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[PATCH 4.4 12/16] arm64: fpsimd: Prevent registers leaking across exec

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4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dave Martin <Dave.Martin@arm.com>

commit 096622104e14d8a1db4860bd557717067a0515d2 upstream.

There are some tricky dependencies between the different stages of
flushing the FPSIMD register state during exec, and these can race
with context switch in ways that can cause the old task's regs to
leak across.  In particular, a context switch during the memset() can
cause some of the task's old FPSIMD registers to reappear.

Disabling preemption for this small window would be no big deal for
performance: preemption is already disabled for similar scenarios
like updating the FPSIMD registers in sigreturn.

So, instead of rearranging things in ways that might swap existing
subtle bugs for new ones, this patch just disables preemption
around the FPSIMD state flushing so that races of this type can't
occur here.  This brings fpsimd_flush_thread() into line with other
code paths.

Fixes: 674c242c9323 ("arm64: flush FP/SIMD state correctly after execve()")
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

For stable only.

3.17.x-4.0.x don't appear active, and this patch isn't sufficient to fix
them (they would need 674c242c9323 also).

 arch/arm64/kernel/fpsimd.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -157,9 +157,11 @@ void fpsimd_thread_switch(struct task_st
 
 void fpsimd_flush_thread(void)
 {
+	preempt_disable();
 	memset(&current->thread.fpsimd_state, 0, sizeof(struct fpsimd_state));
 	fpsimd_flush_task_state(current);
 	set_thread_flag(TIF_FOREIGN_FPSTATE);
+	preempt_enable();
 }
 
 /*

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[PATCH 4.4 00/16] 4.4.86-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 4.4 14/16] scsi: sg: reset res_in_use after unlinking reserved array Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 4.4 06/16] gcov: support GCC 7.1 Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 4.4 08/16] p54: memset(0) whole array Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 4.4 03/16] btrfs: remove duplicate const specifier Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 4.4 05/16] gcov: add support for gcc version >= 6 Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 4.4 13/16] scsi: sg: protect accesses to reserved page array Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 4.4 16/16] f2fs: do more integrity verification for superblock Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
    Re: [PATCH 4.4 16/16] f2fs: do more integrity verification for  superblock Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:40 +0200
  [PATCH 4.4 02/16] ALSA: au88x0: Fix zero clear of stream->resources Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  [PATCH 4.4 12/16] arm64: fpsimd: Prevent registers leaking across exec Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  [PATCH 4.4 10/16] arm64: mm: abort uaccess retries upon fatal signal Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  Re: [PATCH 4.4 00/16] 4.4.86-stable review Shuah Khan <shuahkh@osg.samsung.com> - 2017-08-31 21:10 +0200
  Re: [PATCH 4.4 00/16] 4.4.86-stable review Guenter Roeck <linux@roeck-us.net> - 2017-09-01 04:40 +0200

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