[PATCH] PCI/proc: Warn on writes to kernel-exclusive config space regions

[PATCH] PCI/proc: Warn on writes to kernel-exclusive config space regions

[Krzysztof Wilczyński]

Currently, a driver can claim a region of a device's config space as
exclusive using pci_request_config_region_exclusive(), after which a
write to that region originating from user space is expected to emit a
warning and taint the kernel.  The check is advisory only, as the write
itself is still allowed to proceed.

Since commit 278294798ac9 ("PCI: Allow drivers to request exclusive
config regions"), the sysfs config space attribute performs this check
in pci_write_config(), but the procfs interface was never updated.  A
write performed through /proc/bus/pci/BB/DD.F therefore bypasses the
detection entirely, even though both interfaces offer the same level
of access.

Thus, add the same resource_is_exclusive() check to proc_bus_pci_write().

Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
---
 drivers/pci/proc.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/pci/proc.c b/drivers/pci/proc.c
index 71ad289fcb8e..12200979d3a8 100644
--- a/drivers/pci/proc.c
+++ b/drivers/pci/proc.c
@@ -14,6 +14,8 @@
 #include <linux/capability.h>
 #include <linux/uaccess.h>
 #include <linux/security.h>
+#include <linux/panic.h>
+#include <linux/sched.h>
 #include <asm/byteorder.h>
 #include "pci.h"
 
@@ -122,6 +124,12 @@ static ssize_t proc_bus_pci_write(struct file *file, const char __user *buf,
 	if (ret)
 		return ret;
 
+	if (resource_is_exclusive(&dev->driver_exclusive_resource, pos, nbytes)) {
+		pci_warn_once(dev, "%s: Unexpected write to kernel-exclusive config offset %x",
+			      current->comm, pos);
+		add_taint(TAINT_USER, LOCKDEP_STILL_OK);
+	}
+
 	if (pos >= size)
 		return 0;
 	if (nbytes >= size)
-- 
2.55.0

Re: [PATCH] PCI/proc: Warn on writes to kernel-exclusive config space regions

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Integer truncation of the 64-bit file offset to a 32-bit signed int allows bypassing bounds checks and the new exclusive region check, leading to massive out-of-bounds memory writes.
--

--- Patch [1]: [PATCH] PCI/proc: Warn on writes to kernel-exclusive config space regions ---
commit ae079df2d52cc5cc8959cf281ed5e461ff2fcfd8
Author: Krzysztof Wilczyński <kwilczynski@kernel.org>

PCI/proc: Warn on writes to kernel-exclusive config space regions

This patch updates proc_bus_pci_write() to check if a user space write
targets a kernel-exclusive config space region. If so, it emits a warning
and taints the kernel, bringing the procfs interface to parity with the
sysfs interface.

> diff --git a/drivers/pci/proc.c b/drivers/pci/proc.c
> index 71ad289fcb8e3..12200979d3a82 100644
> --- a/drivers/pci/proc.c
> +++ b/drivers/pci/proc.c
[ ... ]
> @@ -122,6 +124,12 @@ static ssize_t proc_bus_pci_write(struct file *file, const char __user *buf,
>  	if (ret)
>  		return ret;
>  
> +	if (resource_is_exclusive(&dev->driver_exclusive_resource, pos, nbytes)) {
> +		pci_warn_once(dev, "%s: Unexpected write to kernel-exclusive config offset %x",
> +			      current->comm, pos);
> +		add_taint(TAINT_USER, LOCKDEP_STILL_OK);
> +	}
> +

[Severity: High]
This is a pre-existing issue, but does truncating the 64-bit file offset to a
32-bit signed integer earlier in proc_bus_pci_write() allow bypassing this
new exclusive region check?

A large positive 64-bit file offset provided via pwrite64() gets truncated to
a negative value:

drivers/pci/proc.c:proc_bus_pci_write() {
    ...
	int pos = *ppos;
    ...
}

Because resource_is_exclusive() takes a u64 address, the negative pos will
sign-extend into a massive 64-bit value that won't match any exclusive
region, bypassing this detection entirely.

Additionally, this truncation also bypasses the subsequent bounds checks:

	if (pos >= size)
		return 0;
	if (nbytes >= size)
		nbytes = size;
	if (pos + nbytes > size)
		nbytes = size - pos;

The signed comparison if (pos >= size) evaluates to false for a negative
pos. Because nbytes is unsigned, the length clipping if (pos + nbytes > size)
evaluates using unsigned arithmetic and becomes true for a negative pos. This
erroneously increases nbytes up to massive out-of-bounds lengths, potentially
leading to large writes outside the PCI configuration space.

>  	if (pos >= size)
>  		return 0;
>  	if (nbytes >= size)

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260729075413.1215821-1-kwilczynski@kernel.org?part=1

Re: [PATCH] PCI/proc: Warn on writes to kernel-exclusive config space regions

[Krzysztof Wilczyński]

Hello,

> Currently, a driver can claim a region of a device's config space as
> exclusive using pci_request_config_region_exclusive(), after which a
> write to that region originating from user space is expected to emit a
> warning and taint the kernel.  The check is advisory only, as the write
> itself is still allowed to proceed.
> 
> Since commit 278294798ac9 ("PCI: Allow drivers to request exclusive
> config regions"), the sysfs config space attribute performs this check
> in pci_write_config(), but the procfs interface was never updated.  A
> write performed through /proc/bus/pci/BB/DD.F therefore bypasses the
> detection entirely, even though both interfaces offer the same level
> of access.
> 
> Thus, add the same resource_is_exclusive() check to proc_bus_pci_write().

Applied to the procfs branch.

Thank you!

	Krzysztof