How do different commands get executed in CPU x86-64 registers?

Years ago a teacher once said to class that 'everything that gets parsed through the CPU can also be exploited'.

Back then I didn't know too much about the topic, but now the statement is nagging on me and I lack the correct vocabulary to find an answer to this question in the internet myself, so I kindly ask you for help.

We had the lesson about 'cat', 'grep' and 'less' and she said that in the worst case even those commands can cause harm if we parse the wrong content through it.

I don't really understand how she meant that. I do know how CPU registers work, we also had to write an educational buffer overflow so I have seen assembly code in the registers aswell. I still don't get the following:

How do commands get executed in the CPU at all? e.g. I use 'cat' so somehwere there will be a call of the command. But how does the data I enter get parsed to the CPU? If I 'cat' a .txt file which contains 'hello world' - can I find that string in HEX somewhere in the CPU registers? And if yes:
How does the CPU know that said string is NOT to be executed?
Could you think of any scencario where the above commands could get exploited? Afaik only text gets parsed through it, how could that be exploitable? What do I have to be careful about?

Thanks alot!

Solution

Machine code executes by being fetched by the instruction-fetch part of the CPU, at the address pointed to by RIP, the instruction-pointer. CPUs can only execute machine code from memory.

General-purpose registers get loaded with data from data load/store instructions, like mov eax, [rdi]. Having data in registers is totally unrelated to having it execute as machine code. Remember that RIP is a pointer, not actual machine-code bytes. (RIP can be set with jump instructions, including indirect jump to copy a GP register into it, or ret to pop the stack into it).

It would help to learn some basics of assembly language, because you seem to be missing some key concepts there. It's kind of hard to answer the security part of this question when the entire premise seems to be built on some misunderstanding of how computers work. (Which I don't think I can easily clear up here without writing a book on assembly language.) All I can really do is point you at CPU-architecture stuff that answers part of the title question of how instructions get executed. (Not from registers).

How does a computer distinguish between Data and Instructions?
How instructions are differentiated from data?
Modern Microprocessors A 90-Minute Guide! covers the basic fetch/decode/execute cycle of simple pipelines. Modern CPUs might have more complex internals, but from a correctness / security POV are equivalent. (Except for exploits like Spectre and Meltdown that depend on speculative execution).
https://www.realworldtech.com/sandy-bridge/3/ is a deep-dive on Intel's Sandybridge microarchitecture. That page covering instruction-fetch shows how things really work under the hood in real CPUs. (AMD Zen is fairly similar.)

You keep using the word "parse", but I think you just mean "pass". You don't "parse content through" something, but you can "pass content through". Anyway no, cat usually doesn't involve copying or looking-at data in user-space, unless you run cat -n to add line numbers.

See Race condition when piping through x86-64 assembly program for an x86-64 Linux asm implementation of plain cat using read and write system calls. Nothing in it is data-dependent, except for the command-line arg. The data being copied is never loaded into CPU registers in user-space.

Inside the kernel, copy_to_user inside Linux's implementation of a read() system call on x86-64 will normally use rep movsb for the copy, not a loop with separate load/store, so even in kernel the data gets copied from the page-cache, pipe buffer, or whatever, to user-space without actually being in a register. (Same for write copying it to whatever stdout is connected to.)

Other commands, like less and grep, would load data into registers, but that doesn't directly introduce any risk of it being executed as code.