[twtw]

GPUs can only access pinned memory that is intentionally mapped into their address space. Also, each context gets its own virtual address space on the GPU, isolated from other contexts.

There can still be issues, but it isn't quite as much of a free for all as the above comment sounds.

[floatboth]

Fun fact, the Raspberry Pi's GPU can access everything. And to deal with that, the Mesa VC4 driver validates every shader to prevent reading other processes' stuff.

[FraKtus]

Isn't that because in that SOC they use unified memory where GPU and CPU memory is the same. This does not apply to most desktop computers or mobile phones...