Re: [PATCH] mm/usercopy: harden bounds checking for vmalloc allocations

[Uladzislau Rezki]

On Tue, Jul 28, 2026 at 06:22:57AM +0800, Barry Song wrote:
> On Mon, Jul 27, 2026 at 5:32 PM Uladzislau Rezki <urezki@xxxxxxxxx> wrote:
> >
> > On Mon, Jul 27, 2026 at 12:33:49PM +0530, Dev Jain wrote:
> > >
> > >
> > > On 26/07/26 4:16 pm, Matthew Wilcox wrote:
> > > > On Sun, Jul 26, 2026 at 02:23:20PM +0530, Dev Jain wrote:
> > > >> On 22/07/26 7:59 pm, Dev Jain wrote:
> > > >>> The vmalloc allocator stores the actual allocation size inside the
> > > >>> vm_struct structure. We can use this bound in usercopy instead of the
> > > >>> page-aligned va_end to catch usercopy beyond the actual allocation size.
> > > >>>
> > > >>> For vmap, the requested_size field is always page-aligned since it maps a
> > > >>> certain number of pages. Same for vm_map_ram (alongwith, not even having
> > > >>> a vm_struct). So the check is only relevant for vmalloc mappings.
> > > >>>
> > > >>> Because there are early vm areas registered even before vmalloc_init,
> > > >>> requested_size may be zero. So also check whether the requested_size
> > > >>> is set.
> > > >>>
> > > >>> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > > >>> ---
> > > >>
> > > >> Sashiko:
> > > >>
> > > >> 1. "Does this locklessly access area->vm after find_vmap_area() has dropped
> > > >> the busy tree lock?
> > > >>
> > > >> If an out-of-bounds pointer falls into an adjacent vmap_area, and that
> > > >> adjacent area is concurrently freed by another thread, its vm_struct
> > > >> is freed. Additionally, when the vmap_area is moved to the free tree,
> > > >> area->vm (which shares a union with subtree_max_size) is overwritten
> > > >> with an integer size.
> > > >>
> > > >> Would dereferencing vm->flags later in this function cause a use-after-free
> > > >> or a wild pointer dereference?"
> > > >>
> > > >>
> > > >> I don't get it. So usercopy is checking OOB for an object but shouldn't
> > > >> assume the existence of that object while using it?
> > > >>
> > > >> It is a bug in the caller if someone does vfree() while usercopy is operating
> > > >> on the vmalloc object. I don't think usercopy should handle it. For example
> > > >> we don't handle it for slabs currently.
> > > >
> > > > I think the question Sashiko is getting to is how we handle:
> > > >
> > > >     char *p = vmalloc();
> > > >     copy_to_user(p - 4);
> > >
> > > Thanks Willy, I completely misread Sashiko's point.
> > >
> > > I think this is an existing problem? We are using area->va_end currently,
> > > so we can access freed memory.
> > >
> > It is a bit mess here since now we need also take into account a
> > requested_size due to vrealloc. We have requested_size, nr_pages
> > and size in the vm_struct.
> 
> 
> I see that vrealloc() always updates vm->requested_size to the new size.
> So we could always use requested_size? no?
> 
Probably :) But i am not sure what the patch fixes. There is a race anyway?

You check VA on CPU_1 successfully, after that CPU_2 unmaps the pages by
calling vrealloc() and CPU_1 is about to read unmapped pages.

--
Uladzislau Rezki