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Wed, 01 Jul 2026 15:18:00 -0700 (PDT) Received: from localhost.localdomain ([2a01:4f9:2a:1c13::2]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-5aec89dc478sm267767e87.68.2026.07.01.15.17.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 01 Jul 2026 15:17:58 -0700 (PDT) From: Melbin K Mathew To: deller@gmx.de Cc: linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, security@kernel.org, Melbin K Mathew , stable@vger.kernel.org Subject: [PATCH] fbdev: protect mode sysfs reads with lock_fb_info() Date: Thu, 2 Jul 2026 00:17:57 +0200 Message-Id: <20260701221757.231490-1-mlbnkm1@gmail.com> X-Mailer: git-send-email 2.39.5 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit show_mode() dereferences fb_info->mode and show_modes() walks fb_info->modelist without holding lock_fb_info(). store_modes() takes lock_fb_info() while replacing the modelist and freeing the old one. A concurrent reader can load a pointer to an old modelist entry before store_modes() frees it, then dereference freed memory in mode_string(). Take lock_fb_info() in both show_mode() and show_modes() to serialize with store_modes(). In show_mode(), copy the mode to the stack and format the stack copy after dropping the lock. Impact: local kernel UAF read when a privileged writer races with sysfs readers of /sys/class/graphics/fb*/mode and modes. Cc: stable@vger.kernel.org Signed-off-by: Melbin K Mathew --- A userspace reproducer triggering the race is available to maintainers on request. drivers/video/fbdev/core/fbsysfs.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff --git a/drivers/video/fbdev/core/fbsysfs.c b/drivers/video/fbdev/core/fbsysfs.c index ea196603c7..6bdb25f7be 100644 --- a/drivers/video/fbdev/core/fbsysfs.c +++ b/drivers/video/fbdev/core/fbsysfs.c @@ -82,11 +82,20 @@ static ssize_t show_mode(struct device *device, struct device_attribute *attr, char *buf) { struct fb_info *fb_info = dev_get_drvdata(device); + struct fb_videomode mode; + bool have_mode = false; - if (!fb_info->mode) + lock_fb_info(fb_info); + if (fb_info->mode) { + mode = *fb_info->mode; + have_mode = true; + } + unlock_fb_info(fb_info); + + if (!have_mode) return 0; - return mode_string(buf, 0, fb_info->mode); + return mode_string(buf, 0, &mode); } static ssize_t store_modes(struct device *device, @@ -134,10 +143,13 @@ static ssize_t show_modes(struct device *device, struct device_attribute *attr, const struct fb_videomode *mode; i = 0; + lock_fb_info(fb_info); list_for_each_entry(modelist, &fb_info->modelist, list) { mode = &modelist->mode; i += mode_string(buf, i, mode); } + unlock_fb_info(fb_info); + return i; } -- 2.39.5