[v2,next] PCI/P2PDMA: Fix potential undefined behavior bug in struct pci_p2pdma_pagemap

`struct dev_pagemap` is a flexible structure, which means that it
contains a flexible-array member at the bottom. This could potentially
lead to an overwrite of the objects following `pgmap` in `struct
pci_p2pdma_pagemap`, when `nr_range > 1`. This is currently not the
case (notice that `nr_range` is hardcoded to `1`), however as commit
b7b3c01b1915 ("mm/memremap_pages: support multiple ranges per invocation")
mentions in the subject line, this code can `support multiple
ranges per invocation`. So, we'd better prevent any problems that may
arise in the future.

Fix this by placing the declaration of object `pgmap` at the end of
`struct pci_p2pdma_pagemap`.

-Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
ready to enable it globally.

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
Changes in v2:
 - Remove `Fixes:` tags. (Logan Gunthorpe)
 - Update changelog text. Make it clear that `nr_range` is hardcoded to `1`
   (Logan Gunthorpe)
 - Update subject.
 - Add Logan's RB.

v1:
 Link: https://lore.kernel.org/linux-hardening/ZRnf6wVOu0IJQ2Ok@work/

 drivers/pci/p2pdma.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Mon, Oct 02, 2023 at 09:04:15PM +0200, Gustavo A. R. Silva wrote:
> `struct dev_pagemap` is a flexible structure, which means that it
> contains a flexible-array member at the bottom. This could potentially
> lead to an overwrite of the objects following `pgmap` in `struct
> pci_p2pdma_pagemap`, when `nr_range > 1`. This is currently not the
> case (notice that `nr_range` is hardcoded to `1`), however as commit
> b7b3c01b1915 ("mm/memremap_pages: support multiple ranges per invocation")
> mentions in the subject line, this code can `support multiple
> ranges per invocation`. So, we'd better prevent any problems that may
> arise in the future.
> 
> Fix this by placing the declaration of object `pgmap` at the end of
> `struct pci_p2pdma_pagemap`.
> 
> -Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
> ready to enable it globally.
> 
> Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Applied to pci/p2pdma for v6.7.  Since it's not a current problem, I
assume there's no reason to apply it for v6.6 (or tag it for stable);
let me know if otherwise.

> ---
> Changes in v2:
>  - Remove `Fixes:` tags. (Logan Gunthorpe)
>  - Update changelog text. Make it clear that `nr_range` is hardcoded to `1`
>    (Logan Gunthorpe)
>  - Update subject.
>  - Add Logan's RB.
> 
> v1:
>  Link: https://lore.kernel.org/linux-hardening/ZRnf6wVOu0IJQ2Ok@work/
> 
>  drivers/pci/p2pdma.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index fa7370f9561a..ab34d3d36a64 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -28,9 +28,9 @@ struct pci_p2pdma {
>  };
>  
>  struct pci_p2pdma_pagemap {
> -	struct dev_pagemap pgmap;
>  	struct pci_dev *provider;
>  	u64 bus_offset;
> +	struct dev_pagemap pgmap;
>  };
>  
>  static struct pci_p2pdma_pagemap *to_p2p_pgmap(struct dev_pagemap *pgmap)
> -- 
> 2.34.1
>