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

[Hugh Dickins <hughd@google.com>]

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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@redhat.com> 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?

Hugh