Re: CVE-2023-52630: blk-iocost: Fix an UBSAN shift-out-of-bounds warning

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Apr 26, 2024 at 07:34:45PM +0200, Michal Koutný wrote:
> On Tue, Apr 02, 2024 at 08:22:20AM +0200, Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
> > Description
> > ===========
> > 
> > In the Linux kernel, the following vulnerability has been resolved:
> > 
> > blk-iocost: Fix an UBSAN shift-out-of-bounds warning
> > 
> > When iocg_kick_delay() is called from a CPU different than the one which set
> > the delay, @now may be in the past of @iocg->delay_at leading to the
> > following warning:
> > 
> >   UBSAN: shift-out-of-bounds in block/blk-iocost.c:1359:23
> >   shift exponent 18446744073709 is too large for 64-bit type 'u64' (aka 'unsigned long long')
> >   ...
> >   Call Trace:
> >    <TASK>
> >    dump_stack_lvl+0x79/0xc0
> >    __ubsan_handle_shift_out_of_bounds+0x2ab/0x300
> >    iocg_kick_delay+0x222/0x230
> >    ioc_rqos_merge+0x1d7/0x2c0
> >    __rq_qos_merge+0x2c/0x80
> >    bio_attempt_back_merge+0x83/0x190
> >    blk_attempt_plug_merge+0x101/0x150
> >    blk_mq_submit_bio+0x2b1/0x720
> >    submit_bio_noacct_nocheck+0x320/0x3e0
> >    __swap_writepage+0x2ab/0x9d0
> > 
> > The underflow itself doesn't really affect the behavior in any meaningful
> > way; however, the past timestamp may exaggerate the delay amount calculated
> > later in the code, which shouldn't be a material problem given the nature of
> > the delay mechanism.
> 
> The worst implication is unfair or slowed IO but that can't be
> quantified given empirical implementation of the delay mechanism.
> 
> > If @now is in the past, this CPU is racing another CPU which recently set up
> > the delay and there's nothing this CPU can contribute w.r.t. the delay.
> 
> This means the user has limited control (with noise) over such
> placements.
> 
> > Let's bail early from iocg_kick_delay() in such cases.
> > 
> > The Linux kernel CVE team has assigned CVE-2023-52630 to this issue.
> 
> Based on the above I don't think this fix deserves CVE tracking. Shall
> it be rejected?

Makes sense, thanks for looking into this, and sorry for the delay.  Now
rejected.

greg k-h