um: remove process stub VMA

This mostly reverts the old commit 3963333fe676 ("uml: cover stubs
with a VMA") which had added a VMA to the existing PTEs. However,
there's no real reason to have the PTEs in the first place and the
VMA cannot be 'fixed' in place, which leads to bugs that userspace
could try to unmap them and be forcefully killed, or such. Also,
there's a bit of an ugly hole in userspace's address space.

Simplify all this: just install the stub code/page at the top of
the (inner) address space, i.e. put it just above TASK_SIZE. The
pages are simply hard-coded to be mapped in the userspace process
we use to implement an mm context, and they're out of reach of the
inner mmap/munmap/mprotect etc. since they're above TASK_SIZE.

Getting rid of the VMA also makes vma_merge() no longer hit one of
the VM_WARN_ON()s there because we installed a VMA while the code
assumes the stack VMA is the first one.

It also removes a lockdep warning about mmap_sem usage since we no
longer have uml_setup_stubs() and thus no longer need to do any
manipulation that would require mmap_sem in activate_mm().

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

2 files changed, 2 insertions, 28 deletions

diff --git a/arch/um/include/asm/Kbuild b/arch/um/include/asm/Kbuild
index 314979467db1..a58811dc054c 100644
--- a/arch/um/include/asm/Kbuild
+++ b/arch/um/include/asm/Kbuild
@@ -25,3 +25,4 @@ generic-y += topology.h
 generic-y += trace_clock.h
 generic-y += word-at-a-time.h
 generic-y += kprobes.h
+generic-y += mm_hooks.h
diff --git a/arch/um/include/asm/mmu_context.h b/arch/um/include/asm/mmu_context.h
index f8a100770691..68e2eb9cfb47 100644
--- a/arch/um/include/asm/mmu_context.h
+++ b/arch/um/include/asm/mmu_context.h
@@ -10,33 +10,9 @@
 #include <linux/mm_types.h>
 #include <linux/mmap_lock.h>
 
+#include <asm/mm_hooks.h>
 #include <asm/mmu.h>
 
-extern void uml_setup_stubs(struct mm_struct *mm);
-/*
- * Needed since we do not use the asm-generic/mm_hooks.h:
- */
-static inline int arch_dup_mmap(struct mm_struct *oldmm, struct mm_struct *mm)
-{
-	uml_setup_stubs(mm);
-	return 0;
-}
-extern void arch_exit_mmap(struct mm_struct *mm);
-static inline void arch_unmap(struct mm_struct *mm,
-			unsigned long start, unsigned long end)
-{
-}
-static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
-		bool write, bool execute, bool foreign)
-{
-	/* by default, allow everything */
-	return true;
-}
-
-/*
- * end asm-generic/mm_hooks.h functions
- */
-
 extern void force_flush_all(void);
 
 #define activate_mm activate_mm
@@ -47,9 +23,6 @@ static inline void activate_mm(struct mm_struct *old, struct mm_struct *new)
 	 * when the new ->mm is used for the first time.
 	 */
 	__switch_mm(&new->context.id);
-	mmap_write_lock_nested(new, SINGLE_DEPTH_NESTING);
-	uml_setup_stubs(new);
-	mmap_write_unlock(new);
 }
 
 static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,