Messages in this thread | | | Subject | Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region | From | Julien Thierry <> | Date | Wed, 13 Feb 2019 14:24:34 +0000 |
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On 13/02/2019 14:17, Peter Zijlstra wrote: > On Wed, Feb 13, 2019 at 02:00:26PM +0000, Will Deacon wrote: >>> This; how is getting preempted fundamentally different from scheduling >>> ourselves? >> >> The difference is because getting preempted in the sequence above is >> triggered off the back of an interrupt. On arm64, and I think also on x86, >> the user access state (SMAP or PAN) is saved and restored across exceptions >> but not across context switch. Consequently, taking an irq in a >> user_access_{begin,end} section and then scheduling is fine, but calling >> schedule directly within such a section is not. > > So how's this then: > > if (user_access_begin()) { > > preempt_disable(); > > <IRQ> > set_need_resched(); > </IRQ no preempt> > > preempt_enable() > __schedule(); > > user_access_end(); > } > > That _should_ work just fine but explodes with the proposed nonsense.
AFAICT, This does not work properly because when you schedule out this task, you won't be saving the EFLAGS.AF/PSTATE.PAN bit on the stack, and next time you schedule the task back in, it might no longer have the correct flag value (so an unsafe_get/put_user() will fail even though you haven't reached user_access_end()).
One solution is to deal with this in task switching code, but so far I've been told that calling schedule() in such a context is not expected to be supported.
Cheers,
-- Julien Thierry
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