Messages in this thread | | | Subject | Re: [PATCH v2 6/6] powerpc: Implement user_access_begin and friends | From | Christophe Leroy <> | Date | Fri, 24 Jan 2020 12:40:23 +0100 |
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Le 23/01/2020 à 13:31, Michael Ellerman a écrit : > Michael Ellerman <mpe@ellerman.id.au> writes: >> Christophe Leroy <christophe.leroy@c-s.fr> writes: >>> Today, when a function like strncpy_from_user() is called, >>> the userspace access protection is de-activated and re-activated >>> for every word read. >>> >>> By implementing user_access_begin and friends, the protection >>> is de-activated at the beginning of the copy and re-activated at the >>> end. >>> >>> Implement user_access_begin(), user_access_end() and >>> unsafe_get_user(), unsafe_put_user() and unsafe_copy_to_user() >>> >>> For the time being, we keep user_access_save() and >>> user_access_restore() as nops. >> >> That means we will run with user access enabled in a few more places, but >> it's only used sparingly AFAICS: >> >> kernel/trace/trace_branch.c: unsigned long flags = user_access_save(); >> lib/ubsan.c: unsigned long flags = user_access_save(); >> lib/ubsan.c: unsigned long ua_flags = user_access_save(); >> mm/kasan/common.c: unsigned long flags = user_access_save(); >> >> And we don't have objtool checking that user access enablement isn't >> leaking in the first place, so I guess it's OK for us not to implement >> these to begin with? > > It looks like we can implement them on on all three KUAP > implementations. > > For radix and 8xx we just return/set the relevant SPR. > > For book3s/32/kup.h I think we'd just need to add a KUAP_CURRENT case to > allow_user_access()?
Can't do that, we don't want to keep the info in current->thread.kuap after user_access_save(), otherwise we might unexpectedly re-open access through an interrupt.
And if we use KUAP_CURRENT case of prevent_user_access(), it means we'll read current->thread.kuap twice.
So, just regenerate addr and end from the flags, and use allow_user_access() and prevent_user_access() as usual.
I'll have it in v4
Christophe
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