signed-off-by: Alper Nebi Yasak <[email protected]>

mok.c

@@ -423,12 +423,20 @@ mirror_mok_db(CHAR16 *name, CHAR8 *name8, EFI_GUID *guid, UINT32 attrs,
 		}
 
 		/* The name counts towards the size of the variable */
-		max_var_sz -= (StrLen(namen) + 1) * 2;
+		SIZE_T namen_sz = (StrLen(namen) + 1) * 2;
+		if (max_var_sz > namen_sz)
+			max_var_sz -= namen_sz;
+		else
+			max_var_sz = 0;
 		dprint(L"max_var_sz - name: %lx\n", max_var_sz);
 
 		SIZE_T howmany;
-		howmany = MIN((max_var_sz - sizeof(*esl)) / esl->SignatureSize,
-			      (esl_end_pos - pos) / esl->SignatureSize);
+		if (max_var_sz > sizeof(*esl))
+			howmany = MIN((max_var_sz - sizeof(*esl)) / esl->SignatureSize,
+				      (esl_end_pos - pos) / esl->SignatureSize);
+		else
+			howmany = 0;
+
 		if (howmany == 0) {
 			/* No signatures from this ESL can be mirrored in to a
 			 * single variable, so skip it.