[tip:x86/urgent] x86/fpu: Fix 32-bit signal frame handling

From: tip-bot for Dave Hansen
Date: Thu Nov 12 2015 - 08:29:21 EST

Commit-ID: ab6b52947545a5355154f64f449f97af9d05845f
Gitweb: http://git.kernel.org/tip/ab6b52947545a5355154f64f449f97af9d05845f
Author: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
AuthorDate: Tue, 10 Nov 2015 16:23:54 -0800
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 12 Nov 2015 09:23:45 +0100

x86/fpu: Fix 32-bit signal frame handling

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


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"

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.


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.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: dave@xxxxxxxx
Cc: fenghua.yu@xxxxxxxxx
Cc: yu-cheng.yu@xxxxxxxxx
Link: http://lkml.kernel.org/r/20151111002354.A0799571@xxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
arch/x86/kernel/fpu/signal.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index ef29b74..31c6a60 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -385,20 +385,19 @@ fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
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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