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SubjectRe: [PATCH] mm: fix use-after-free in sys_remap_file_pages
On Thu, 12 Dec 2013 14:07:57 -0800
Kees Cook <keescook@chromium.org> wrote:

> From: PaX Team <pageexec@freemail.hu>
>
> http://lkml.org/lkml/2013/9/17/30
>
> SyS_remap_file_pages() calls mmap_region(), which calls remove_vma_list(),
> which calls remove_vma(), which frees the vma. Later (after out label)
> SyS_remap_file_pages() accesses the freed vma in vm_flags = vma->vm_flags.

> index 5bff08147768..afad07b85ef2 100644
> --- a/mm/fremap.c
> +++ b/mm/fremap.c
> @@ -218,6 +218,8 @@ get_write_lock:
> BUG_ON(addr != start);
> err = 0;
> }
> + vm_flags = vma->vm_flags;
> + vma = NULL;
> goto out;


If the vma has been freed by the time the code jumps to the
out label (because it was freed by a function called from
mmap_region), surely it will also already have been freed
by the time this patch dereferences it?

Also, setting vma = NULL to avoid the if (vma) branch at
the out: label is unnecessarily obfuscated. Lets make things
clear by documenting what is going on, and having a label
after that dereference.

Maybe something like this patch:

---8<---

Subject: [PATCH] mm: fix use-after-free in sys_remap_file_pages

remap_file_pages calls mmap_region, which may merge the VMA with other existing
VMAs, and free "vma". This can lead to a use-after-free bug. Avoid the bug by
remembering vm_flags before calling mmap_region, and not trying to dereference
vma later.

Signed-off-by: Rik van Riel <riel@redhat.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: stable@vger.kernel.org
---
mm/fremap.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/fremap.c b/mm/fremap.c
index 87da359..d28ede5 100644
--- a/mm/fremap.c
+++ b/mm/fremap.c
@@ -203,6 +203,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
if (mapping_cap_account_dirty(mapping)) {
unsigned long addr;
struct file *file = get_file(vma->vm_file);
+ vm_flags = vma->vm_flags;

addr = mmap_region(file, start, size,
vma->vm_flags, pgoff);
@@ -213,7 +214,8 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
BUG_ON(addr != start);
err = 0;
}
- goto out;
+ /* mmap_region may have freed vma */
+ goto out_freed;
}
mutex_lock(&mapping->i_mmap_mutex);
flush_dcache_mmap_lock(mapping);
@@ -248,6 +250,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
out:
if (vma)
vm_flags = vma->vm_flags;
+out_freed:
if (likely(!has_write_lock))
up_read(&mm->mmap_sem);
else

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