linux-kernel - Re: [RFC PATCH RESEND v2 1/2] mm/memfd: Add support for F_SEAL_FUTURE

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Message-ID: <CAH5fLgifNkTFTVHbsp7wXBgRQmXQ3+r3xD03bZq06gU7eOfDOw@mail.gmail.com>
Date: Wed, 8 Jan 2025 14:57:20 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Jeff Xu <jeffxu@...omium.org>
Cc: Isaac Manjarres <isaacmanjarres@...gle.com>, Jann Horn <jannh@...gle.com>, 
	Kees Cook <keescook@...omium.org>, lorenzo.stoakes@...cle.com, 
	Jeff Layton <jlayton@...nel.org>, Chuck Lever <chuck.lever@...cle.com>, 
	Alexander Aring <alex.aring@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Shuah Khan <shuah@...nel.org>, surenb@...gle.com, kaleshsingh@...gle.com, 
	jstultz@...gle.com, jeffxu@...gle.com, kees@...nel.org, 
	kernel-team@...roid.com, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH RESEND v2 1/2] mm/memfd: Add support for
 F_SEAL_FUTURE_EXEC to memfd

On Tue, Jan 7, 2025 at 6:21 AM Jeff Xu <jeffxu@...omium.org> wrote:
> Do you know which code checks for VM_MAYEXEC flag in the mprotect code
> path ?  it isn't obvious to me, i.e. when I grep the VM_MAYEXEC inside
> mm path, it only shows one place in mprotect and that doesn't do the
> work.
>
> ~/mm/mm$ grep VM_MAYEXEC *
> mmap.c: mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
> mmap.c: vm_flags &= ~VM_MAYEXEC;
> mprotect.c: if (rier && (vma->vm_flags & VM_MAYEXEC))
> nommu.c: vm_flags |= VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
> nommu.c: vm_flags |= VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;

The check happens here:

/* newflags >> 4 shift VM_MAY% in place of VM_% */
if ((newflags & ~(newflags >> 4)) & VM_ACCESS_FLAGS) {
    error = -EACCES;
    break;
}

Alice