Merge tag 'x86-urgent-2022-05-08' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fix from Thomas Gleixner:
 "A fix and an email address update:

   - Prevent FPU state corruption.

     The condition in irq_fpu_usable() grants FPU usage when the FPU is
     not used in the kernel. That's just wrong as it does not take the
     fpregs_lock()'ed regions into account. If FPU usage happens within
     such a region from interrupt context, then the FPU state gets
     corrupted.

     That's a long standing bug, which got unearthed by the recent
     changes to the random code.

   - Josh wants to use his kernel.org email address"

* tag 'x86-urgent-2022-05-08' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/fpu: Prevent FPU state corruption
  MAINTAINERS: Update Josh Poimboeuf's email address

2 files changed, 31 insertions, 46 deletions

diff --git a/MAINTAINERS b/MAINTAINERS
index 97fb18c7c26c..e8c52d0192a6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -7499,7 +7499,7 @@ F:	Documentation/hwmon/f71805f.rst
 F:	drivers/hwmon/f71805f.c
 
 FADDR2LINE
-M:	Josh Poimboeuf <jpoimboe@redhat.com>
+M:	Josh Poimboeuf <jpoimboe@kernel.org>
 S:	Maintained
 F:	scripts/faddr2line
 
@@ -11348,7 +11348,7 @@ F:	drivers/mmc/host/litex_mmc.c
 N:	litex
 
 LIVE PATCHING
-M:	Josh Poimboeuf <jpoimboe@redhat.com>
+M:	Josh Poimboeuf <jpoimboe@kernel.org>
 M:	Jiri Kosina <jikos@kernel.org>
 M:	Miroslav Benes <mbenes@suse.cz>
 M:	Petr Mladek <pmladek@suse.com>
@@ -14224,7 +14224,7 @@ F:	lib/objagg.c
 F:	lib/test_objagg.c
 
 OBJTOOL
-M:	Josh Poimboeuf <jpoimboe@redhat.com>
+M:	Josh Poimboeuf <jpoimboe@kernel.org>
 M:	Peter Zijlstra <peterz@infradead.org>
 S:	Supported
 F:	tools/objtool/
@@ -18792,7 +18792,7 @@ F:	include/dt-bindings/reset/starfive-jh7100.h
 
 STATIC BRANCH/CALL
 M:	Peter Zijlstra <peterz@infradead.org>
-M:	Josh Poimboeuf <jpoimboe@redhat.com>
+M:	Josh Poimboeuf <jpoimboe@kernel.org>
 M:	Jason Baron <jbaron@akamai.com>
 R:	Steven Rostedt <rostedt@goodmis.org>
 R:	Ard Biesheuvel <ardb@kernel.org>
@@ -21444,7 +21444,7 @@ F:	arch/x86/kernel/apic/x2apic_uv_x.c
 F:	arch/x86/platform/uv/
 
 X86 STACK UNWINDING
-M:	Josh Poimboeuf <jpoimboe@redhat.com>
+M:	Josh Poimboeuf <jpoimboe@kernel.org>
 M:	Peter Zijlstra <peterz@infradead.org>
 S:	Supported
 F:	arch/x86/include/asm/unwind*.h
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index c049561f373a..e28ab0ecc537 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -41,17 +41,7 @@ struct fpu_state_config fpu_user_cfg __ro_after_init;
  */
 struct fpstate init_fpstate __ro_after_init;
 
-/*
- * Track whether the kernel is using the FPU state
- * currently.
- *
- * This flag is used:
- *
- *   - by IRQ context code to potentially use the FPU
- *     if it's unused.
- *
- *   - to debug kernel_fpu_begin()/end() correctness
- */
+/* Track in-kernel FPU usage */
 static DEFINE_PER_CPU(bool, in_kernel_fpu);
 
 /*
@@ -59,42 +49,37 @@ static DEFINE_PER_CPU(bool, in_kernel_fpu);
  */
 DEFINE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);
 
-static bool kernel_fpu_disabled(void)
-{
-	return this_cpu_read(in_kernel_fpu);
-}
-
-static bool interrupted_kernel_fpu_idle(void)
-{
-	return !kernel_fpu_disabled();
-}
-
-/*
- * Were we in user mode (or vm86 mode) when we were
- * interrupted?
- *
- * Doing kernel_fpu_begin/end() is ok if we are running
- * in an interrupt context from user mode - we'll just
- * save the FPU state as required.
- */
-static bool interrupted_user_mode(void)
-{
-	struct pt_regs *regs = get_irq_regs();
-	return regs && user_mode(regs);
-}
-
 /*
  * Can we use the FPU in kernel mode with the
  * whole "kernel_fpu_begin/end()" sequence?
- *
- * It's always ok in process context (ie "not interrupt")
- * but it is sometimes ok even from an irq.
  */
 bool irq_fpu_usable(void)
 {
-	return !in_interrupt() ||
-		interrupted_user_mode() ||
-		interrupted_kernel_fpu_idle();
+	if (WARN_ON_ONCE(in_nmi()))
+		return false;
+
+	/* In kernel FPU usage already active? */
+	if (this_cpu_read(in_kernel_fpu))
+		return false;
+
+	/*
+	 * When not in NMI or hard interrupt context, FPU can be used in:
+	 *
+	 * - Task context except from within fpregs_lock()'ed critical
+	 *   regions.
+	 *
+	 * - Soft interrupt processing context which cannot happen
+	 *   while in a fpregs_lock()'ed critical region.
+	 */
+	if (!in_hardirq())
+		return true;
+
+	/*
+	 * In hard interrupt context it's safe when soft interrupts
+	 * are enabled, which means the interrupt did not hit in
+	 * a fpregs_lock()'ed critical region.
+	 */
+	return !softirq_count();
 }
 EXPORT_SYMBOL(irq_fpu_usable);