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

[Michal Koutný]

On Tue, Apr 02, 2024 at 08:22:20AM +0200, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> 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?

Michal

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