x86/fsgsbase: Revert FSGSBASE support

The FSGSBASE series turned out to have serious bugs and there is still an
open issue which is not fully understood yet.

The confidence in those changes has become close to zero especially as the
test cases which have been shipped with that series were obviously never
run before sending the final series out to LKML.

  ./fsgsbase_64 >/dev/null
  Segmentation fault

As the merge window is close, the only sane decision is to revert FSGSBASE
support. The revert is necessary as this branch has been merged into
perf/core already and rebasing all of that a few days before the merge
window is not the most brilliant idea.

I could definitely slap myself for not noticing the test case fail when
merging that series, but TBH my expectations weren't that low back
then. Won't happen again.

Revert the following commits:
539bca535dec ("x86/entry/64: Fix and clean up paranoid_exit")
2c7b5ac5d5a9 ("Documentation/x86/64: Add documentation for GS/FS addressing mode")
f987c955c745 ("x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2")
2032f1f96ee0 ("x86/cpu: Enable FSGSBASE on 64bit by default and add a chicken bit")
5bf0cab60ee2 ("x86/entry/64: Document GSBASE handling in the paranoid path")
708078f65721 ("x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit")
79e1932fa3ce ("x86/entry/64: Introduce the FIND_PERCPU_BASE macro")
1d07316b1363 ("x86/entry/64: Switch CR3 before SWAPGS in paranoid entry")
f60a83df4593 ("x86/process/64: Use FSGSBASE instructions on thread copy and ptrace")
1ab5f3f7fe3d ("x86/process/64: Use FSBSBASE in switch_to() if available")
a86b4625138d ("x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions")
8b71340d702e ("x86/fsgsbase/64: Add intrinsics for FSGSBASE instructions")
b64ed19b93c3 ("x86/cpu: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE")

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>

2 files changed, 12 insertions, 129 deletions

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 637c9117d5ae..dad20bc891d5 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -366,22 +366,6 @@ out:
 	cr4_clear_bits(X86_CR4_UMIP);
 }
 
-static __init int x86_nofsgsbase_setup(char *arg)
-{
-	/* Require an exact match without trailing characters. */
-	if (strlen(arg))
-		return 0;
-
-	/* Do not emit a message if the feature is not present. */
-	if (!boot_cpu_has(X86_FEATURE_FSGSBASE))
-		return 1;
-
-	setup_clear_cpu_cap(X86_FEATURE_FSGSBASE);
-	pr_info("FSGSBASE disabled via kernel command line\n");
-	return 1;
-}
-__setup("nofsgsbase", x86_nofsgsbase_setup);
-
 /*
  * Protection Keys are not available in 32-bit mode.
  */
@@ -1386,12 +1370,6 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	setup_smap(c);
 	setup_umip(c);
 
-	/* Enable FSGSBASE instructions if available. */
-	if (cpu_has(c, X86_FEATURE_FSGSBASE)) {
-		cr4_set_bits(X86_CR4_FSGSBASE);
-		elf_hwcap2 |= HWCAP2_FSGSBASE;
-	}
-
 	/*
 	 * The vendor-specific functions might have changed features.
 	 * Now we do "generic changes."
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 8f239091c15d..250e4c4ac6d9 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -162,40 +162,6 @@ enum which_selector {
 };
 
 /*
- * Out of line to be protected from kprobes. It is not used on Xen
- * paravirt. When paravirt support is needed, it needs to be renamed
- * with native_ prefix.
- */
-static noinline unsigned long __rdgsbase_inactive(void)
-{
-	unsigned long gsbase;
-
-	lockdep_assert_irqs_disabled();
-
-	native_swapgs();
-	gsbase = rdgsbase();
-	native_swapgs();
-
-	return gsbase;
-}
-NOKPROBE_SYMBOL(__rdgsbase_inactive);
-
-/*
- * Out of line to be protected from kprobes. It is not used on Xen
- * paravirt. When paravirt support is needed, it needs to be renamed
- * with native_ prefix.
- */
-static noinline void __wrgsbase_inactive(unsigned long gsbase)
-{
-	lockdep_assert_irqs_disabled();
-
-	native_swapgs();
-	wrgsbase(gsbase);
-	native_swapgs();
-}
-NOKPROBE_SYMBOL(__wrgsbase_inactive);
-
-/*
  * 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
@@ -244,22 +210,8 @@ static __always_inline void save_fsgs(struct task_struct *task)
 {
 	savesegment(fs, task->thread.fsindex);
 	savesegment(gs, task->thread.gsindex);
-	if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
-		unsigned long flags;
-
-		/*
-		 * If FSGSBASE is enabled, we can't make any useful guesses
-		 * about the base, and user code expects us to save the current
-		 * value.  Fortunately, reading the base directly is efficient.
-		 */
-		task->thread.fsbase = rdfsbase();
-		local_irq_save(flags);
-		task->thread.gsbase = __rdgsbase_inactive();
-		local_irq_restore(flags);
-	} else {
-		save_base_legacy(task, task->thread.fsindex, FS);
-		save_base_legacy(task, task->thread.gsindex, GS);
-	}
+	save_base_legacy(task, task->thread.fsindex, FS);
+	save_base_legacy(task, task->thread.gsindex, GS);
 }
 
