From: Nicholas Piggin npiggin@gmail.com
[ 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@gmail.com Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20210622053036.474678-1-npiggin@gmail.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 1a1d2657fe8d..3064694afea1 100644 --- a/arch/powerpc/kernel/process.c +++ b/arch/powerpc/kernel/process.c @@ -1227,6 +1227,19 @@ 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 @@ -1272,30 +1285,33 @@ 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;