RE: [PATCH v2] vduse: Fix NULL pointer dereference on sysfs access

From: Yongji Xie <redacted>
Sent: Wednesday, June 8, 2022 9:26 AM

On Wed, Jun 8, 2022 at 8:52 PM Michael S. Tsirkin [off-list ref] wrote:

quoted

On Tue, Apr 26, 2022 at 10:07:38AM +0200, Greg KH wrote:

quoted

On Tue, Apr 26, 2022 at 03:36:56PM +0800, Xie Yongji wrote:

quoted

The control device has no drvdata. So we will get a NULL pointer
dereference when accessing control device's msg_timeout attribute
via sysfs:

[ 132.841881][ T3644] BUG: kernel NULL pointer dereference,
address: 00000000000000f8 [ 132.850619][ T3644] RIP:
0010:msg_timeout_show (drivers/vdpa/vdpa_user/vduse_dev.c:1271)
[ 132.869447][ T3644] dev_attr_show (drivers/base/core.c:2094) [
132.870215][ T3644] sysfs_kf_seq_show (fs/sysfs/file.c:59) [
132.871164][ T3644] ? device_remove_bin_file
(drivers/base/core.c:2088) [ 132.872082][ T3644] kernfs_seq_show
(fs/kernfs/file.c:164) [ 132.872838][ T3644] seq_read_iter
(fs/seq_file.c:230) [ 132.873578][ T3644] ? __vmalloc_area_node
(mm/vmalloc.c:3041) [ 132.874532][ T3644] kernfs_fop_read_iter
(fs/kernfs/file.c:238) [ 132.875513][ T3644] __kernel_read
(fs/read_write.c:440 (discriminator 1)) [ 132.876319][ T3644]
kernel_read (fs/read_write.c:459) [ 132.877129][ T3644]
kernel_read_file (fs/kernel_read_file.c:94) [ 132.877978][ T3644]
kernel_read_file_from_fd (include/linux/file.h:45
fs/kernel_read_file.c:186) [ 132.879019][ T3644]
__do_sys_finit_module (kernel/module.c:4207) [ 132.879930][ T3644]
__ia32_sys_finit_module (kernel/module.c:4189) [ 132.880930][
T3644] do_int80_syscall_32 (arch/x86/entry/common.c:112
arch/x86/entry/common.c:132) [ 132.881847][ T3644]
entry_INT80_compat (arch/x86/entry/entry_64_compat.S:419)

To fix it, don't create the unneeded attribute for control device
anymore.

Fixes: c8a6153b6c59 ("vduse: Introduce VDUSE - vDPA Device in
Userspace")
Reported-by: kernel test robot <redacted>
Cc: stable@vger.kernel.org
Signed-off-by: Xie Yongji <redacted>
---
 drivers/vdpa/vdpa_user/vduse_dev.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c

b/drivers/vdpa/vdpa_user/vduse_dev.c
index f85d1a08ed87..160e40d03084 100644

--- a/drivers/vdpa/vdpa_user/vduse_dev.c
+++ b/drivers/vdpa/vdpa_user/vduse_dev.c

@@ -1344,9 +1344,9 @@ static int vduse_create_dev(struct

vduse_dev_config *config,

    dev->minor = ret;
    dev->msg_timeout = VDUSE_MSG_DEFAULT_TIMEOUT;
-   dev->dev = device_create(vduse_class, NULL,
-                            MKDEV(MAJOR(vduse_major), dev->minor),
-                            dev, "%s", config->name);
+   dev->dev = device_create_with_groups(vduse_class, NULL,
+                           MKDEV(MAJOR(vduse_major), dev->minor),
+                           dev, vduse_dev_groups, "%s",
+ config->name);
    if (IS_ERR(dev->dev)) {
            ret = PTR_ERR(dev->dev);
            goto err_dev;

@@ -1595,7 +1595,6 @@ static int vduse_init(void)
            return PTR_ERR(vduse_class);

    vduse_class->devnode = vduse_devnode;
-   vduse_class->dev_groups = vduse_dev_groups;

Ok, this looks much better.

But wow, there are some problems in this code overall.  I see a
number of flat-out-wrong things in there that should have been
caught by code reviews.  Some examples:
      - empty release() callbacks.  That is a huge sign the code
        design is wrong and broken and you are just trying to make the
        driver core quiet for some reason.  The documentation in the
        kernel explains why this is not ok.
      - __module_get(THIS_MODULE);  That's racy, buggy, and doesn't do
        what you think it does.  Please never ever ever do that.  It
        too is a sign of a broken design.
      - no Documentation/ABI/ entries for the sysfs files here.  I
        think it's burried in some other documentation file but that's
        not the correct place for it and if you run scripts/get_abi.pl
        with the code loaded it will rightly complain about this.

Do you want to address these, or do you want patches for them?

thanks,

greg k-h

So, any patches?

For empty release() callbacks, I think Parav is working on it based on the
discussion [1]. I can help test and send the patch if Parav wants.

As there are no more comments, I will send the patch for vduse this week based on [1] which was previously in email form.

For Documentation, I have sent a patch [2].

[1] https://www.spinics.net/lists/linux-virtualization/msg56518.html
[2]
https://lore.kernel.org/all/CACGkMEuJeU6c1z8+_FqGtovbF+Sq8w_eQUcG8
SHm_GXV5q7yNA@mail.gmail.com/

Thanks,
Yongji

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