Re: [PATCH 4/9] mm: use mm_populate() for blocking remap_file_pages()

[Michel Lespinasse <walken@google.com>]

(Sorry for the late reply)

On Mon, Mar 11, 2013 at 4:03 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Sun, 10 Mar 2013 20:55:21 +0200 Tommi Rantala <tt.rantala@gmail.com> wrote:
>
>> 2012/12/21 Michel Lespinasse <walken@google.com>:
>> > Signed-off-by: Michel Lespinasse <walken@google.com>
>>
>> Hello, this patch introduced the following bug, seen while fuzzing with trinity:
>>
>> [  396.825414] BUG: unable to handle kernel NULL pointer dereference
>> at 0000000000000050

Good catch...

> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: mm/fremap.c: fix oops on error path
>
> If find_vma() fails, sys_remap_file_pages() will dereference `vma', which
> contains NULL.  Fix it by checking the pointer.
>
> (We could alternatively check for err==0, but this seems more direct)
>
> (The vm_flags change is to squish a bogus used-uninitialised warning
> without adding extra code).
>
> Reported-by: Tommi Rantala <tt.rantala@gmail.com>
> Cc: Michel Lespinasse <walken@google.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
>  mm/fremap.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff -puN mm/fremap.c~mm-fremapc-fix-oops-on-error-path mm/fremap.c
> --- a/mm/fremap.c~mm-fremapc-fix-oops-on-error-path
> +++ a/mm/fremap.c
> @@ -163,7 +163,8 @@ SYSCALL_DEFINE5(remap_file_pages, unsign
>          * and that the remapped range is valid and fully within
>          * the single existing vma.
>          */
> -       if (!vma || !(vma->vm_flags & VM_SHARED))
> +       vm_flags = vma->vm_flags;
> +       if (!vma || !(vm_flags & VM_SHARED))
>                 goto out;

Your commit message indicates the vm_flags load here doesn't generate any code, but this seems very brittle and compiler dependent. If the compiler was to generate an actual load here, the issue with vma == NULL would reappear.

>         if (!vma->vm_ops || !vma->vm_ops->remap_pages)
> @@ -254,7 +255,8 @@ get_write_lock:
>          */
>
>  out:
> -       vm_flags = vma->vm_flags;
> +       if (vma)
> +               vm_flags = vma->vm_flags;
>         if (likely(!has_write_lock))
>                 up_read(&mm->mmap_sem);
>         else



Would the following work ? I think it's simpler, and with the compiler
I'm using here it doesn't emit warnings:

diff --git a/mm/fremap.c b/mm/fremap.c
index 0cd4c11488ed..329507e832fb 100644
--- a/mm/fremap.c
+++ b/mm/fremap.c
@@ -254,7 +254,8 @@ get_write_lock:
 	 */
 
 out:
-	vm_flags = vma->vm_flags;
+	if (!err)
+		vm_flags = vma->vm_flags;
 	if (likely(!has_write_lock))
 		up_read(&mm->mmap_sem);
 	else

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

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