x86/arch_prctl/64: Remove FSBASE/GSBASE < 4G optimization

As far as I know, the optimization doesn't work on any modern distro
because modern distros use high addresses for ASLR.  Remove it.

The ptrace code was either wrong or very strange, but the behavior
with this patch should be essentially identical to the behavior
without this patch unless user code goes out of its way to mislead
ptrace.

On newer CPUs, once the FSGSBASE instructions are enabled, we won't
want to use the optimized variant anyway.

This isn't actually much of a performance regression, it has no effect
on normal dynamically linked programs, and it's a considerably
simplification. It also removes some nasty special cases from code
that is already way too full of special cases for comfort.

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/dd1599b08866961dba9d2458faa6bbd7fba471d7.1461698311.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 6 insertions, 38 deletions

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 0f4d2a5..e72ab40 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -303,29 +303,11 @@ static int set_segment_reg(struct task_struct *task,
 
 	switch (offset) {
 	case offsetof(struct user_regs_struct,fs):
-		/*
-		 * If this is setting fs as for normal 64-bit use but
-		 * setting fs_base has implicitly changed it, leave it.
-		 */
-		if ((value == FS_TLS_SEL && task->thread.fsindex == 0 &&
-		     task->thread.fs != 0) ||
-		    (value == 0 && task->thread.fsindex == FS_TLS_SEL &&
-		     task->thread.fs == 0))
-			break;
 		task->thread.fsindex = value;
 		if (task == current)
 			loadsegment(fs, task->thread.fsindex);
 		break;
 	case offsetof(struct user_regs_struct,gs):
-		/*
-		 * If this is setting gs as for normal 64-bit use but
-		 * setting gs_base has implicitly changed it, leave it.
-		 */
-		if ((value == GS_TLS_SEL && task->thread.gsindex == 0 &&
-		     task->thread.gs != 0) ||
-		    (value == 0 && task->thread.gsindex == GS_TLS_SEL &&
-		     task->thread.gs == 0))
-			break;
 		task->thread.gsindex = value;
 		if (task == current)
 			load_gs_index(task->thread.gsindex);
@@ -453,31 +435,17 @@ static unsigned long getreg(struct task_struct *task, unsigned long offset)
 #ifdef CONFIG_X86_64
 	case offsetof(struct user_regs_struct, fs_base): {
 		/*
-		 * do_arch_prctl may have used a GDT slot instead of
-		 * the MSR.  To userland, it appears the same either
-		 * way, except the %fs segment selector might not be 0.
+		 * XXX: This will not behave as expected if called on
+		 * current or if fsindex != 0.
 		 */
-		unsigned int seg = task->thread.fsindex;
-		if (task->thread.fs != 0)
-			return task->thread.fs;
-		if (task == current)
-			asm("movl %%fs,%0" : "=r" (seg));
-		if (seg != FS_TLS_SEL)
-			return 0;
-		return get_desc_base(&task->thread.tls_array[FS_TLS]);
+		return task->thread.fs;
 	}
 	case offsetof(struct user_regs_struct, gs_base): {
 		/*
-		 * Exactly the same here as the %fs handling above.
+		 * XXX: This will not behave as expected if called on
+		 * current or if fsindex != 0.
 		 */
-		unsigned int seg = task->thread.gsindex;
-		if (task->thread.gs != 0)
-			return task->thread.gs;
-		if (task == current)
-			asm("movl %%gs,%0" : "=r" (seg));
-		if (seg != GS_TLS_SEL)
-			return 0;
-		return get_desc_base(&task->thread.tls_array[GS_TLS]);
+		return task->thread.gs;
 	}
 #endif
 	}