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

[Thomas Zimmermann]

Hi,

thanks for following through with the bug and sending the patch

Am 19.04.24 um 21:00 schrieb 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.

Please tone down the drama. :)

> Fix this by taking the mapping offset into account.
>
> Reported-and-tested-by: Harshit Mogalapalli <harshit.m.mogalapalli@xxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-fbdev/271372d6-e665-4e7f-b088-dee5f4ab341a@xxxxxxxxxx
> Fixes: 56c134f7f1b5 ("fbdev: Track deferred-I/O pages in pageref struct")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Nam Cao <namcao@xxxxxxxxxxxxx>
> ---
>   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);

The page-fault handler at [1] use vm_fault.pgoff to retrieve the page 
structure. Can we do the same here and avoid that computation?

Best regards
Thomas

[1] 
https://elixir.bootlin.com/linux/v6.8/source/drivers/video/fbdev/core/fb_defio.c#L100

>   	struct page *page = vmf->page;
>   
>   	file_update_time(vmf->vma->vm_file);

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)