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powerpc/64s: Fix copy-paste data exposure into newly created tasks 

copy-paste contains implicit "copy buffer" state that can contain
arbitrary user data (if the user process executes a copy instruction).
This could be snooped by another process if a context switch hits while
the state is live. So cp_abort is executed on context switch to clear
out possible sensitive data and prevent the leak.

cp_abort is done after the low level _switch(), which means it is never
reached by newly created tasks, so they could snoop on this buffer
between their first and second context switch.

Fix this by doing the cp_abort before calling _switch. Add some
comments which should make the issue harder to miss.

Fixes: 07d2a628bc ("powerpc/64s: Avoid cpabort in context switch when possible")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210622053036.474678-1-npiggin@gmail.com
This commit is contained in:
Nicholas Piggin 2021-06-22 15:30:36 +10:00 committed by Michael Ellerman
parent a27755d57e
commit f35d2f249e
1 changed files with 32 additions and 16 deletions
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48
arch/powerpc/kernel/process.c
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@ -1223,6 +1223,19 @@ struct task_struct *__switch_to(struct task_struct *prev,
__flush_tlb_pending(batch);
batch->active = 0;
}
/*
* On POWER9 the copy-paste buffer can only paste into
* foreign real addresses, so unprivileged processes can not
* see the data or use it in any way unless they have
* foreign real mappings. If the new process has the foreign
* real address mappings, we must issue a cp_abort to clear
* any state and prevent snooping, corruption or a covert
* channel. ISA v3.1 supports paste into local memory.
*/
if (new->mm && (cpu_has_feature(CPU_FTR_ARCH_31) ||
atomic_read(&new->mm->context.vas_windows)))
asm volatile(PPC_CP_ABORT);
#endif /* CONFIG_PPC_BOOK3S_64 */
#ifdef CONFIG_PPC_ADV_DEBUG_REGS
@ -1273,30 +1286,33 @@ struct task_struct *__switch_to(struct task_struct *prev,
#endif
last = _switch(old_thread, new_thread);
/*
* Nothing after _switch will be run for newly created tasks,
* because they switch directly to ret_from_fork/ret_from_kernel_thread
* etc. Code added here should have a comment explaining why that is
* okay.
*/
#ifdef CONFIG_PPC_BOOK3S_64
/*
* This applies to a process that was context switched while inside
* arch_enter_lazy_mmu_mode(), to re-activate the batch that was
* deactivated above, before _switch(). This will never be the case
* for new tasks.
*/
if (current_thread_info()->local_flags & _TLF_LAZY_MMU) {
current_thread_info()->local_flags &= ~_TLF_LAZY_MMU;
batch = this_cpu_ptr(&ppc64_tlb_batch);
batch->active = 1;
}
if (current->thread.regs) {
/*
* Math facilities are masked out of the child MSR in copy_thread.
* A new task does not need to restore_math because it will
* demand fault them.
*/
if (current->thread.regs)
restore_math(current->thread.regs);
/*
* On POWER9 the copy-paste buffer can only paste into
* foreign real addresses, so unprivileged processes can not
* see the data or use it in any way unless they have
* foreign real mappings. If the new process has the foreign
* real address mappings, we must issue a cp_abort to clear
* any state and prevent snooping, corruption or a covert
* channel. ISA v3.1 supports paste into local memory.
*/
if (current->mm &&
(cpu_has_feature(CPU_FTR_ARCH_31) ||
atomic_read(&current->mm->context.vas_windows)))
asm volatile(PPC_CP_ABORT);
}
#endif /* CONFIG_PPC_BOOK3S_64 */
return last;