x86/fpu: Reinitialize FPU registers if restoring FPU state fails

Userspace can change the FPU state of a task using the ptrace() or
rt_sigreturn() system calls.  Because reserved bits in the FPU state can
cause the XRSTOR instruction to fail, the kernel has to carefully
validate that no reserved bits or other invalid values are being set.

Unfortunately, there have been bugs in this validation code.  For
example, we were not checking that the 'xcomp_bv' field in the
xstate_header was 0.  As-is, such bugs are exploitable to read the FPU
registers of other processes on the system.  To do so, an attacker can
create a task, assign to it an invalid FPU state, then spin in a loop
and monitor the values of the FPU registers.  Because the task's FPU
registers are not being restored, sometimes the FPU registers will have
the values from another process.

This is likely to continue to be a problem in the future because the
validation done by the CPU instructions like XRSTOR is not immediately
visible to kernel developers.  Nor will invalid FPU states ever be
encountered during ordinary use --- they will only be seen during
fuzzing or exploits.  There can even be reserved bits outside the
xstate_header which are easy to forget about.  For example, the MXCSR
register contains reserved bits, which were not validated by the
KVM_SET_XSAVE ioctl until commit a575813bfe4b ("KVM: x86: Fix load
damaged SSEx MXCSR register").

Therefore, mitigate this class of vulnerability by restoring the FPU
registers from init_fpstate if restoring from the task's state fails.

We actually used to do this, but it was (perhaps unwisely) removed by
commit 9ccc27a5d297 ("x86/fpu: Remove error return values from
copy_kernel_to_*regs() functions").  This new patch is also a bit
different.  First, it only clears the registers, not also the bad
in-memory state; this is simpler and makes it easier to make the
mitigation cover all callers of __copy_kernel_to_fpregs().  Second, it
does the register clearing in an exception handler so that no extra
instructions are added to context switches.  In fact, we *remove*
instructions, since previously we were always zeroing the register
containing 'err' even if CONFIG_X86_DEBUG_FPU was disabled.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Kevin Hao <haokexin@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Halcrow <mhalcrow@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <wanpeng.li@hotmail.com>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: kernel-hardening@lists.openwall.com
Link: http://lkml.kernel.org/r/20170922174156.16780-4-ebiggers3@gmail.com
Link: http://lkml.kernel.org/r/20170923130016.21448-27-mingo@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 24 insertions, 0 deletions

diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index c076f710de4c..c3521e2be396 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -2,6 +2,7 @@
 #include <linux/uaccess.h>
 #include <linux/sched/debug.h>
 
+#include <asm/fpu/internal.h>
 #include <asm/traps.h>
 #include <asm/kdebug.h>
 
@@ -78,6 +79,29 @@ bool ex_handler_refcount(const struct exception_table_entry *fixup,
 }
 EXPORT_SYMBOL_GPL(ex_handler_refcount);
 
+/*
+ * Handler for when we fail to restore a task's FPU state.  We should never get
+ * here because the FPU state of a task using the FPU (task->thread.fpu.state)
+ * should always be valid.  However, past bugs have allowed userspace to set
+ * reserved bits in the XSAVE area using PTRACE_SETREGSET or sys_rt_sigreturn().
+ * These caused XRSTOR to fail when switching to the task, leaking the FPU
+ * registers of the task previously executing on the CPU.  Mitigate this class
+ * of vulnerability by restoring from the initial state (essentially, zeroing
+ * out all the FPU registers) if we can't restore from the task's FPU state.
+ */
+bool ex_handler_fprestore(const struct exception_table_entry *fixup,
+			  struct pt_regs *regs, int trapnr)
+{
+	regs->ip = ex_fixup_addr(fixup);
+
+	WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing FPU registers.",
+		  (void *)instruction_pointer(regs));
+
+	__copy_kernel_to_fpregs(&init_fpstate, -1);
+	return true;
+}
+EXPORT_SYMBOL_GPL(ex_handler_fprestore);
+
 bool ex_handler_ext(const struct exception_table_entry *fixup,
 		   struct pt_regs *regs, int trapnr)
 {