binder: validate alloc->mm in ->mmap() handler

Since commit 1da52815d5f1 ("binder: fix alloc->vma_vm_mm null-ptr
dereference") binder caches a pointer to the current->mm during open().
This fixes a null-ptr dereference reported by syzkaller. Unfortunately,
it also opens the door for a process to update its mm after the open(),
(e.g. via execve) making the cached alloc->mm pointer invalid.

Things get worse when the process continues to mmap() a vma. From this
point forward, binder will attempt to find this vma using an obsolete
alloc->mm reference. Such as in binder_update_page_range(), where the
wrong vma is obtained via vma_lookup(), yet binder proceeds to happily
insert new pages into it.

To avoid this issue fail the ->mmap() callback if we detect a mismatch
between the vma->vm_mm and the original alloc->mm pointer. This prevents
alloc->vm_addr from getting set, so that any subsequent vma_lookup()
calls fail as expected.

Fixes: 1da52815d5f1 ("binder: fix alloc->vma_vm_mm null-ptr dereference")
Reported-by: Jann Horn <[email protected]>
Cc: <[email protected]> # 5.15+
Signed-off-by: Carlos Llamas <[email protected]>
Acked-by: Todd Kjos <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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