Re: [PATCH v2] tee: add overflow check in register_shm_helper()

[Linus Torvalds]

On Thu, Aug 18, 2022 at 4:09 AM Jens Wiklander
<jens.wiklander@xxxxxxxxxx> wrote:
>
> Fix this by adding an overflow check when calculating the end of the
> memory range. Also add an explicit call to access_ok() in
> tee_shm_register_user_buf() to catch an invalid user space address
> early.

I applied the access_ok() part of this which was clearly missing.

The check_add_overflow() should be pointless with that.

And the "roundup() overflows" check should just check for a zero
result - if it is actually needed. Which I don't think it is on any
relevant platform (the TEE subsystem only works on arm and x86).

I do think it might be worth discussing whether
ALTERNATE_USER_ADDRESS_SPACE (and no-MMU) architectures should still
have access_ok() check that it doesn't actually wrap around in the
address space, so I've added linux-arch here.

That's m68k, PA-RISC, S390 and sparc.

In fact, I wonder if some or all of those might want to have the
TASK_SIZE limit anyway - they may have a separate user address space,
but several ones have some limits even then, and probably should have
access_ok() check them rather than depend on the hardware then giving
page fault.

For example, sparc32 has a user address space, but defines TASK_SIZE
to 0xF0000000. m68k has several different case. parisc also has an
actual limit.

And s390 uses

    #define TASK_SIZE_MAX         (-PAGE_SIZE)

which is a good value and leaves a guard page at the top.

So I think the "roundup overflows" would probably be best fixed by
just admitting that every architecture in practice has a TASK_SIZE_MAX
anyway, and we should just make access_ok() check it.

              Linus