linux-kernel - Re: BUG: unable to handle kernel NULL pointer dereference (drm

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Message-ID: <87y75kaeg7.fsf@saeurebad.de>
Date:	Thu, 05 Jun 2008 02:54:48 +0200
From:	Johannes Weiner <hannes@...urebad.de>
To:	Sitsofe Wheeler <sitsofe@...oo.com>
CC:	David Airlie <airlied@...hat.com>
Subject: Re: BUG: unable to handle kernel NULL pointer dereference (drm_getunique)

Hi,

Sitsofe Wheeler <sitsofe@...oo.com> writes:

> While flipping back and forth between the vts/Xorg and doing iperf test
> over the wifi connection the following error appeared in next-20080604.
>
> [ 4305.767435] BUG: unable to handle kernel NULL pointer dereference at 00000000
> [ 4305.767452] IP: [<c02511d4>] drm_getunique+0xc/0x30
> [ 4305.767466] *pde = 00000000
> [ 4305.767474] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
> [ 4305.767483] last sysfs file: /sys/devices/system/cpu/cpu1/cache/index2/shared_cpu_map
> [ 4305.767492] Modules linked in:
> [ 4305.767498]
> [ 4305.767503] Pid: 8373, comm: unix2_chkpwd Tainted: G        W (2.6.26-rc4-next-20080604skw #177)
> [ 4305.767513] EIP: 0060:[<c02511d4>] EFLAGS: 00210246 CPU: 0
> [ 4305.767521] EIP is at drm_getunique+0xc/0x30
> [ 4305.767527] EAX: f7d8ea70 EBX: 00000000 ECX: 00000028 EDX: 00000000
> [ 4305.767535] ESI: f7d8ea70 EDI: 00005401 EBP: ed383f34 ESP: ed383f2c
> [ 4305.767543]  DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
> [ 4305.767550] Process unix2_chkpwd (pid: 8373, ti=ed382000 task=f6efdee0 task.ti=ed382000)
> [ 4305.767558] Stack: 00000000 c04940a4 ed383f58 c0250637 f1e03f50 f7d8ea70 f7d8eaf0 c02511c8
> [ 4305.767574]        c04944c4 d603b200 bfda0500 ed383f74 c017ebba bfda0500 00005401 d603b200
> [ 4305.767589]        d603b200 bfda0500 ed383f98 c017ee0a 00000001 ffffffea f1c0c100 ed383fb0
> [ 4305.767605] Call Trace:
> [ 4305.767611]  [<c0250637>] ? drm_ioctl+0x1b0/0x225
> [ 4305.767622]  [<c02511c8>] ? drm_getunique+0x0/0x30
> [ 4305.767632]  [<c017ebba>] ? vfs_ioctl+0x4e/0x67
> [ 4305.767643]  [<c017ee0a>] ? do_vfs_ioctl+0x237/0x245
> [ 4305.767652]  [<c017ee44>] ? sys_ioctl+0x2c/0x48
> [ 4305.767661]  [<c0102c35>] ? sysenter_past_esp+0x6a/0xa5

Hm, in drm_getunique, dev is not NULL as it was already dereferenced in
drm_ioctl.  file_priv is not used at all.  So only data is left.

data is kdata in drm_ioctl and only NULL if the ioctl request is neither
input nor output.

I have not found a check on cmd in the callpath that would filter out
malformed requests.  So the user would be allowed to pass in a valid
request number with wrong flags here, correct?

	Hannes

---
drm: check ioctl direction before dispatching

The userland might specify a valid ioctl request number with a wrong
direction.  Precheck the direction of the dispatcher function beforehand
because we prepare arguments based on this parameter and the dispatched
function relies on them being valid.

Signed-off-by: Johannes Weiner <hannes@...urebad.de>
CC: David Airlie <airlied@...hat.com>
---

Compile-time tested only!

diff --git a/drivers/char/drm/drm_drv.c b/drivers/char/drm/drm_drv.c
index fc54140..1a27d04 100644
--- a/drivers/char/drm/drm_drv.c
+++ b/drivers/char/drm/drm_drv.c
@@ -475,6 +475,10 @@ int drm_ioctl(struct inode *inode, struct file *filp,
 	else
 		goto err_i1;
 
+	/* Make sure the direction is correct */
+	if (_IOC_DIR(ioctl->cmd) != _IOC_DIR(cmd))
+		goto err_i1;
+
 	func = ioctl->func;
 	/* is there a local override? */
 	if ((nr == DRM_IOCTL_NR(DRM_IOCTL_DMA)) && dev->driver->dma_ioctl)
--
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