Re: [PATCH] mm/mmap: Map MAP_STACK to VM_STACK

[Waiman Long]

On 4/18/23 21:36, Hugh Dickins wrote:
> On Tue, 18 Apr 2023, Waiman Long wrote:
> > On 4/18/23 17:18, Andrew Morton wrote:
> > > On Tue, 18 Apr 2023 17:02:30 -0400 Waiman Long <longman@xxxxxxxxxx> wrote:
> > >
> > > > One of the flags of mmap(2) is MAP_STACK to request a memory segment
> > > > suitable for a process or thread stack. The kernel currently ignores
> > > > this flags. Glibc uses MAP_STACK when mmapping a thread stack. However,
> > > > selinux has an execstack check in selinux_file_mprotect() which disallows
> > > > a stack VMA to be made executable.
> > > >
> > > > Since MAP_STACK is a noop, it is possible for a stack VMA to be merged
> > > > with an adjacent anonymous VMA. With that merging, using mprotect(2)
> > > > to change a part of the merged anonymous VMA to make it executable may
> > > > fail. This can lead to sporadic failure of applications that need to
> > > > make those changes.
> > > "Sporadic failure of applications" sounds quite serious.  Can you
> > > provide more details?
> > The problem boils down to the fact that it is possible for user code to mmap a
> > region of memory and then for the kernel to merge the VMA for that memory with
> > the VMA for one of the application's thread stacks. This is causing random
> > SEGVs with one of our large customer application.
> >
> > At a high level, this is what's happening:
> >
> >   1) App runs creating lots of threads.
> >   2) It mmap's 256K pages of anonymous memory.
> >   3) It writes executable code to that memory.
> >   4) It calls mprotect() with PROT_EXEC on that memory so
> >      it can subsequently execute the code.
> >
> > The above mprotect() will fail if the mmap'd region's VMA gets merged with the
> > VMA for one of the thread stacks.  That's because the default RHEL SELinux
> > policy is to not allow executable stacks.
> Then wouldn't the bug be at the SELinux end?  VMAs may have been merged
> already, but the mprotect() with PROT_EXEC of the good non-stack range
> will then split that area off from the stack again - maybe the SELinux
> check does not understand that must happen?

The SELinux check is done per VMA, not a region within a VMA. After VMA 
merging, SELinux is probably not able to determine which part of a VMA 
is a stack unless we keep that information somewhere and provide an API 
for SELinux to query. That can be quite a lot of work. So the easiest 
way to prevent this problem is to avoid merging a stack VMA with a 
regular anonymous VMA.

Cheers,
Longman