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SubjectRe: Memory leak in 2.4.27 kernel, using mmap raw packet sockets
Hi Andrea,

On Thu, Nov 25, 2004 at 09:32:48PM +0100, Andrea Arcangeli wrote:
> On Thu, Nov 25, 2004 at 01:02:06PM -0200, Marcelo Tosatti wrote:
> > On Sun, Oct 17, 2004 at 03:39:26AM +0100, Alan Cox wrote:
> > > On Gwe, 2004-10-15 at 19:23, Marcelo Tosatti wrote:
> > > > I prefer doing the "if (PageReserved(page)) put_page_testzero(page)" as
> > > > you propose instead of changing get_user_pages(), as there are several
> > > > users which rely on its behaviour.
> > > >
> > > > I have applied your fix to the 2.4 BK tree.
> > >
> > > That isnt sufficient. Consider anything else taking a reference to the
> > > page and the refcount going negative. And yes 2.6.x has this problem and
> > > far worse in some ways, but it also has the mechanism to fix it.
> > >
> > > 2.6.x uses VM_IO as a VMA flag which tells the kernel two things
> > > a) get_user_pages fails on it
> > > b) core dumping of it is forbidden
> > >
> > > 2.6.x is missing a whole pile of these (fixed in the 2.6.9-ac tree I'm
> > > putting together). I *think* remap_page_range() in 2.6.x can just set
> > > VM_IO, but older kernels didn't pass the vma so all the users would need
> > > fixing (OSS audio, media/video, usb audio, usb video, frame buffer
> > > etc).
> >
> > I dont see any practical problem with 2.4.x right now.
> >
> > get_user_pages() wont be called on driver created VMA's with PageReserved
> > pages because of the VM_IO bit which is set at remap_page_range().
> >
> > Its not possible to have any vma mapped by a driver without VM_IO set.
> >
> > But the network packet mmap was an isolated case, so I'll apply Andrea's
> > fix just for safety, although I can't find any offender in the tree.
> >
> >
> > --- memory.c 2004-10-22 15:58:28.000000000 -0200
> > +++ memory.c 2004-10-28 14:32:26.585813200 -0200
> > @@ -499,7 +499,7 @@
> > /* FIXME: call the correct function,
> > * depending on the type of the found page
> > */
> > - if (!pages[i])
> > + if (!pages[i] || PageReserved(pages[i]))
> > goto bad_page;
> > page_cache_get(pages[i]);
> > }
>
> this needs to be modified to take the ZERO_PAGE(start) into account.
> It's a minor detail, but we should allow that. (my 2.4-aa tree was
> already checking the zeropage for other reasons, so it didn't need any
> change)
>
> in short the above fix should be modified like this:
>
> if (!pages[i] || PageReserved(pages[i])) {
> if (pages[i] != ZERO_PAGE(start))
> goto bad_page;
> } else
> page_cache_get(pages[i]);
>
> the zero page is guaranteed to remain reserved. the major bug is that
> get_user_pages was allowing _temporarily_ reserved pages to be pinned.
> __free_pages isn't checking the VM_IO, and as such get_user_pages should
> be robust against its __free_pages counterpart, this is why I believe
> the above fix is the right fix.
>
> ZEROPAGE is special because:
>
> 1) it's guaranteed to never be unpinned
> 2) it's the only reserved page that handle_mm_fault is allowed to
> istantiate for a filesystem data mapping
>
> The VM_IO enforcment is a nice improvement on top of the above.

get_user_pages() bails out if ! VM_IO.

Is that what you mean with "VM_IO enforcement" ?

> Checking the PageReserved bitflag is a good thing at least for the
> zeropage, so we don't overflow the zeropage count, which isn't nice.
>
> If you really want to fix it only using VM_IO, I still recommend to
> apply the above patch, and to turn 'goto bad_page' into BUG().
> It'd be a bad idea not to at least add the above code as a robustness
> check.
>
> Comments welcome. thanks.

I thought about the BUG() to catch potential offenders, but I was
not sure if it was possible for a PG_reserved page to be part of VMA's
which was being get_user_pages'd.

Now you tell me it is possible, and thats only the ZERO page. Fine.

This is what you suggests plus some extra hopefully useful debugging


--- memory.c.orig 2004-11-25 14:51:00.074508952 -0200
+++ memory.c 2004-11-25 15:08:38.026675776 -0200
@@ -454,8 +454,9 @@
int get_user_pages(struct task_struct *tsk, struct mm_struct *mm, unsigned long start,
int len, int write, int force, struct page **pages, struct vm_area_struct **vmas)
{
- int i;
+ int i, s;
unsigned int flags;
+ struct vm_area_struct *savevma = NULL;

/*
* Require read or write permissions.
@@ -463,7 +464,7 @@
*/
flags = write ? (VM_WRITE | VM_MAYWRITE) : (VM_READ | VM_MAYREAD);
flags &= force ? (VM_MAYREAD | VM_MAYWRITE) : (VM_READ | VM_WRITE);
- i = 0;
+ i = s = 0;

do {
struct vm_area_struct * vma;
@@ -499,9 +500,13 @@
/* FIXME: call the correct function,
* depending on the type of the found page
*/
- if (!pages[i] || PageReserved(pages[i]))
- goto bad_page;
- page_cache_get(pages[i]);
+ if (!pages[i] || PageReserved(pages[i])) {
+ if (pages[i] != ZERO_PAGE(start)) {
+ savevma = vma;
+ goto bad_page;
+ }
+ } else
+ page_cache_get(pages[i]);
}
if (vmas)
vmas[i] = vma;
@@ -520,9 +525,15 @@
*/
bad_page:
spin_unlock(&mm->page_table_lock);
+ s = i;
while (i--)
page_cache_release(pages[i]);
- i = -EFAULT;
+ /* catch bad uses of PG_reserved on !VM_IO vma's */
+ printk(KERN_ERR "get_user_pages PG_reserved page on"
+ "vma:%p flags:%lx page:%d\n", savevma,
+ savevma->flags, s);
+ BUG();
+ i = -EFAULT;
goto out;
}
-
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