diff options
| -rw-r--r-- | arch/x86/kernel/process_64.c | 227 |
1 files changed, 122 insertions, 105 deletions
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index 02fa4701cc2e..0887d2ae3797 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -136,6 +136,123 @@ void release_thread(struct task_struct *dead_task) } } +enum which_selector { + FS, + GS +}; + +/* + * Saves the FS or GS base for an outgoing thread if FSGSBASE extensions are + * not available. The goal is to be reasonably fast on non-FSGSBASE systems. + * It's forcibly inlined because it'll generate better code and this function + * is hot. + */ +static __always_inline void save_base_legacy(struct task_struct *prev_p, + unsigned short selector, + enum which_selector which) +{ + if (likely(selector == 0)) { + /* + * On Intel (without X86_BUG_NULL_SEG), the segment base could + * be the pre-existing saved base or it could be zero. On AMD + * (with X86_BUG_NULL_SEG), the segment base could be almost + * anything. + * + * This branch is very hot (it's hit twice on almost every + * context switch between 64-bit programs), and avoiding + * the RDMSR helps a lot, so we just assume that whatever + * value is already saved is correct. This matches historical + * Linux behavior, so it won't break existing applications. + * + * To avoid leaking state, on non-X86_BUG_NULL_SEG CPUs, if we + * report that the base is zero, it needs to actually be zero: + * see the corresponding logic in load_seg_legacy. + */ + } else { + /* + * If the selector is 1, 2, or 3, then the base is zero on + * !X86_BUG_NULL_SEG CPUs and could be anything on + * X86_BUG_NULL_SEG CPUs. In the latter case, Linux + * has never attempted to preserve the base across context + * switches. + * + * If selector > 3, then it refers to a real segment, and + * saving the base isn't necessary. + */ + if (which == FS) + prev_p->thread.fsbase = 0; + else + prev_p->thread.gsbase = 0; + } +} + +static __always_inline void save_fsgs(struct task_struct *task) +{ + savesegment(fs, task->thread.fsindex); + savesegment(gs, task->thread.gsindex); + save_base_legacy(task, task->thread.fsindex, FS); + save_base_legacy(task, task->thread.gsindex, GS); +} + +static __always_inline void loadseg(enum which_selector which, + unsigned short sel) +{ + if (which == FS) + loadsegment(fs, sel); + else + load_gs_index(sel); +} + +static __always_inline void load_seg_legacy(unsigned short prev_index, + unsigned long prev_base, + unsigned short next_index, + unsigned long next_base, + enum which_selector which) +{ + if (likely(next_index <= 3)) { + /* + * The next task is using 64-bit TLS, is not using this + * segment at all, or is having fun with arcane CPU features. + */ + if (next_base == 0) { + /* + * Nasty case: on AMD CPUs, we need to forcibly zero + * the base. + */ + if (static_cpu_has_bug(X86_BUG_NULL_SEG)) { + loadseg(which, __USER_DS); + loadseg(which, next_index); + } else { + /* + * We could try to exhaustively detect cases + * under which we can skip the segment load, + * but there's really only one case that matters + * for performance: if both the previous and + * next states are fully zeroed, we can skip + * the load. + * + * (This assumes that prev_base == 0 has no + * false positives. This is the case on + * Intel-style CPUs.) + */ + if (likely(prev_index | next_index | prev_base)) + loadseg(which, next_index); + } + } else { + if (prev_index != next_index) + loadseg(which, next_index); + wrmsrl(which == FS ? MSR_FS_BASE : MSR_KERNEL_GS_BASE, + next_base); + } + } else { + /* + * The next task is using a real segment. Loading the selector + * is sufficient. + */ + loadseg(which, next_index); + } +} + int copy_thread_tls(unsigned long clone_flags, unsigned long sp, unsigned long arg, struct task_struct *p, unsigned long tls) { @@ -273,7 +390,6 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) struct fpu *next_fpu = &next->fpu; int cpu = smp_processor_id(); struct tss_struct *tss = &per_cpu(cpu_tss, cpu); - unsigned prev_fsindex, prev_gsindex; fpu_switch_t fpu_switch; fpu_switch = switch_fpu_prepare(prev_fpu, next_fpu, cpu); @@ -283,8 +399,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) * * (e.g. xen_load_tls()) */ - savesegment(fs, prev_fsindex); - savesegment(gs, prev_gsindex); + save_fsgs(prev_p); /* * Load TLS before restoring any segments so that segment loads @@ -323,108 +438,10 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) if (unlikely(next->ds | prev->ds)) loadsegment(ds, next->ds); - /* - * Switch FS and GS. - * - * These are even more complicated than DS and ES: they have - * 64-bit bases are that controlled by arch_prctl. The bases - * don't necessarily match the selectors, as user code can do - * any number of things to cause them to be inconsistent. - * - * We don't promise to preserve the bases if the selectors are - * nonzero. We also don't promise to preserve the base if the - * selector is zero and the base doesn't match whatever was - * most recently passed to ARCH_SET_FS/GS. (If/when the - * FSGSBASE instructions are enabled, we'll need to offer - * stronger guarantees.) - * - * As an invariant, - * (fsbase != 0 && fsindex != 0) || (gsbase != 0 && gsindex != 0) is - * impossible. - */ - if (next->fsindex) { - /* Loading a nonzero value into FS sets the index and base. */ - loadsegment(fs, next->fsindex); - } else { - if (next->fsbase) { - /* Next index is zero but next base is nonzero. */ - if (prev_fsindex) - loadsegment(fs, 0); - wrmsrl(MSR_FS_BASE, next->fsbase); - } else { - /* Next base and index are both zero. */ - if (static_cpu_has_bug(X86_BUG_NULL_SEG)) { - /* - * We don't know the previous base and can't - * find out without RDMSR. Forcibly clear it. - */ - loadsegment(fs, __USER_DS); - loadsegment(fs, 0); - } else { - /* - * If the previous index is zero and ARCH_SET_FS - * didn't change the base, then the base is - * also zero and we don't need to do anything. - */ - if (prev->fsbase || prev_fsindex) - loadsegment(fs, 0); - } - } - } - /* - * Save the old state and preserve the invariant. - * NB: if prev_fsindex == 0, then we can't reliably learn the base - * without RDMSR because Intel user code can zero it without telling - * us and AMD user code can program any 32-bit value without telling - * us. - */ - if (prev_fsindex) - prev->fsbase = 0; - prev->fsindex = prev_fsindex; - - if (next->gsindex) { - /* Loading a nonzero value into GS sets the index and base. */ - load_gs_index(next->gsindex); - } else { - if (next->gsbase) { - /* Next index is zero but next base is nonzero. */ - if (prev_gsindex) - load_gs_index(0); - wrmsrl(MSR_KERNEL_GS_BASE, next->gsbase); - } else { - /* Next base and index are both zero. */ - if (static_cpu_has_bug(X86_BUG_NULL_SEG)) { - /* - * We don't know the previous base and can't - * find out without RDMSR. Forcibly clear it. - * - * This contains a pointless SWAPGS pair. - * Fixing it would involve an explicit check - * for Xen or a new pvop. - */ - load_gs_index(__USER_DS); - load_gs_index(0); - } else { - /* - * If the previous index is zero and ARCH_SET_GS - * didn't change the base, then the base is - * also zero and we don't need to do anything. - */ - if (prev->gsbase || prev_gsindex) - load_gs_index(0); - } - } - } - /* - * Save the old state and preserve the invariant. - * NB: if prev_gsindex == 0, then we can't reliably learn the base - * without RDMSR because Intel user code can zero it without telling - * us and AMD user code can program any 32-bit value without telling - * us. - */ - if (prev_gsindex) - prev->gsbase = 0; - prev->gsindex = prev_gsindex; + load_seg_legacy(prev->fsindex, prev->fsbase, + next->fsindex, next->fsbase, FS); + load_seg_legacy(prev->gsindex, prev->gsbase, + next->gsindex, next->gsbase, GS); switch_fpu_finish(next_fpu, fpu_switch); |