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Date: Thu, 19 Oct 2023 10:30:20 -0700
From: Jeff Xu <jeffxu@...gle.com>
To: Pedro Falcato <pedro.falcato@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>, jeffxu@...omium.org, akpm@...ux-foundation.org, 
	keescook@...omium.org, sroettger@...gle.com, jorgelo@...omium.org, 
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	dave.hansen@...el.com, luto@...nel.org, linux-hardening@...r.kernel.org
Subject: Re: [RFC PATCH v1 0/8] Introduce mseal() syscall

Hi Pedro

Some followup on mmap() + mprotect():

On Wed, Oct 18, 2023 at 11:20 AM Jeff Xu <jeffxu@...gle.com> wrote:
>
> On Tue, Oct 17, 2023 at 3:35 PM Pedro Falcato <pedro.falcato@...il.com> wrote:
> >
> > > >
> > > > I think it's worth pointing out that this suggestion (with PROT_*)
> > > > could easily integrate with mmap() and as such allow for one-shot
> > > > mmap() + mseal().
> > > > If we consider the common case as 'addr = mmap(...); mseal(addr);', it
> > > > definitely sounds like a performance win as we halve the number of
> > > > syscalls for a sealed mapping. And if we trivially look at e.g OpenBSD
> > > > ld.so code, mmap() + mimmutable() and mprotect() + mimmutable() seem
> > > > like common patterns.
> > > >
> > > Yes. mmap() can support sealing as well, and memory is allocated as
> > > immutable from begining.
> > > This is orthogonal to mseal() though.
> >
> > I don't see how this can be orthogonal to mseal().
> > In the case we opt for adding PROT_ bits, we should more or less only
> > need to adapt calc_vm_prot_bits(), and the rest should work without
> > issues.
> > vma merging won't merge vmas with different prots. The current
> > interfaces (mmap and mprotect) would work just fine.
> > In this case, mseal() or mimmutable() would only be needed if you need
> > to set immutability over a range of VMAs with different permissions.
> >
> Agreed. By orthogonal, I meant we can have two APIs:
> mmap() and mseal()/mprotect()
> i.e. we can't just rely on mmap() only without mseal()/mprotect()/mimmutable().
> Sealing can be applied after initial memory creation.
>
> > Note: modifications should look kinda like this: https://godbolt.org/z/Tbjjd14Pe
> > The only annoying wrench in my plans here is that we have effectively
> > run out of vm_flags bits in 32-bit architectures, so this approach as
> > I described is not compatible with 32-bit.
> >
> > > In case of ld.so, iiuc, memory can be first allocated as W, then later
> > > changed to RO, for example, during symbol resolution.
> > > The important point is that the application can decide what type of
> > > sealing it wants, and when to apply it.  There needs to be an api(),
> > > that can be mseal() or mprotect2() or mimmutable(), the naming is not
> > > important to me.
> > >
> > > mprotect() in linux have the following signature:
> > > int mprotect(void addr[.len], size_t len, int prot);
> > > the prot bitmasks are all taken here.
> > > I have not checked the prot field in mmap(), there might be bits left,
> > > even not, we could have mmap2(), so that is not an issue.
> >
> > I don't see what you mean. We have plenty of prot bits left (32-bits,
> > and we seem to have around 8 different bits used).
> > And even if we didn't, prot is the same in mprotect and mmap and mmap2 :)
> >
> > The only issue seems to be that 32-bit ran out of vm_flags, but that
> > can probably be worked around if need be.
> >
> Ah, you are right about this. vm_flags is full, and prot in mprotect() is not.
> Apology that I was wrong previously and caused confusion.
>
> There is a slight difference in the syntax of mprotect and mseal.
> Each time when mprotect() is called, the kernel takes all of RWX bits
> and updates vm_flags,
> In other words, the application sets/unset each RWX, and kernel takes it.
>
> In the mseal() case, the kernel will remember which seal types were
> applied previously, and the application doesn’t need to repeat all
> existing seal types in the next mseal().  Once a seal type is applied,
> it can’t be unsealed.
>
> So if we want to use mprotect() for sealing, developers need to think
> of sealing bits differently than the rest of prot bits. It is a
> different programming model, might or might not be an obvious concept
> to developers.
>
This probably doesn't matter much to developers.
We can enforce the sealing bit to be the same as the rest of PROT bits.
If mprotect() tries to unset sealing, it will fail.

> There is a difference in input check and error handling as well.
> for mseal(), if a given address range has a gap (unallocated memory),
> or if one of VMA is sealed with MM_SEAL_SEAL flag, none of VMAs is
> updated.
> For mprotect(), some VMAs can be updated, till an error happens to a VMA.
>
This difference doesn't matter much.

For mprotect()/mmap(), is Linux implementation limited by POSIX ?
This can be made backward compatible.
If there is no objection to adding linux specific values in mmap() and
mprotect(),
This works for me.

Thanks for your input.
-Jeff

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