linux-kernel - [tip:x86/urgent] x86/fpu: Load xsave pointer *after* initialization

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Message-ID: <tip-18ecb3bfa5a9f6fffbb3eeb4369f0b9463438ec0@git.kernel.org>
Date:	Sat, 18 Apr 2015 03:14:44 -0700
From:	tip-bot for Borislav Petkov <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	bp@...e.de, torvalds@...ux-foundation.org, mingo@...nel.org,
	riel@...hat.com, linux-kernel@...r.kernel.org, oleg@...hat.com,
	tglx@...utronix.de, luto@...capital.net, taviso@...gle.com,
	hpa@...or.com
Subject: [tip:x86/urgent] x86/fpu:
  Load xsave pointer *after* initialization

Commit-ID:  18ecb3bfa5a9f6fffbb3eeb4369f0b9463438ec0
Gitweb:     http://git.kernel.org/tip/18ecb3bfa5a9f6fffbb3eeb4369f0b9463438ec0
Author:     Borislav Petkov <bp@...e.de>
AuthorDate: Thu, 16 Apr 2015 20:41:37 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 17 Apr 2015 10:15:47 +0200

x86/fpu: Load xsave pointer *after* initialization

So I was playing with gdb today and did this simple thing:

	gdb /bin/ls

	...

	(gdb) run

Box exploded with this splat:

	BUG: unable to handle kernel NULL pointer dereference at 00000000000001d0
	IP: [<ffffffff8100fe5a>] xstateregs_get+0x7a/0x120
	[...]

	Call Trace:
	 ptrace_regset
	 ptrace_request
	 ? wait_task_inactive
	 ? preempt_count_sub
	 arch_ptrace
	 ? ptrace_get_task_struct
	 SyS_ptrace
	 system_call_fastpath

... because we do cache &target->thread.fpu.state->xsave into the
local variable xsave but that pointer is NULL at that time and
it gets initialized later, in init_fpu(), see:

	e7f180dcd8ab ("x86/fpu: Change xstateregs_get()/set() to use ->xsave.i387 rather than ->fxsave")

The fix is simple: load xsave *after* init_fpu() has run.

Also do the same in xstateregs_set(), as suggested by Oleg Nesterov.

Signed-off-by: Borislav Petkov <bp@...e.de>
Acked-by: Oleg Nesterov <oleg@...hat.com>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: Tavis Ormandy <taviso@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/1429209697-5902-1-git-send-email-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/i387.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index 367f39d..00918327 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -341,7 +341,7 @@ int xstateregs_get(struct task_struct *target, const struct user_regset *regset,
 		unsigned int pos, unsigned int count,
 		void *kbuf, void __user *ubuf)
 {
-	struct xsave_struct *xsave = &target->thread.fpu.state->xsave;
+	struct xsave_struct *xsave;
 	int ret;
 
 	if (!cpu_has_xsave)
@@ -351,6 +351,8 @@ int xstateregs_get(struct task_struct *target, const struct user_regset *regset,
 	if (ret)
 		return ret;
 
+	xsave = &target->thread.fpu.state->xsave;
+
 	/*
 	 * Copy the 48bytes defined by the software first into the xstate
 	 * memory layout in the thread struct, so that we can copy the entire
@@ -369,7 +371,7 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
 		  unsigned int pos, unsigned int count,
 		  const void *kbuf, const void __user *ubuf)
 {
-	struct xsave_struct *xsave = &target->thread.fpu.state->xsave;
+	struct xsave_struct *xsave;
 	int ret;
 
 	if (!cpu_has_xsave)
@@ -379,6 +381,8 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
 	if (ret)
 		return ret;
 
+	xsave = &target->thread.fpu.state->xsave;
+
 	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, xsave, 0, -1);
 	/*
 	 * mxcsr reserved bits must be masked to zero for security reasons.
--
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