On Thu, Apr 18, 2019 at 2:01 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > On Thu, Apr 18, 2019 at 01:30:07AM -0500, Kees Cook wrote: > > > Anything running with READ_IMPLIES_EXEC (i.e. a gnu stack marked WITH > > execute) should be considered broken. Now, the trouble is that this > > personality flag is carried across execve(), so if you have a launcher > > that doesn't fix up the personality for children, you'll see this > > spread all over your process tree. What is doing rdma mmap calls with > > an executable stack? That really feels to me like the real source of > > the problem. > > Apparently the Fortran runtime forces the READ_IMPLIES_EXEC and > requires it for some real reason or another - Fortran and RDMA go > together in alot of cases. That's pretty unfortunate for the security of the resulting proceses. :( > > Is the file for the driver coming out of /dev? Seems like that should > > be mounted noexec and it would solve this too. (Though now I wonder > > why /dev isn't noexec by default? /dev/pts is noexec... > > Yes - maybe? I've found why /dev isn't noexec: to support old tools that mapped /dev/zero with VM_EXEC to get executable mappings (instead of using MAP_ANON). Seems like maybe this could change now? > > Regardless, if you wanted to add a "ignore READ_IMPLIES_EXEC" flag to > > struct file, maybe this bit could be populated by drivers? > > This would solve our problem.. How about a flag in struct > file_operations? Oh! That seems like it'd be pretty clean, I think. There's no flags field currently, which vaguely surprises me... I wonder if we could simply make devtmpfs ignore READ_IMPLIES_EXEC entirely, though? And I wonder if we could defang READ_IMPLIES_EXEC a bit in general. It was _supposed_ to be for the cases where binaries were missing exec bits and a processor was just gaining NX ability. I know this has been discussed before... ah-ha, here it is: http://lkml.kernel.org/r/1462963502-11636-1-git-send-email-hecmargi@xxxxxx > Do you agree it is worth drivers banning VM_EXEC for these truely > non-executable pages? I do: I think it's reasonable defense-in-depth. -- Kees Cook