 #if IS_ENABLED(CONFIG_KVM)
@@ -338,22 +290,10 @@ static __always_inline void load_seg_legacy(unsigned short prev_index,
 static __always_inline void x86_fsgsbase_load(struct thread_struct *prev,
 					      struct thread_struct *next)
 {
-	if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
-		/* Update the FS and GS selectors if they could have changed. */
-		if (unlikely(prev->fsindex || next->fsindex))
-			loadseg(FS, next->fsindex);
-		if (unlikely(prev->gsindex || next->gsindex))
-			loadseg(GS, next->gsindex);
-
-		/* Update the bases. */
-		wrfsbase(next->fsbase);
-		__wrgsbase_inactive(next->gsbase);
-	} else {
-		load_seg_legacy(prev->fsindex, prev->fsbase,
-				next->fsindex, next->fsbase, FS);
-		load_seg_legacy(prev->gsindex, prev->gsbase,
-				next->gsindex, next->gsbase, GS);
-	}
+	load_seg_legacy(prev->fsindex, prev->fsbase,
+			next->fsindex, next->fsbase, FS);
+	load_seg_legacy(prev->gsindex, prev->gsbase,
+			next->gsindex, next->gsbase, GS);
 }
 
 static unsigned long x86_fsgsbase_read_task(struct task_struct *task,
@@ -399,46 +339,13 @@ static unsigned long x86_fsgsbase_read_task(struct task_struct *task,
 	return base;
 }
 
-unsigned long x86_gsbase_read_cpu_inactive(void)
-{
-	unsigned long gsbase;
-
-	if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
-		unsigned long flags;
-
-		/* Interrupts are disabled here. */
-		local_irq_save(flags);
-		gsbase = __rdgsbase_inactive();
-		local_irq_restore(flags);
-	} else {
-		rdmsrl(MSR_KERNEL_GS_BASE, gsbase);
-	}
-
-	return gsbase;
-}
-
-void x86_gsbase_write_cpu_inactive(unsigned long gsbase)
-{
-	if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
-		unsigned long flags;
-
-		/* Interrupts are disabled here. */
-		local_irq_save(flags);
-		__wrgsbase_inactive(gsbase);
-		local_irq_restore(flags);
-	} else {
-		wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
-	}
-}
-
 unsigned long x86_fsbase_read_task(struct task_struct *task)
 {
 	unsigned long fsbase;
 
 	if (task == current)
 		fsbase = x86_fsbase_read_cpu();
-	else if (static_cpu_has(X86_FEATURE_FSGSBASE) ||
-		 (task->thread.fsindex == 0))
+	else if (task->thread.fsindex == 0)
 		fsbase = task->thread.fsbase;
 	else
 		fsbase = x86_fsgsbase_read_task(task, task->thread.fsindex);
@@ -452,8 +359,7 @@ unsigned long x86_gsbase_read_task(struct task_struct *task)
 
 	if (task == current)
 		gsbase = x86_gsbase_read_cpu_inactive();
-	else if (static_cpu_has(X86_FEATURE_FSGSBASE) ||
-		 (task->thread.gsindex == 0))
+	else if (task->thread.gsindex == 0)
 		gsbase = task->thread.gsbase;
 	else
 		gsbase = x86_fsgsbase_read_task(task, task->thread.gsindex);
@@ -493,11 +399,10 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
 	p->thread.sp = (unsigned long) fork_frame;
 	p->thread.io_bitmap_ptr = NULL;
 
-	save_fsgs(me);
-	p->thread.fsindex = me->thread.fsindex;
-	p->thread.fsbase = me->thread.fsbase;
-	p->thread.gsindex = me->thread.gsindex;
-	p->thread.gsbase = me->thread.gsbase;
+	savesegment(gs, p->thread.gsindex);
+	p->thread.gsbase = p->thread.gsindex ? 0 : me->thread.gsbase;
+	savesegment(fs, p->thread.fsindex);
+	p->thread.fsbase = p->thread.fsindex ? 0 : me->thread.fsbase;
 	savesegment(es, p->thread.es);
 	savesegment(ds, p->thread.ds);
 	memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));