From 45e876f794e8e566bf827c25ef0791875081724f Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Tue, 26 Apr 2016 12:23:26 -0700 Subject: [PATCH] x86/segments/64: When loadsegment(fs, ...) fails, clear the base On AMD CPUs, a failed loadsegment currently may not clear the FS base. Fix it. While we're at it, prevent loadsegment(gs, xyz) from even compiling on 64-bit kernels. It shouldn't be used. Signed-off-by: Andy Lutomirski Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/a084c1b93b7b1408b58d3fd0b5d6e47da8e7d7cf.1461698311.git.luto@kernel.org Signed-off-by: Ingo Molnar --- arch/x86/include/asm/segment.h | 42 +++++++++++++++++++++++++++++++--- arch/x86/kernel/cpu/common.c | 2 +- arch/x86/mm/extable.c | 10 ++++++++ 3 files changed, 50 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h index 7d5a1929d76b..e1a4afd20223 100644 --- a/arch/x86/include/asm/segment.h +++ b/arch/x86/include/asm/segment.h @@ -2,6 +2,7 @@ #define _ASM_X86_SEGMENT_H #include +#include /* * Constructor for a conventional segment GDT (or LDT) entry. @@ -249,10 +250,13 @@ extern const char early_idt_handler_array[NUM_EXCEPTION_VECTORS][EARLY_IDT_HANDL #endif /* - * Load a segment. Fall back on loading the zero - * segment if something goes wrong.. + * Load a segment. Fall back on loading the zero segment if something goes + * wrong. This variant assumes that loading zero fully clears the segment. + * This is always the case on Intel CPUs and, even on 64-bit AMD CPUs, any + * failure to fully clear the cached descriptor is only observable for + * FS and GS. */ -#define loadsegment(seg, value) \ +#define __loadsegment_simple(seg, value) \ do { \ unsigned short __val = (value); \ \ @@ -269,6 +273,38 @@ do { \ : "+r" (__val) : : "memory"); \ } while (0) +#define __loadsegment_ss(value) __loadsegment_simple(ss, (value)) +#define __loadsegment_ds(value) __loadsegment_simple(ds, (value)) +#define __loadsegment_es(value) __loadsegment_simple(es, (value)) + +#ifdef CONFIG_X86_32 + +/* + * On 32-bit systems, the hidden parts of FS and GS are unobservable if + * the selector is NULL, so there's no funny business here. + */ +#define __loadsegment_fs(value) __loadsegment_simple(fs, (value)) +#define __loadsegment_gs(value) __loadsegment_simple(gs, (value)) + +#else + +static inline void __loadsegment_fs(unsigned short value) +{ + asm volatile(" \n" + "1: movw %0, %%fs \n" + "2: \n" + + _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_clear_fs) + + : : "rm" (value) : "memory"); +} + +/* __loadsegment_gs is intentionally undefined. Use load_gs_index instead. */ + +#endif + +#define loadsegment(seg, value) __loadsegment_ ## seg (value) + /* * Save a segment register away: */ diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index 6bfa36de6d9f..088106140c4b 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -430,7 +430,7 @@ void load_percpu_segment(int cpu) #ifdef CONFIG_X86_32 loadsegment(fs, __KERNEL_PERCPU); #else - loadsegment(gs, 0); + __loadsegment_simple(gs, 0); wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, cpu)); #endif load_stack_canary_segment(); diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c index aaeda3ffaafe..4bb53b89f3c5 100644 --- a/arch/x86/mm/extable.c +++ b/arch/x86/mm/extable.c @@ -70,6 +70,16 @@ bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup, } EXPORT_SYMBOL(ex_handler_wrmsr_unsafe); +bool ex_handler_clear_fs(const struct exception_table_entry *fixup, + struct pt_regs *regs, int trapnr) +{ + if (static_cpu_has(X86_BUG_NULL_SEG)) + asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS)); + asm volatile ("mov %0, %%fs" : : "rm" (0)); + return ex_handler_default(fixup, regs, trapnr); +} +EXPORT_SYMBOL(ex_handler_clear_fs); + bool ex_has_fault_handler(unsigned long ip) { const struct exception_table_entry *e;