Re: [PATCH 6.6 00/28] fix CVE-2024-46701

[Yu Kuai]

Hi,

在 2024/11/07 22:41, Chuck Lever 写道:
> On Thu, Nov 07, 2024 at 08:57:23AM +0800, Yu Kuai wrote:
> > Hi,
> >
> > 在 2024/11/06 23:19, Chuck Lever III 写道:
> > > > On Nov 6, 2024, at 1:16 AM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > On Thu, Oct 24, 2024 at 09:19:41PM +0800, Yu Kuai wrote:
> > > > > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> > > > >
> > > > > Fix patch is patch 27, relied patches are from:
> > >
> > > I assume patch 27 is:
> > >
> > > libfs: fix infinite directory reads for offset dir
> > >
> > > https://lore.kernel.org/stable/20241024132225.2271667-12-yukuai1@xxxxxxxxxxxxxxx/
> > >
> > > I don't think the Maple tree patches are a hard
> > > requirement for this fix. And note that libfs did
> > > not use Maple tree originally because I was told
> > > at that time that Maple tree was not yet mature.
> > >
> > > So, a better approach might be to fit the fix
> > > onto linux-6.6.y while sticking with xarray.
> >
> > The painful part is that using xarray is not acceptable, the offet
> > is just 32 bit and if it overflows, readdir will read nothing. That's
> > why maple_tree has to be used.
>
> A 32-bit range should be entirely adequate for this usage.
>
>   - The offset allocator wraps when it reaches the maximum, it
>     doesn't overflow unless there are actually billions of extant
>     entries in the directory, which IMO is not likely.

Yes, it's not likely, but it's possible, and not hard to trigger for
test. And please notice that the offset will increase for each new file,
and file can be removed, while offset stays the same.
>   - The offset values are dense, so the directory can use all 2- or
>     4- billion in the 32-bit integer range before wrapping.

A simple math, if user create and remove 1 file in each seconds, it will
cost about 130 years to overflow. And if user create and remove 1000
files in each second, it will cost about 1 month to overflow.

maple tree use 64 bit value for the offset, which is impossible to
overflow for the rest of our lifes.
>   - No-one complained about this limitation when offset_readdir() was
>     first merged. The xarray was replaced for performance reasons,
>     not because of the 32-bit range limit.
>
> It is always possible that I have misunderstood your concern!

The problem is that if the next_offset overflows to 0, then after patch
27, offset_dir_open() will record the 0, and later offset_readdir will
return directly, while there can be many files.

Thanks,
Kuai