[PATCH] msr: Fix NULL pointer deref due to msr_open on nonexistent CPUs

[PATCH] msr: Fix NULL pointer deref due to msr_open on nonexistent CPUs

[Darrick J. Wong]

msr_open tests for someone trying to open a device for a nonexistent CPU.
However, the function always returns 0, not ret like it should, hence userspace
can BUG the kernel trivially.  This bug was introduced by the cdev lock_kernel
pushdown patch last May.

The BUG can be reproduced with these commands:

# mknod fubar c 202 8 <-- pick a number less than NR_CPUS that is not
                          the number of an online CPU
# cat fubar

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
---

 arch/x86/kernel/msr.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/msr.c b/arch/x86/kernel/msr.c
index 9fd8095..e439380 100644
--- a/arch/x86/kernel/msr.c
+++ b/arch/x86/kernel/msr.c
@@ -131,7 +131,7 @@ static int msr_open(struct inode *inode, struct file *file)
 		ret = -EIO;	/* MSR not supported */
 out:
 	unlock_kernel();
-	return 0;
+	return ret;
 }
 
 /*

Re: [PATCH] msr: Fix NULL pointer deref due to msr_open on nonexistent CPUs

[Ingo Molnar]

* Darrick J. Wong <djwong@us.ibm.com> wrote:

> msr_open tests for someone trying to open a device for a nonexistent CPU.
> However, the function always returns 0, not ret like it should, hence userspace
> can BUG the kernel trivially.  This bug was introduced by the cdev lock_kernel
> pushdown patch last May.
> 
> The BUG can be reproduced with these commands:
> 
> # mknod fubar c 202 8 <-- pick a number less than NR_CPUS that is not
>                           the number of an online CPU
> # cat fubar

applied to tip/x86/urgent - thanks Darrick.

	Ingo

Re: [PATCH] msr: Fix NULL pointer deref due to msr_open on nonexistent CPUs

[Jonathan Corbet]

On Thu, 14 Aug 2008 15:43:33 -0700
"Darrick J. Wong" <djwong@us.ibm.com> wrote:

> msr_open tests for someone trying to open a device for a nonexistent
> CPU. However, the function always returns 0, not ret like it should,
> hence userspace can BUG the kernel trivially.  This bug was
> introduced by the cdev lock_kernel pushdown patch last May.

> -	return 0;
> +	return ret;

Oops, that was pretty silly.  Apologies for the screwup.

jon