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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@suse.de>
AuthorDate: Thu, 16 Apr 2015 20:41:37 +0200
Committer: Ingo Molnar <mingo@kernel.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@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>
---
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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