Re: [PATCH RFC 3/8] openat2: explicitly return -E2BIG for (usize > PAGE_SIZE)

[Aleksa Sarai]

On 2024-09-02, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Mon, Sep 2, 2024, at 07:06, Aleksa Sarai wrote:
> > While we do currently return -EFAULT in this case, it seems prudent to
> > follow the behaviour of other syscalls like clone3. It seems quite
> > unlikely that anyone depends on this error code being EFAULT, but we can
> > always revert this if it turns out to be an issue.
> 
> Right, it's probably a good idea to have a limit there rather than
> having a busy loop with a user-provided length when the only bound is
> the available virtual memory.
> 
> >  	if (unlikely(usize < OPEN_HOW_SIZE_VER0))
> >  		return -EINVAL;
> > +	if (unlikely(usize > PAGE_SIZE))
> > +		return -E2BIG;
> > 
> 
> Is PAGE_SIZE significant here? If there is a need to enforce a limit,
> I would expect this to be the same regardless of kernel configuration,
> since the structure layout is also independent of the configuration.

PAGE_SIZE is what clone3, perf_event_open, sched_setattr, bpf, etc all
use. The idea was that PAGE_SIZE is the absolute limit of any reasonable
extensible structure size because we are never going to have argument
structures that are larger than a page (I think this was discussed in
the original copy_struct_from_user() patchset thread in late 2019, but I
can't find the reference at the moment.)

I simply forgot to add this when I first submitted openat2, the original
intention was to just match the other syscalls.

> Where is the current -EFAULT for users passing more than a page?
> I only see it for reads beyond the VMA, but not e.g. when checking
> terabytes of zero pages from an anonymous mapping.

I meant that we in practice return -EFAULT if you pass a really large
size (because you end up running off the end of mapped memory). There is
no explicit -EFAULT for large sizes, which is exactly the problem. :P

> 
>     Arnd

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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