linux-kernel - Re: [PATCH] secretmem: disable memfd_secret() if arch cannot set direct map

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Message-ID: <ZvzV_GVjp05rcqxj@kernel.org>
Date: Wed, 2 Oct 2024 08:11:24 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Patrick Roy <roypat@...zon.co.uk>, david@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, graf@...zon.com, jgowans@...zon.com
Subject: Re: [PATCH] secretmem: disable memfd_secret() if arch cannot set
 direct map

On Tue, Oct 01, 2024 at 03:04:38PM -0700, Andrew Morton wrote:
> On Tue, 1 Oct 2024 09:00:41 +0100 Patrick Roy <roypat@...zon.co.uk> wrote:
> 
> > Return -ENOSYS from memfd_secret() syscall if !can_set_direct_map().
> > This is the case for example on some arm64 configurations, where marking
> > 4k PTEs in the direct map not present can only be done if the direct map
> > is set up at 4k granularity in the first place (as ARM's
> > break-before-make semantics do not easily allow breaking apart
> > large/gigantic pages).
> > 
> > More precisely, on arm64 systems with !can_set_direct_map(),
> > set_direct_map_invalid_noflush() is a no-op, however it returns success
> > (0) instead of an error. This means that memfd_secret will seemingly
> > "work" (e.g. syscall succeeds, you can mmap the fd and fault in pages),
> > but it does not actually achieve its goal of removing its memory from
> > the direct map.
> > 
> > Note that with this patch, memfd_secret() will start erroring on systems
> > where can_set_direct_map() returns false (arm64 with
> > CONFIG_RODATA_FULL_DEFAULT_ENABLED=n, CONFIG_DEBUG_PAGEALLOC=n and
> > CONFIG_KFENCE=n), but that still seems better than the current silent
> > failure. Since CONFIG_RODATA_FULL_DEFAULT_ENABLED defaults to 'y', most
> > arm64 systems actually have a working memfd_secret() and aren't be
> > affected.
> > 
> > >From going through the iterations of the original memfd_secret patch
> > series, it seems that disabling the syscall in these scenarios was the
> > intended behavior [1] (preferred over having
> > set_direct_map_invalid_noflush return an error as that would result in
> > SIGBUSes at page-fault time), however the check for it got dropped
> > between v16 [2] and v17 [3], when secretmem moved away from CMA
> > allocations.
> > 
> > [1]: https://lore.kernel.org/lkml/20201124164930.GK8537@kernel.org/
> > [2]: https://lore.kernel.org/lkml/20210121122723.3446-11-rppt@kernel.org/#t
> > [3]: https://lore.kernel.org/lkml/20201125092208.12544-10-rppt@kernel.org/
> 
> Thanks.
> 
> > Fixes: 1507f51255c9 ("mm: introduce memfd_secret system call to create "secret" memory areas")
> 
> So I'm thinking this fix should be backported into kernels which
> contain 1507f51255c9, agree?

Yes 

-- 
Sincerely yours,
Mike.