[RFC PATCH 0/2] mm: introduce MAP_FIXED_SAFE

[Michal Hocko]

[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