Messages in this thread | | | Date | Fri, 6 Jan 2023 12:30:24 +0100 | From | Greg Kroah-Hartman <> | Subject | Re: [PATCH] tty: vt: add some NULL checks for vc_data |
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On Tue, Jan 03, 2023 at 10:01:15PM -0500, Hang Zhang wrote: > On Tue, Jan 3, 2023 at 4:24 AM Jiri Slaby <jirislaby@kernel.org> wrote: > > > > On 29. 12. 22, 7:41, Hang Zhang wrote: > > > vc_selection(), do_blank_screen() and scrollfront() all access "vc_data" > > > structures obtained from the global "vc_cons[fg_console].d", which can > > > be freed and nullified (e.g., in the error path of vc_allocate()). But > > > these functions don't have any NULL checks against the pointers before > > > dereferencing them, causing potentially use-after-free or null pointer > > > dereference. > > > > Could you elaborate under what circumstances is fg_console set to a > > non-allocated console? > > Hi, Jiri, thank you for your reply! I am not a developer for tty > subsystem, so the reasoning here is based on my best-effort code > reading. Please correct me if I am wrong. > > This patch is based on several observations: > > (1) at the beginning of vc_selection() (where one NULL check is > inserted in this patch), poke_blanked_console() is invoked, which > explicitly checks whether "vc_cons[fg_console].d" is NULL, suggesting > the possibility of "fg_console" associated with an unallocated console > at this point. However, poke_blanked_console() returns "void", so > even if "fg_console" is NULL, after returning to vc_selection(), > it will just keep executing, resulting in the possible NULL pointer > dereference later ("vc" in vc_selection() can be "vc_cons[fg_console].d" > if called from set_selection_kernel()). So this patch actually tries > to make the already existing NULL check take effect on the control > flow (e.g., early return if NULL).
But again, how can that value ever be NULL?
And why are you returning "success" if it is?
> (2) a similar NULL check for "vc_cons[fg_console].d" can also be found > in do_unblank_screen() ("if (!vc_cons_allocated(fg_console))") before > accessing the corresponding "vc_data". I do notice that the NULL check > has a comment "/* impossible */", but the check has not been removed so > far. My guess is that there might still be a chance that it can be > unallocated at that point.
Please verify that this really ever could be NULL or not.
thanks,
greg k-h
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