Re: [PATCH v7 0/4] Introduce mseal()

[Jeff Xu]

On Mon, Jan 22, 2024 at 7:49 AM Theo de Raadt <deraadt@xxxxxxxxxxx> wrote:
>
> Regarding these pieces
>
> > The PROT_SEAL bit in prot field of mmap(). When present, it marks
> > the map sealed since creation.
>
> OpenBSD won't be doing this.  I had PROT_IMMUTABLE as a draft.  In my
> research I found basically zero circumstances when you userland does
> that.  The most common circumstance is you create a RW mapping, fill it,
> and then change to a more restrictve mapping, and lock it.
>
> There are a few regions in the addressspace that can be locked while RW.
> For instance, the stack.  But the kernel does that, not userland.  I
> found regions where the kernel wants to do this to the address space,
> but there is no need to export useless functionality to userland.
>
I have a feeling that most apps that need to use mmap() in their code
are likely using RW mappings. Adding sealing to mmap() could stop
those mappings from being executable. Of course, those apps would
need to change their code. We can't do it for them.

Also, I believe adding this to mmap() has no downsides, only
performance gain, as Pedro Falcato pointed out in [1].

[1] https://lore.kernel.org/lkml/CAKbZUD2A+=bp_sd+Q0Yif7NJqMu8p__eb4yguq0agEcmLH8SDQ@xxxxxxxxxxxxxx/

> OpenBSD now uses this for a high percent of the address space.  It might
> be worth re-reading a description of the split of responsibility regarding
> who locks different types of memory in a process;
> - kernel (the majority, based upon what ELF layout tell us),
> - shared library linker (the next majority, dealing with shared
>   library mappings and left-overs not determinable at kernel time),
> - libc (a small minority, mostly regarding forced mutable objects)
> - and the applications themselves (only 1 application today)
>
>     https://lwn.net/Articles/915662/
>
> > The MAP_SEALABLE bit in the flags field of mmap(). When present, it marks
> > the map as sealable. A map created without MAP_SEALABLE will not support
> > sealing, i.e. mseal() will fail.
>
> We definately won't be doing this.  We allow a process to lock any and all
> it's memory that isn't locked already, even if it means it is shooting
> itself in the foot.
>
> I think you are going to severely hurt the power of this mechanism,
> because you won't be able to lock memory that has been allocated by a
> different callsite not under your source-code control which lacks the
> MAP_SEALABLE flag.  (Which is extremely common with the system-parts of
> a process, meaning not just libc but kernel allocated objects).
>
MAP_SEALABLE was an open discussion item called out on V3 [2] and V4 [3].

I acknowledge that additional coordination would be required if
mapping were to be allocated by one software component and sealed in
another. However, this is feasible.

Considering the side effect of not having this flag (as discussed in
V3/V4) and the significant implications of altering the lifetime of
the mapping (since unmapping would not be possible), I believe it is
reasonable to expect developers to exercise additional care and
caution when utilizing memory sealing.

[2] https://lore.kernel.org/linux-mm/20231212231706.2680890-2-jeffxu@xxxxxxxxxxxx/
[3] https://lore.kernel.org/all/20240104185138.169307-1-jeffxu@xxxxxxxxxxxx/

> It may be fine inside a program like chrome, but I expect that flag to make
> it harder to use in libc, and it will hinder adoption.
>
In the case of glibc and linux, as stated in the cover letter, Stephen
is working on a change to glibc to add sealing support to the dynamic
linker,  also I plan to make necessary code changes in the linux kernel.