[PATCH 3/3] Add fb_open_adj_file() to bochsdrmfb to avoid use-after-free

['Max Staudt]

From: Max Staudt <mstaudt@xxxxxxx>

Currently, when using bochsdrm(fb), opening /dev/drm/card0 will adjust
file->f_mapping, but opening /dev/fb0 will not.
This may result in dangling pointers from user space when memory is
evicted, and therefore in use-after-free crashes when using the emulated
framebuffer device.

Bochs is the only driver to use the TTM fbdev integration
(ttm_fbdev_mmap()), and thus the only one able to trigger this case.
It is highlighted by the WARN_ON() in ttm_bo_vm_open() in
drivers/gpu/drm/ttm/ttm_bo_vm.c which is printed upon splitting
a VMA (see reproducer code below):

WARN_ON(bo->bdev->dev_mapping != vma->vm_file->f_mapping);

This happens because ttm_fbdev_mmap() sets the VMA's vm_ops pointer
to point to the TTM memory handling functions, without the file's
f_mapping having been adjusted. This means that file->f_mapping would
still point to the address_space for the inode in the file system.

This patch avoids dangling/use-after-free pointers that remain after
TTM decides to evict the memory referenced by bo->bdev->dev_mapping,
as the VMAs of a mmap()ed /dev/fb0 belong to /dev/fb0 instead of the
anonymous inode dev->anon_inode used by DRM.
It basically mimics the adjustment of file->f_mapping that is also
performed in drm_open() in drivers/gpu/drm/drm_fops.c.

For the original report, see:
https://lkml.kernel.org/g/s5hy49ue4y0.wl-tiwai@xxxxxxx

The reproducer code is as follows:

----

int main(void)
{
        void *addr;

        int fd = open("/dev/fb0", O_RDONLY);
        if (fd < 0)
                err(1, "open");

        addr = mmap(0, 8192, PROT_READ, MAP_SHARED, fd, 0);
        if (addr == MAP_FAILED)
                err(1, "1st mmap");

        if (mmap(addr, 4096, PROT_READ, MAP_SHARED | MAP_FIXED | MAP_ANONYMOUS, -1, 0) == MAP_FAILED)
                err(1, "2nd mmap");

        return 0;
}

----

Signed-off-by: Max Staudt <mstaudt@xxxxxxx>
---
 drivers/gpu/drm/bochs/bochs.h       |  1 +
 drivers/gpu/drm/bochs/bochs_fbdev.c | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/drivers/gpu/drm/bochs/bochs.h b/drivers/gpu/drm/bochs/bochs.h
index 19b5ada..c37d30a 100644
--- a/drivers/gpu/drm/bochs/bochs.h
+++ b/drivers/gpu/drm/bochs/bochs.h
@@ -1,5 +1,6 @@
 #include <linux/io.h>
 #include <linux/fb.h>
+#include <linux/fs.h>
 #include <linux/console.h>
 
 #include <drm/drmP.h>
diff --git a/drivers/gpu/drm/bochs/bochs_fbdev.c b/drivers/gpu/drm/bochs/bochs_fbdev.c
index 7520bf8..fd70449 100644
--- a/drivers/gpu/drm/bochs/bochs_fbdev.c
+++ b/drivers/gpu/drm/bochs/bochs_fbdev.c
@@ -20,6 +20,24 @@ static int bochsfb_mmap(struct fb_info *info,
 	return ttm_fbdev_mmap(vma, &bo->bo);
 }
 
+static int bochsfb_open_adj_file(struct fb_info *info, struct file *file)
+{
+	struct drm_fb_helper *helper = (struct drm_fb_helper *)info->par;
+	struct bochs_device *bochs =
+		container_of(helper, struct bochs_device, fb.helper);
+	struct drm_device *dev = bochs->dev;
+
+	/* Correct f_mapping here so it's consistent across the
+	 * fd's lifetime.
+	 * If this isn't done, the WARN_ON() in ttm_bo_vm_open()
+	 * will trip upon vma_split(), hinting that other memory
+	 * management nastiness may occur when TTM evicts.
+	 */
+	file->f_mapping = dev->anon_inode->i_mapping;
+
+	return 0;
+}
+
 static struct fb_ops bochsfb_ops = {
 	.owner = THIS_MODULE,
 	.fb_check_var = drm_fb_helper_check_var,
@@ -31,6 +49,7 @@ static struct fb_ops bochsfb_ops = {
 	.fb_blank = drm_fb_helper_blank,
 	.fb_setcmap = drm_fb_helper_setcmap,
 	.fb_mmap = bochsfb_mmap,
+	.fb_open_adj_file = bochsfb_open_adj_file,
 };
 
 static int bochsfb_create_object(struct bochs_device *bochs,
-- 
2.6.6