Re: [PATCH bpf 1/2] mm: Fix copy_from_user_nofault().

[Kees Cook <keescook@chromium.org>]

On Tue, Jan 17, 2023 at 09:14:42PM -0800, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> There are several issues with copy_from_user_nofault():
> 
> - access_ok() is designed for user context only and for that reason
> it has WARN_ON_IN_IRQ() which triggers when bpf, kprobe, eprobe
> and perf on ppc are calling it from irq.
> 
> - it's missing nmi_uaccess_okay() which is a nop on all architectures
> except x86 where it's required.
> The comment in arch/x86/mm/tlb.c explains the details why it's necessary.
> Calling copy_from_user_nofault() from bpf, [ke]probe without this check is not safe.
> 
> - __copy_from_user_inatomic() under CONFIG_HARDENED_USERCOPY is calling
> check_object_size()->__check_object_size()->check_heap_object()->find_vmap_area()->spin_lock()
> which is not safe to do from bpf, [ke]probe and perf due to potential deadlock.

Er, this drops check_object_size() -- that needs to stay. The vmap area
test in check_object_size is likely what needs fixing. It was discussed
before:
https://lore.kernel.org/lkml/YySML2HfqaE%2FwXBU@casper.infradead.org/

The only reason it was ultimately tolerable to remove the check from
the x86-only _nmi function was because it was being used on compile-time
sized copies.

We need to fix the vmap lookup so the checking doesn't regress --
especially for trace, bpf, etc, where we could have much more interested
dest/source/size combinations. :)

-Kees

-- 
Kees Cook