Messages in this thread | | | Date | Wed, 28 Jun 2023 09:32:20 -0700 | From | Kees Cook <> | Subject | Re: [PATCH v3 2/2] tty: Allow TIOCSTI to be disabled |
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On Wed, Jun 28, 2023 at 08:07:16AM +0200, Samuel Thibault wrote: > Kees Cook, le mar. 27 juin 2023 19:48:45 -0700, a ecrit: > > On Sun, Jun 25, 2023 at 05:56:25PM +0200, Samuel Thibault wrote: > > > > Can we perhaps just introduce a CAP_TIOCSTI that the brltty daemon would > > > > be able to use? We could even make it only allow TIOCSTI on the linux > > > > console (tty->ops == con_ops). > > > > Does brltty run with CAP_SYS_ADMIN? > > ATM most often, yes, though we are trying to reduce the CAP_* privileges > to what it actually needs. > > > > *Please* comment on this so we can progress. ATM people are > > > advising each other to set dev.tty.legacy_tiocsti=1, which is just > > > counter-productive in terms of security... > > > > So is there really no solution for brltty and TIOCSTI being disabled? > > No, there is no way to simulate characters on the Linux console. The > alternative would be to use uinput, but that simulates keycodes, not > characters, thus requiring backtranslating first, which is very fragile. > > > What is FreeBSD doing? I imagine it's the same situation there too, > > though maybe there is just no support? > > There is just no support in the kernel, only a patch against "screen". > > > > Really, this a serious regression for the people affected by this. > > > > Can you send a patch adding a CAP_SYS_ADMIN exception? > > Sure!
Thanks! (And be sure to use file->f_cred for the check[1], not "current", that way brltty can open the tty and drop caps and still do the ioctl.)
-Kees
https://docs.kernel.org/security/credentials.html?highlight=confused+deputy#open-file-credentials
-- Kees Cook
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