Re: [PATCH AUTOSEL for 3.18 044/102] fix race in drivers/char/random.c:get_reg()

[Geert Uytterhoeven]

Hi Sasha,

On Mon, Mar 19, 2018 at 5:12 PM, Sasha Levin
<Alexander.Levin@xxxxxxxxxxxxx> wrote:
> From: Michael Schmitz <schmitzmic@xxxxxxxxx>
>
> [ Upstream commit 9dfa7bba35ac08a63565d58c454dccb7e1bb0a08 ]
>
> get_reg() can be reentered on architectures with prioritized interrupts
> (m68k in this case), causing f->reg_index to be incremented after the
> range check. Out of bounds memory access past the pt_regs struct results.
> This will go mostly undetected unless access is beyond end of memory.
>
> Prevent the race by disabling interrupts in get_reg().
>
> Tested on m68k (Atari Falcon, and ARAnyM emulator).
>
> Kudos to Geert Uytterhoeven for helping to trace this race.
>
> Signed-off-by: Michael Schmitz <schmitzmic@xxxxxxxxx>
> Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
> Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>

You probably want to apply follow-up commit 92e75428ffc90e2a
("random: use lockless method of accessing and updating f->reg_idx"), too.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds