linux-kernel - [PATCH v2 1/2] drm: Use size_t type for len variable in drm_copy

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Message-Id: <20220705100215.572498-2-javierm@redhat.com>
Date:   Tue,  5 Jul 2022 12:02:13 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>,
        Peter Robinson <pbrobinson@...il.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        dri-devel@...ts.freedesktop.org
Subject: [PATCH v2 1/2] drm: Use size_t type for len variable in drm_copy_field()

The strlen() function returns a size_t which is an unsigned int on 32-bit
arches and an unsigned long on 64-bit arches. But in the drm_copy_field()
function, the strlen() return value is assigned to an 'int len' variable.

Later, the len variable is passed as copy_from_user() third argument that
is an unsigned long parameter as well.

In theory, this can lead to an integer overflow via type conversion. Since
the assignment happens to a signed int lvalue instead of a size_t lvalue.

In practice though, that's unlikely since the values copied are set by DRM
drivers and not controlled by userspace. But using a size_t for len is the
correct thing to do anyways.

Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
Tested-by: Peter Robinson <pbrobinson@...il.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>
---

Changes in v2:
- Add Peter Robinson Tested-by and Thomas Zimmermann Reviewed-by tags.

 drivers/gpu/drm/drm_ioctl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 8faad23dc1d8..e1b9a03e619c 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -472,7 +472,7 @@ EXPORT_SYMBOL(drm_invalid_op);
  */
 static int drm_copy_field(char __user *buf, size_t *buf_len, const char *value)
 {
-	int len;
+	size_t len;

 	/* don't overflow userbuf */
 	len = strlen(value);
-- 
2.36.1