From 3e2b68d752c9e09c40d76442aa94d3b8e421b0f1 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Thu, 7 Apr 2016 17:31:47 -0700 Subject: [PATCH] x86/asm, sched/x86: Rewrite the FS and GS context switch code The old code was incomprehensible and was buggy on AMD CPUs. Signed-off-by: Andy Lutomirski Reviewed-by: Borislav Petkov Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Rudolf Marek Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/5f6bde874c6fe6831c6711b5b1522a238ba035b4.1460075211.git.luto@kernel.org Signed-off-by: Ingo Molnar --- arch/x86/kernel/process_64.c | 148 ++++++++++++++++++++++------------- 1 file changed, 93 insertions(+), 55 deletions(-) diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index c671b9b015c0..50337eac1ca2 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -282,7 +282,7 @@ __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 fsindex, gsindex; + unsigned prev_fsindex, prev_gsindex; fpu_switch_t fpu_switch; fpu_switch = switch_fpu_prepare(prev_fpu, next_fpu, cpu); @@ -292,8 +292,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) * * (e.g. xen_load_tls()) */ - savesegment(fs, fsindex); - savesegment(gs, gsindex); + savesegment(fs, prev_fsindex); + savesegment(gs, prev_gsindex); /* * Load TLS before restoring any segments so that segment loads @@ -336,66 +336,104 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) * Switch FS and GS. * * These are even more complicated than DS and ES: they have - * 64-bit bases are that controlled by arch_prctl. Those bases - * only differ from the values in the GDT or LDT if the selector - * is 0. + * 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. * - * Loading the segment register resets the hidden base part of - * the register to 0 or the value from the GDT / LDT. If the - * next base address zero, writing 0 to the segment register is - * much faster than using wrmsr to explicitly zero the base. + * 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.) * - * The thread_struct.fs and thread_struct.gs values are 0 - * if the fs and gs bases respectively are not overridden - * from the values implied by fsindex and gsindex. They - * are nonzero, and store the nonzero base addresses, if - * the bases are overridden. - * - * (fs != 0 && fsindex != 0) || (gs != 0 && gsindex != 0) should - * be impossible. - * - * Therefore we need to reload the segment registers if either - * the old or new selector is nonzero, and we need to override - * the base address if next thread expects it to be overridden. - * - * This code is unnecessarily slow in the case where the old and - * new indexes are zero and the new base is nonzero -- it will - * unnecessarily write 0 to the selector before writing the new - * base address. - * - * Note: This all depends on arch_prctl being the only way that - * user code can override the segment base. Once wrfsbase and - * wrgsbase are enabled, most of this code will need to change. + * As an invariant, + * (fs != 0 && fsindex != 0) || (gs != 0 && gsindex != 0) is + * impossible. */ - if (unlikely(fsindex | next->fsindex | prev->fs)) { + if (next->fsindex) { + /* Loading a nonzero value into FS sets the index and base. */ loadsegment(fs, next->fsindex); - - /* - * If user code wrote a nonzero value to FS, then it also - * cleared the overridden base address. - * - * XXX: if user code wrote 0 to FS and cleared the base - * address itself, we won't notice and we'll incorrectly - * restore the prior base address next time we reschdule - * the process. - */ - if (fsindex) - prev->fs = 0; + } else { + if (next->fs) { + /* Next index is zero but next base is nonzero. */ + if (prev_fsindex) + loadsegment(fs, 0); + wrmsrl(MSR_FS_BASE, next->fs); + } 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->fs || prev_fsindex) + loadsegment(fs, 0); + } + } } - if (next->fs) - wrmsrl(MSR_FS_BASE, next->fs); - prev->fsindex = fsindex; + /* + * 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->fs = 0; + prev->fsindex = prev_fsindex; - if (unlikely(gsindex | next->gsindex | prev->gs)) { + if (next->gsindex) { + /* Loading a nonzero value into GS sets the index and base. */ load_gs_index(next->gsindex); - - /* This works (and fails) the same way as fsindex above. */ - if (gsindex) - prev->gs = 0; + } else { + if (next->gs) { + /* Next index is zero but next base is nonzero. */ + if (prev_gsindex) + load_gs_index(0); + wrmsrl(MSR_KERNEL_GS_BASE, next->gs); + } 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->gs || prev_gsindex) + load_gs_index(0); + } + } } - if (next->gs) - wrmsrl(MSR_KERNEL_GS_BASE, next->gs); - prev->gsindex = gsindex; + /* + * 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->gs = 0; + prev->gsindex = prev_gsindex; switch_fpu_finish(next_fpu, fpu_switch);