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 offset 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.
Correct the calculation by using vmf->pgoff instead. With this change, the variable "offset" will no longer hold the exact offset value, but it is rounded down to multiples of PAGE_SIZE. But this is still correct, because this variable is only used to calculate the page offset.
Reported-by: Harshit Mogalapalli harshit.m.mogalapalli@oracle.com Closes: https://lore.kernel.org/linux-fbdev/271372d6-e665-4e7f-b088-dee5f4ab341a@ora... Fixes: 56c134f7f1b5 ("fbdev: Track deferred-I/O pages in pageref struct") Cc: stable@vger.kernel.org Signed-off-by: Nam Cao namcao@linutronix.de --- v2: - simplify the patch by using vfg->pgoff - remove tested-by tag, as the patch is now different
drivers/video/fbdev/core/fb_defio.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c index 1ae1d35a5942..b9607d5a370d 100644 --- a/drivers/video/fbdev/core/fb_defio.c +++ b/drivers/video/fbdev/core/fb_defio.c @@ -196,7 +196,7 @@ 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->pgoff << PAGE_SHIFT; struct page *page = vmf->page;
file_update_time(vmf->vma->vm_file);