From c4a7bba29bc6d9fcd5842d712b6b3d493b682b17 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Mon, 6 Oct 2014 12:36:47 -0700 Subject: [PATCH] sched/x86: Add a comment clarifying LDT context switching The code is correct, but only for a rather subtle reason. This confused me for quite a while when I read switch_mm, so clarify the code to avoid confusing other people, too. TBH, I wouldn't be surprised if this code was only correct by accident. [ I wouldn't normally send a comment-only patch, but it took me a long time to first figure out wtf was going on here, and then to figure out why this wasn't exploitable by malicious code, and then to figure out why this oddity had no user-visible effect at all. Let's spare future readers the same confusion. ] Signed-off-by: Andy Lutomirski Cc: Linus Torvalds Link: http://lkml.kernel.org/r/36275c99801a87d8dcf0502a41cf4e2ad81aae46.1412623954.git.luto@amacapital.net Signed-off-by: Ingo Molnar --- arch/x86/include/asm/mmu_context.h | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h index 166af2a8e865..04478103df37 100644 --- a/arch/x86/include/asm/mmu_context.h +++ b/arch/x86/include/asm/mmu_context.h @@ -53,7 +53,16 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, /* Stop flush ipis for the previous mm */ cpumask_clear_cpu(cpu, mm_cpumask(prev)); - /* Load the LDT, if the LDT is different: */ + /* + * Load the LDT, if the LDT is different. + * + * It's possible leave_mm(prev) has been called. If so, + * then prev->context.ldt could be out of sync with the + * LDT descriptor or the LDT register. This can only happen + * if prev->context.ldt is non-null, since we never free + * an LDT. But LDTs can't be shared across mms, so + * prev->context.ldt won't be equal to next->context.ldt. + */ if (unlikely(prev->context.ldt != next->context.ldt)) load_LDT_nolock(&next->context); } -- 2.20.1