From: Andy Lutomirski luto@kernel.org
commit a493d1ca1a03b532871f1da27f8dbda2b28b04c4 upstream.
sync_core_before_usermode() had an incorrect optimization. If the kernel returns from an interrupt, it can get to usermode without IRET. It just has to schedule to a different task in the same mm and do SYSRET. Fortunately, there were no callers of sync_core_before_usermode() that could have had in_irq() or in_nmi() equal to true, because it's only ever called from the scheduler.
While at it, clarify a related comment.
Fixes: 70216e18e519 ("membarrier: Provide core serializing command, *_SYNC_CORE") Signed-off-by: Andy Lutomirski luto@kernel.org Signed-off-by: Thomas Gleixner tglx@linutronix.de Reviewed-by: Mathieu Desnoyers mathieu.desnoyers@efficios.com Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/5afc7632be1422f91eaf7611aaaa1b5b8580a086.160705830... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/include/asm/sync_core.h | 9 +++++---- arch/x86/mm/tlb.c | 10 ++++++++-- 2 files changed, 13 insertions(+), 6 deletions(-)
--- a/arch/x86/include/asm/sync_core.h +++ b/arch/x86/include/asm/sync_core.h @@ -16,12 +16,13 @@ static inline void sync_core_before_user /* With PTI, we unconditionally serialize before running user code. */ if (static_cpu_has(X86_FEATURE_PTI)) return; + /* - * Return from interrupt and NMI is done through iret, which is core - * serializing. + * Even if we're in an interrupt, we might reschedule before returning, + * in which case we could switch to a different thread in the same mm + * and return using SYSRET or SYSEXIT. Instead of trying to keep + * track of our need to sync the core, just sync right away. */ - if (in_irq() || in_nmi()) - return; sync_core(); }
--- a/arch/x86/mm/tlb.c +++ b/arch/x86/mm/tlb.c @@ -321,8 +321,14 @@ void switch_mm_irqs_off(struct mm_struct /* * The membarrier system call requires a full memory barrier and * core serialization before returning to user-space, after - * storing to rq->curr. Writing to CR3 provides that full - * memory barrier and core serializing instruction. + * storing to rq->curr, when changing mm. This is because + * membarrier() sends IPIs to all CPUs that are in the target mm + * to make them issue memory barriers. However, if another CPU + * switches to/from the target mm concurrently with + * membarrier(), it can cause that CPU not to receive an IPI + * when it really should issue a memory barrier. Writing to CR3 + * provides that full memory barrier and core serializing + * instruction. */ if (real_prev == next) { VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=