[PATCH 5.14 147/151] x86/fpu: Restore the masking out of reserved MXCSR bits

Greg Kroah-Hartman
2021-10-11T14:13:55Z

From: Borislav Petkov <bp@suse.de>

commit d298b03506d3e161f7492c440babb0bfae35e650 upstream.

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 only for 32-bit kernels where you have
ancient machines with buggy hardware. For 32-bit programs on 64-bit
kernels, user space which supplies wrong MXCSR values is considered
malicious so fail the sigframe restoration there.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kernel/fpu/signal.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -379,9 +379,14 @@ static int __fpu_restore_sig(void __user
sizeof(fpu->state.fxsave)))
return -EFAULT;

- /* Reject invalid MXCSR values. */
- if (fpu->state.fxsave.mxcsr & ~mxcsr_feature_mask)
- return -EINVAL;
+ if (IS_ENABLED(CONFIG_X86_64)) {
+ /* Reject invalid MXCSR values. */
+ if (fpu->state.fxsave.mxcsr & ~mxcsr_feature_mask)
+ return -EINVAL;
+ } else {
+ /* Mask invalid bits out for historical reasons (broken hardware). */
+ fpu->state.fxsave.mxcsr &= ~mxcsr_feature_mask;
+ }

/* Enforce XFEATURE_MASK_FPSSE when XSAVE is enabled */
if (use_xsave())