[PATCH] x86, fpu: fix 32-bit signal frame handling

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

(This should have gone to LKML originally. Sorry for the extra
 noise, folks on the cc.)

Background:

Signal frames on x86 have two formats:
1. For 32-bit executables (whether on a real 32-bit kernel or
   under ia32 emulation on a 64-bit kernel) we have a
  'fpregset_t' that includes the "FSAVE" registers.
2. For 64-bit executables (on 64-bit kernels obviously), the
   'fpregset_t' is smaller and does not contain the "FSAVE"
   state.

When creating the signal frame, we have to be aware of whether
we are running a 32 or 64-bit executable so we create the correct
format signal frame.

Problem:

save_xstate_epilog() uses 'fx_sw_reserved_ia32' whenever it is
called for a 32-bit executable.  This is for real 32-bit and
ia32 emulation.

But, fpu__init_prepare_fx_sw_frame() only initializes
'fx_sw_reserved_ia32' when emulation is enabled, *NOT* for real
32-bit kernels.

This leads to really wierd situations where 32-bit programs
lose their extended state when returning from a signal handler.
The kernel copies the uninitialized (zero) 'fx_sw_reserved_ia32'
out to userspace in save_xstate_epilog().  But when returning
from the signal, the kernel errors out in check_for_xstate()
when it does not see FP_XSTATE_MAGIC1 present (because it was
zeroed).  This leads to the FPU/XSAVE state being initialized.

For MPX, this leads to the most permissive state and means we
silently lose bounds violations.  I think this would also mean
that we could lose *ANY* FPU/SSE/AVX state.  I'm not sure why
no one has spotted this bug.

I believe this was broken by:

	72a671ced: x86, fpu: Unify signal handling code paths for x86 and x86_64 kernels

way back in 2012.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: luto@xxxxxxxxxx
Cc: fenghua.yu@xxxxxxxxx
Cc: yu-cheng.yu@xxxxxxxxx
---

 b/arch/x86/kernel/fpu/signal.c |   11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff -puN arch/x86/kernel/fpu/signal.c~only-fix-signal-bug arch/x86/kernel/fpu/signal.c
--- a/arch/x86/kernel/fpu/signal.c~only-fix-signal-bug	2015-11-10 14:30:19.910135621 -0800
+++ b/arch/x86/kernel/fpu/signal.c	2015-11-10 14:30:19.913135757 -0800
@@ -385,20 +385,19 @@ fpu__alloc_mathframe(unsigned long sp, i
  */
 void fpu__init_prepare_fx_sw_frame(void)
 {
-	int fsave_header_size = sizeof(struct fregs_state);
 	int size = xstate_size + FP_XSTATE_MAGIC2_SIZE;
 
-	if (config_enabled(CONFIG_X86_32))
-		size += fsave_header_size;
-
 	fx_sw_reserved.magic1 = FP_XSTATE_MAGIC1;
 	fx_sw_reserved.extended_size = size;
 	fx_sw_reserved.xfeatures = xfeatures_mask;
 	fx_sw_reserved.xstate_size = xstate_size;
 
-	if (config_enabled(CONFIG_IA32_EMULATION)) {
+	if (config_enabled(CONFIG_IA32_EMULATION) ||
+	    config_enabled(CONFIG_X86_32)) {
+		int fsave_header_size = sizeof(struct fregs_state);
+
 		fx_sw_reserved_ia32 = fx_sw_reserved;
-		fx_sw_reserved_ia32.extended_size += fsave_header_size;
+		fx_sw_reserved_ia32.extended_size = size + fsave_header_size;
 	}
 }
 
_
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