[tip: x86/urgent] x86/fpu: Restore the masking out of reserved MXCSR bits

["tip-bot2 for Borislav Petkov" <tip-bot2@linutronix.de>]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     908d969f88bfcf6d8538a0159e502567c7678775
Gitweb:        https://git.kernel.org/tip/908d969f88bfcf6d8538a0159e502567c7678775
Author:        Borislav Petkov <bp@suse.de>
AuthorDate:    Wed, 06 Oct 2021 18:33:52 +02:00
Committer:     Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 06 Oct 2021 18:46:06 +02:00

x86/fpu: Restore the masking out of reserved MXCSR bits

Ser Olmy reported a boot failure:

  init[1] bad frame in sigreturn frame:(ptrval) ip:b7c9fbe6 sp:bf933310 orax:ffffffff \
	  in libc-2.33.so[b7bed000+156000]
  Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
  CPU: 0 PID: 1 Comm: init Tainted: G        W         5.14.9 #1
  Hardware name: Hewlett-Packard HP PC/HP Board, BIOS  JD.00.06 12/06/2001
  Call Trace:
   dump_stack_lvl
   dump_stack
   panic
   do_exit.cold
   do_group_exit
   get_signal
   arch_do_signal_or_restart
   ? force_sig_info_to_task
   ? force_sig
   exit_to_user_mode_prepare
   syscall_exit_to_user_mode
   do_int80_syscall_32
   entry_INT80_32

on an old 32-bit Intel CPU:

  vendor_id       : GenuineIntel
  cpu family      : 6
  model           : 6
  model name      : Celeron (Mendocino)
  stepping        : 5
  microcode       : 0x3

Ser bisected the problem to the commit in Fixes.

tglx suggested reverting the rejection of invalid MXCSR values which
this commit introduced and replacing it with what the old code did -
simply masking them out to zero.

Further debugging confirmed his suggestion:

  fpu->state.fxsave.mxcsr: 0xb7be13b4, mxcsr_feature_mask: 0xffbf
  WARNING: CPU: 0 PID: 1 at arch/x86/kernel/fpu/signal.c:384 __fpu_restore_sig+0x51f/0x540

so restore the original behavior.

Fixes: 6f9866a166cd ("x86/fpu/signal: Let xrstor handle the features to init")
Reported-by: Ser Olmy <ser.olmy@protonmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Ser Olmy <ser.olmy@protonmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/YVtA67jImg3KlBTw@zn.tnic
---
 arch/x86/kernel/fpu/signal.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 445c57c..684be34 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -379,9 +379,8 @@ static int __fpu_restore_sig(void __user *buf, void __user *buf_fx,
 				     sizeof(fpu->state.fxsave)))
 			return -EFAULT;
 
-		/* Reject invalid MXCSR values. */
-		if (fpu->state.fxsave.mxcsr & ~mxcsr_feature_mask)
-			return -EINVAL;
+		/* Mask out reserved MXCSR bits. */
+		fpu->state.fxsave.mxcsr &= mxcsr_feature_mask;
 
 		/* Enforce XFEATURE_MASK_FPSSE when XSAVE is enabled */
 		if (use_xsave())