On Fri, 13 Dec 2013 12:10:17 +0100
"PaX Team" <pageexec@xxxxxxxxxxx> wrote:
> pass in vm_flags instead of vma->vm_flags just to prevent someone
> from 'optimizing' away the read in the future?
In that case, we should probably also use ACCESS_ONCE, if
only to be explicit about it.
> perhaps {copy,move} this comment above the previous hunk since that's
> where the relevant action is?
See the new version below:
---8<---
Subject: 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@xxxxxxxxxx>
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
mm/fremap.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/mm/fremap.c b/mm/fremap.c
index 87da359..c85e2ec 100644
--- a/mm/fremap.c
+++ b/mm/fremap.c
@@ -203,9 +203,10 @@ 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);
+ /* mmap_region may free vma; grab the info now */
+ vm_flags = ACCESS_ONCE(vma->vm_flags);
- addr = mmap_region(file, start, size,
- vma->vm_flags, pgoff);
+ addr = mmap_region(file, start, size, vm_flags, pgoff);
fput(file);
if (IS_ERR_VALUE(addr)) {
err = addr;
@@ -213,7 +214,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
BUG_ON(addr != start);
err = 0;
}
- goto out;
+ goto out_freed;
}
mutex_lock(&mapping->i_mmap_mutex);
flush_dcache_mmap_lock(mapping);
@@ -248,6 +249,7 @@ 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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