[PATCH] holepunch: fix mmap_sem i_mutex deadlock

sys_madvise has down_write of mmap_sem, then madvise_remove calls
vmtruncate_range which takes i_mutex and i_alloc_sem: no, we can easily devise
deadlocks from that ordering.

madvise_remove drop mmap_sem while calling vmtruncate_range: luckily, since
madvise_remove doesn't split or merge vmas, it's easy to handle this case with
a NULL prev, without restructuring sys_madvise.  (Though sad to retake
mmap_sem when it's unlikely to be needed, and certainly down_read is
sufficient for MADV_REMOVE, unlike the other madvices.)

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Miklos Szeredi <mszeredi@suse.cz>
Cc: Badari Pulavarty <pbadari@us.ibm.com>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 5 deletions

diff --git a/mm/madvise.c b/mm/madvise.c
index 77916e9fc52b..603c5257ed6e 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -159,9 +159,10 @@ static long madvise_remove(struct vm_area_struct *vma,
                                 unsigned long start, unsigned long end)
 {
         struct address_space *mapping;
-        loff_t offset, endoff;
+        loff_t offset, endoff;
+        int error;
 
-        *prev = vma;
+        *prev = NULL;   /* tell sys_madvise we drop mmap_sem */
 
         if (vma->vm_flags & (VM_LOCKED|VM_NONLINEAR|VM_HUGETLB))
                 return -EINVAL;
@@ -180,7 +181,12 @@ static long madvise_remove(struct vm_area_struct *vma,
                         + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
         endoff = (loff_t)(end - vma->vm_start - 1)
                         + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
-        return  vmtruncate_range(mapping->host, offset, endoff);
+
+        /* vmtruncate_range needs to take i_mutex and i_alloc_sem */
+        up_write(&current->mm->mmap_sem);
+        error = vmtruncate_range(mapping->host, offset, endoff);
+        down_write(&current->mm->mmap_sem);
+        return error;
 }
 
 static long
@@ -315,12 +321,15 @@ asmlinkage long sys_madvise(unsigned long start, size_t len_in, int behavior)
                 if (error)
                         goto out;
                 start = tmp;
-                if (start < prev->vm_end)
+                if (prev && start < prev->vm_end)
                         start = prev->vm_end;
                 error = unmapped_error;
                 if (start >= end)
                         goto out;
-                vma = prev->vm_next;
+                if (prev)
+                        vma = prev->vm_next;
+                else    /* madvise_remove dropped mmap_sem */
+                        vma = find_vma(current->mm, start);
         }
 out:
         up_write(&current->mm->mmap_sem);