Patch "x86/fpu: Fix 32-bit signal frame handling" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

x86/fpu: Fix 32-bit signal frame handling

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     x86-fpu-fix-32-bit-signal-frame-handling.patch

and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Dave Hansen <[email protected]>
Date: Tue, 10 Nov 2015 16:23:54 -0800
Subject: x86/fpu: Fix 32-bit signal frame handling

From: Dave Hansen <[email protected]>

commit ab6b52947545a5355154f64f449f97af9d05845f upstream.

(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 32-bit 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:

	72a671ced66d ("x86, fpu: Unify signal handling code paths for x86 and x86_64 kernels")

way back in 2012.

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

@@ -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;
 	}
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/x86-fpu-fix-get_xsave_addr-behavior-under-virtualization.patch queue-4.2/x86-mpx-do-proper-get_user-when-running-32-bit-binaries-on-64-bit-kernels.patch queue-4.2/x86-mpx-fix-32-bit-address-space-calculation.patch queue-4.2/x86-fpu-fix-32-bit-signal-frame-handling.patch