[PATCH] PCI/proc: check __get_user() return value in proc_bus_pci_write()

[PATCH] PCI/proc: check __get_user() return value in proc_bus_pci_write()

[Deepanshu Kartikey]

proc_bus_pci_write() does not check the return value of __get_user().
On a faulting user pointer the extable fixup zeros the destination,
and the function writes those zeros to PCI configuration space.

syzbot exploits this by writev()-ing a NULL iov_base to
/proc/bus/pci/00/03.0 (the virtio-blk controller in the syzkaller VM):
zero is written to the Command register, clearing Bus Master Enable,
and the disk stops responding. In-flight journal writes never complete
and jbd2 hangs in wait_on_buffer() indefinitely.

Check __get_user() and return -EFAULT on failure.

Reported-by: syzbot+c7604c9fdd7580cca4e0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c7604c9fdd7580cca4e0
Tested-by: syzbot+c7604c9fdd7580cca4e0@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 drivers/pci/proc.c | 33 +++++++++++++++++++++++++--------
 1 file changed, 25 insertions(+), 8 deletions(-)

diff --git a/drivers/pci/proc.c b/drivers/pci/proc.c
index ce36e35681e8..54052157c276 100644
--- a/drivers/pci/proc.c
+++ b/drivers/pci/proc.c
@@ -136,7 +136,10 @@ static ssize_t proc_bus_pci_write(struct file *file, const char __user *buf,
 
 	if ((pos & 1) && cnt) {
 		unsigned char val;
-		__get_user(val, buf);
+		if (__get_user(val, buf)) {
+			ret = -EFAULT;
+			goto out;
+		}
 		pci_user_write_config_byte(dev, pos, val);
 		buf++;
 		pos++;
@@ -145,7 +148,10 @@ static ssize_t proc_bus_pci_write(struct file *file, const char __user *buf,
 
 	if ((pos & 3) && cnt > 2) {
 		__le16 val;
-		__get_user(val, (__le16 __user *) buf);
+		if (__get_user(val, (__le16 __user *) buf)) {
+			ret = -EFAULT;
+			goto out;
+		}
 		pci_user_write_config_word(dev, pos, le16_to_cpu(val));
 		buf += 2;
 		pos += 2;
@@ -154,7 +160,10 @@ static ssize_t proc_bus_pci_write(struct file *file, const char __user *buf,
 
 	while (cnt >= 4) {
 		__le32 val;
-		__get_user(val, (__le32 __user *) buf);
+		if (__get_user(val, (__le32 __user *) buf)) {
+			ret = -EFAULT;
+			goto out;
+		}
 		pci_user_write_config_dword(dev, pos, le32_to_cpu(val));
 		buf += 4;
 		pos += 4;
@@ -163,7 +172,10 @@ static ssize_t proc_bus_pci_write(struct file *file, const char __user *buf,
 
 	if (cnt >= 2) {
 		__le16 val;
-		__get_user(val, (__le16 __user *) buf);
+		if (__get_user(val, (__le16 __user *) buf)) {
+			ret = -EFAULT;
+			goto out;
+		}
 		pci_user_write_config_word(dev, pos, le16_to_cpu(val));
 		buf += 2;
 		pos += 2;
@@ -172,16 +184,21 @@ static ssize_t proc_bus_pci_write(struct file *file, const char __user *buf,
 
 	if (cnt) {
 		unsigned char val;
-		__get_user(val, buf);
+		if (__get_user(val, buf)) {
+			ret = -EFAULT;
+			goto out;
+		}
 		pci_user_write_config_byte(dev, pos, val);
 		pos++;
 	}
 
+	ret = nbytes;
+out:
 	pci_config_pm_runtime_put(dev);
-
 	*ppos = pos;
-	i_size_write(ino, dev->cfg_size);
-	return nbytes;
+	if (ret > 0)
+		i_size_write(ino, dev->cfg_size);
+	return ret;
 }
 
 #ifdef HAVE_PCI_MMAP
-- 
2.43.0

Re: [PATCH] PCI/proc: check __get_user() return value in proc_bus_pci_write()

[Krzysztof Wilczyński]

Hello,

> Check __get_user() and return -EFAULT on failure.
[...]
> @@ -136,7 +136,10 @@ static ssize_t proc_bus_pci_write(struct file *file, const char __user *buf,
>  
>  	if ((pos & 1) && cnt) {
>  		unsigned char val;
> -		__get_user(val, buf);
> +		if (__get_user(val, buf)) {
> +			ret = -EFAULT;
> +			goto out;
> +		}

We could move to get_user() here.  This would allow you to drop
the access_ok(), too, as get_user() would return -EFAULT on error.

So, something simple, like:

  if (get_user(val, buf))
  	goto err;

> +out:

Use "err" for a single goto label for the error path.

>  	pci_config_pm_runtime_put(dev);
> -
>  	*ppos = pos;
> -	i_size_write(ino, dev->cfg_size);
> -	return nbytes;
> +	if (ret > 0)
> +		i_size_write(ino, dev->cfg_size);
> +	return ret;

This can be kept simple:

  err:
  	pci_config_pm_runtime_put(dev);
  	return -EFAULT;

The i_size_write() is such an unfortunate band-aid, but unless we have a
way to set the size before the procfs entry is created/made visible, then
the problem that this aims to fix is here to stay for now, see:

  ecb3908046ce ("pci: write file size to inode on proc bus file write")

Having said all that, since you are looking at proc_bus_pci_write(),
then perhaps an update to proc_bus_pci_read() to use put_user() would
also be prudent.

Thank you!

	Krzysztof

Re: [PATCH] PCI/proc: check __get_user() return value in proc_bus_pci_write()

[Deepanshu Kartikey]

On Mon, May 4, 2026 at 12:16 AM Krzysztof Wilczyński <kw@linux.com> wrote:
>
> Hello,
>
> > Check __get_user() and return -EFAULT on failure.
> [...]
> > @@ -136,7 +136,10 @@ static ssize_t proc_bus_pci_write(struct file *file, const char __user *buf,
> >
> >       if ((pos & 1) && cnt) {
> >               unsigned char val;
> > -             __get_user(val, buf);
> > +             if (__get_user(val, buf)) {
> > +                     ret = -EFAULT;
> > +                     goto out;
> > +             }
>
> We could move to get_user() here.  This would allow you to drop
> the access_ok(), too, as get_user() would return -EFAULT on error.
>
> So, something simple, like:
>
>   if (get_user(val, buf))
>         goto err;
>
> > +out:
>
> Use "err" for a single goto label for the error path.
>
> >       pci_config_pm_runtime_put(dev);
> > -
> >       *ppos = pos;
> > -     i_size_write(ino, dev->cfg_size);
> > -     return nbytes;
> > +     if (ret > 0)
> > +             i_size_write(ino, dev->cfg_size);
> > +     return ret;
>
> This can be kept simple:
>
>   err:
>         pci_config_pm_runtime_put(dev);
>         return -EFAULT;
>
> The i_size_write() is such an unfortunate band-aid, but unless we have a
> way to set the size before the procfs entry is created/made visible, then
> the problem that this aims to fix is here to stay for now, see:
>
>   ecb3908046ce ("pci: write file size to inode on proc bus file write")
>
> Having said all that, since you are looking at proc_bus_pci_write(),
> then perhaps an update to proc_bus_pci_read() to use put_user() would
> also be prudent.
>
> Thank you!
>
>         Krzysztof

Thanks for the detailed feedback. I will send patch v2 shortly.

Thanks

Deepanshu Kartikey