Commit 18ecb3bf authored Apr 16, 2015 by Borislav Petkov Committed by Ingo Molnar Apr 17, 2015

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:

	e7f180dc ("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@suse.de>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Tavis Ormandy <taviso@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1429209697-5902-1-git-send-email-bp@alien8.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent fd0f86b6

arch/x86/kernel/i387.c

+6 −2

Original line number	Diff line number	Diff line
		@@ -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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