From: Nicholas Piggin <npiggin@xxxxxxxxx>
[ Upstream commit f35d2f249ef05b9671e7898f09ad89aa78f99122 ]
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: 07d2a628bc000 ("powerpc/64s: Avoid cpabort in context switch when possible")
Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20210622053036.474678-1-npiggin@xxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
arch/powerpc/kernel/process.c | 48 +++++++++++++++++++++++------------
1 file changed, 32 insertions(+), 16 deletions(-)
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 3231c2df9e26..03d7261e1492 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1212,6 +1212,19 @@ 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
@@ -1257,30 +1270,33 @@ struct task_struct *__switch_to(struct task_struct *prev,
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(¤t->mm->context.vas_windows)))
- asm volatile(PPC_CP_ABORT);
- }
#endif /* CONFIG_PPC_BOOK3S_64 */
return last;
--
2.30.2
