[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 > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
