linux-kernel - Re: [PATCH 4/9] mm: use mm_populate() for blocking remap_file

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Date:	Mon, 11 Mar 2013 16:03:22 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tommi Rantala <tt.rantala@...il.com>
Cc:	Michel Lespinasse <walken@...gle.com>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>, Jorn_Engel <joern@...fs.org>,
	Rik van Riel <riel@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: [PATCH 4/9] mm: use mm_populate() for blocking
 remap_file_pages()

On Sun, 10 Mar 2013 20:55:21 +0200 Tommi Rantala <tt.rantala@...il.com> wrote:

> 2012/12/21 Michel Lespinasse <walken@...gle.com>:
> > Signed-off-by: Michel Lespinasse <walken@...gle.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
>
> ...
>
> > +       vm_flags = vma->vm_flags;
> 
> When find_vma() fails, vma is NULL here.

Yup, thanks.

This, methinks:


From: Andrew Morton <akpm@...ux-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@...il.com>
Cc: Michel Lespinasse <walken@...gle.com>
Signed-off-by: Andrew Morton <akpm@...ux-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;
 
 	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
_

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