linux-hardening - Re: [PATCH 3/6] PKEY: Apply PKEY_ENFORCE

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Date:   Tue, 16 May 2023 16:36:49 -0700
From:   Jeff Xu <jeffxu@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     jeffxu@...omium.org, luto@...nel.org, jorgelo@...omium.org,
        keescook@...omium.org, groeck@...omium.org, jannh@...gle.com,
        sroettger@...gle.com, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-mm@...ck.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 3/6] PKEY: Apply PKEY_ENFORCE_API to mprotect

On Tue, May 16, 2023 at 4:19 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 5/15/23 06:05, jeffxu@...omium.org wrote:
> >  /*
> >   * pkey==-1 when doing a legacy mprotect()
> > + * syscall==true if this is called by syscall from userspace.
> > + * Note: this is always true for now, added as a reminder in case that
> > + * do_mprotect_pkey is called directly by kernel in the future.
> > + * Also it is consistent with __do_munmap().
> >   */
> >  static int do_mprotect_pkey(unsigned long start, size_t len,
> > -             unsigned long prot, int pkey)
> > +             unsigned long prot, int pkey, bool syscall)
> >  {
>
> The 'syscall' seems kinda silly (and a bit confusing).  It's easy to
> check if the caller is a kthread or has a current->mm==NULL.  If you
> *really* want a warning, I'd check for those rather than plumb a
> apparently unused argument in here.
>
> BTW, this warning is one of those things that will probably cause some
> amount of angst.  I'd move it to the end of the series or just axe it
> completely.

Agreed. syscall is not a good name here.
The intention is to check this at the system call entry point
For example, munmap can get called inside mremap(), but by that time
mremap() should already check that all the memory is writeable.

I will remove "syscall" from do_mprotect_pkey signature, it seems it caused
more confusion than helpful.  I will keep the comments/note in place to remind
future developer.