Re: [PATCH] staging: vme_user: fix bounds check when vme_get_size() returns zero

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Aug 06, 2026 at 08:25:06PM -0500, Som Tripathi wrote:
> vme_get_size() returns zero on failure, as its kerneldoc in vme.c
> states. vme_user_read() and vme_user_write() assign it to a size_t and
> check the file position with:
> 
> 	if ((*ppos < 0) || (*ppos > (image_size - 1)))
> 
> When image_size is zero, image_size - 1 wraps to SIZE_MAX. The test is
> then never true, so the check does nothing. The following statement,
> 
> 	count = image_size - *ppos;
> 
> wraps the same way whenever *ppos is greater than zero.
> 
> This is not an out-of-bounds access. resource_to_user() and
> resource_from_user() clamp count to size_buf, buffer_to_user() and
> buffer_from_user() clamp it to size_buf - *ppos, and vme_master_read()
> and vme_master_write() reject an offset greater than the window
> length. What happens instead is that read() and write() operate on a
> window whose size the driver failed to read, rather than returning at
> the check.
> 
> Compare *ppos against image_size directly. The two forms agree for a
> non-zero size, the new one is also correct for zero, and both wraps go
> away.
> 
> Found by reading the code after Dan Carpenter listed this as one of
> three outstanding bugs in this driver; see the Link below. Compile
> tested only. I have no VME hardware.

Please see:
	https://lore.kernel.org/all/2026080354-skater-urgent-31b2@gregkh/T/#u
for why I can't take this.

thanks,

greg k-h