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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 <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/a084c1b93b7b1408b58d3fd0b5d6e47da8e7d7cf.1461698311.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2016-04-26 12:23:26 -07:00 committed by Ingo Molnar
parent f005f5d860
commit 45e876f794
3 changed files with 50 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

42
arch/x86/include/asm/segment.h
View File

@ -2,6 +2,7 @@
#define _ASM_X86_SEGMENT_H
#include <linux/const.h>
#include <asm/alternative.h>
/*
* 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:
*/

2
arch/x86/kernel/cpu/common.c
View File

@ -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();

10
arch/x86/mm/extable.c
View File

@ -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;