Re: [RFC PATCH 0/2] mm: introduce MAP_FIXED_SAFE

[Michal Hocko]

Are there any more concerns? So far the biggest one was the name. The
other which suggests a flag as a modifier has been sorted out hopefully.
Is there anymore more before we can consider this for merging? Well
except for man page update which I will prepare of course. Can we target
this to 4.16?

On Thu 16-11-17 13:14:38, Michal Hocko wrote:
> [Ups, managed to screw the subject - fix it]
> 
> On Thu 16-11-17 11:18:58, Michal Hocko wrote:
> > Hi,
> > this has started as a follow up discussion [1][2] resulting in the
> > runtime failure caused by hardening patch [3] which removes MAP_FIXED
> > from the elf loader because MAP_FIXED is inherently dangerous as it
> > might silently clobber and existing underlying mapping (e.g. stack). The
> > reason for the failure is that some architectures enforce an alignment
> > for the given address hint without MAP_FIXED used (e.g. for shared or
> > file backed mappings).
> > 
> > One way around this would be excluding those archs which do alignment
> > tricks from the hardening [4]. The patch is really trivial but it has
> > been objected, rightfully so, that this screams for a more generic
> > solution. We basically want a non-destructive MAP_FIXED.
> > 
> > The first patch introduced MAP_FIXED_SAFE which enforces the given
> > address but unlike MAP_FIXED it fails with ENOMEM if the given range
> > conflicts with an existing one. The flag is introduced as a completely
> > new flag rather than a MAP_FIXED extension because of the backward
> > compatibility. We really want a never-clobber semantic even on older
> > kernels which do not recognize the flag. Unfortunately mmap sucks wrt.
> > flags evaluation because we do not EINVAL on unknown flags. On those
> > kernels we would simply use the traditional hint based semantic so the
> > caller can still get a different address (which sucks) but at least not
> > silently corrupt an existing mapping. I do not see a good way around
> > that. Except we won't export expose the new semantic to the userspace at
> > all. It seems there are users who would like to have something like that
> > [5], though. Atomic address range probing in the multithreaded programs
> > sounds like an interesting thing to me as well, although I do not have
> > any specific usecase in mind.
> > 
> > The second patch simply replaces MAP_FIXED use in elf loader by
> > MAP_FIXED_SAFE. I believe other places which rely on MAP_FIXED should
> > follow. Actually real MAP_FIXED usages should be docummented properly
> > and they should be more of an exception.
> > 
> > Does anybody see any fundamental reasons why this is a wrong approach?
> > 
> > Diffstat says
> >  arch/alpha/include/uapi/asm/mman.h   |  2 ++
> >  arch/metag/kernel/process.c          |  6 +++++-
> >  arch/mips/include/uapi/asm/mman.h    |  2 ++
> >  arch/parisc/include/uapi/asm/mman.h  |  2 ++
> >  arch/powerpc/include/uapi/asm/mman.h |  1 +
> >  arch/sparc/include/uapi/asm/mman.h   |  1 +
> >  arch/tile/include/uapi/asm/mman.h    |  1 +
> >  arch/xtensa/include/uapi/asm/mman.h  |  2 ++
> >  fs/binfmt_elf.c                      | 12 ++++++++----
> >  include/uapi/asm-generic/mman.h      |  1 +
> >  mm/mmap.c                            | 11 +++++++++++
> >  11 files changed, 36 insertions(+), 5 deletions(-)
> > 
> > [1] http://lkml.kernel.org/r/20171107162217.382cd754@xxxxxxxxxxxxxxxx
> > [2] http://lkml.kernel.org/r/1510048229.12079.7.camel@xxxxxxxxxxxxxxxx
> > [3] http://lkml.kernel.org/r/20171023082608.6167-1-mhocko@xxxxxxxxxx
> > [4] http://lkml.kernel.org/r/20171113094203.aofz2e7kueitk55y@xxxxxxxxxxxxxx
> > [5] http://lkml.kernel.org/r/87efp1w7vy.fsf@xxxxxxxxxxxxxxxxxxxxxxxx
> > 
> > 
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> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs