[PATCH] fbdev: fix incorrect address computation in deferred IO

[Nam Cao]

With deferred IO enabled, a page fault happens when data is written to the
framebuffer device. Then driver determines which page is being updated by
calculating the offset of the written virtual address within the virtual
memory area, and uses this offset to get the updated page within the
internal buffer. This page is later copied to hardware (thus the name
"deferred IO").

This calculation is only correct if the virtual memory area is mapped to
the beginning of the internal buffer. Otherwise this is wrong. For example,
if users do:
    mmap(ptr, 4096, PROT_WRITE, MAP_FIXED | MAP_SHARED, fd, 0xff000);

Then the virtual memory area will mapped at offset 0xff000 within the
internal buffer. This offset 0xff000 is not accounted for, and wrong page
is updated. This will lead to wrong pixels being updated on the device.

However, it gets worse: if users do 2 mmap to the same virtual address, for
example:

    int fd = open("/dev/fb0", O_RDWR, 0);
    char *ptr = (char *) 0x20000000ul;
    mmap(ptr, 4096, PROT_WRITE, MAP_FIXED | MAP_SHARED, fd, 0xff000);
    *ptr = 0; // write #1
    mmap(ptr, 4096, PROT_WRITE, MAP_FIXED | MAP_SHARED, fd, 0);
    *ptr = 0; // write #2

In this case, both write #1 and write #2 apply to the same virtual address
(0x20000000ul), and the driver mistakenly thinks the same page is being
written to. When the second write happens, the driver thinks this is the
same page as the last time, and reuse the page from write #1. The driver
then lock_page() an incorrect page, and returns VM_FAULT_LOCKED with the
correct page unlocked. It is unclear what will happen with memory
management subsystem after that, but likely something terrible.

Fix this by taking the mapping offset into account.

Reported-and-tested-by: Harshit Mogalapalli <harshit.m.mogalapalli at oracle.com>
Closes: https://lore.kernel.org/linux-fbdev/271372d6-e665-4e7f-b088-dee5f4ab341a@oracle.com
Fixes: 56c134f7f1b5 ("fbdev: Track deferred-I/O pages in pageref struct")
Cc: stable at vger.kernel.org
Signed-off-by: Nam Cao <namcao at linutronix.de>
---
 drivers/video/fbdev/core/fb_defio.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
index dae96c9f61cf..d5d6cd9e8b29 100644
--- a/drivers/video/fbdev/core/fb_defio.c
+++ b/drivers/video/fbdev/core/fb_defio.c
@@ -196,7 +196,8 @@ static vm_fault_t fb_deferred_io_track_page(struct fb_info *info, unsigned long
  */
 static vm_fault_t fb_deferred_io_page_mkwrite(struct fb_info *info, struct vm_fault *vmf)
 {
-	unsigned long offset = vmf->address - vmf->vma->vm_start;
+	unsigned long offset = vmf->address - vmf->vma->vm_start
+			+ (vmf->vma->vm_pgoff << PAGE_SHIFT);
 	struct page *page = vmf->page;
 
 	file_update_time(vmf->vma->vm_file);
-- 
2.39.2