Messages in this thread | | | Date | Tue, 23 Apr 2024 10:37:41 +0200 | Subject | Re: [PATCH] fbdev: fix incorrect address computation in deferred IO | From | Thomas Zimmermann <> |
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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@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@vger.kernel.org > Signed-off-by: Nam Cao <namcao@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);
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)
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