Re: [PATCH] [RFC] xfrm: work around a clang-19 fortifiy-string false-positive

[Nathan Chancellor]

Hi Arnd,

On Fri, Feb 16, 2024 at 09:26:40PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> clang-19 recently got branched from clang-18 and is not yet released.
> The current version introduces exactly one new warning that I came
> across in randconfig testing, in the copy_to_user_tmpl() function:
> 
> include/linux/fortify-string.h:420:4: error: call to '__write_overflow_field' declared with 'warning' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror,-Wattribute-warning]
>   420 |                         __write_overflow_field(p_size_field, size);
> 
> I have not yet produced a minimized test case for it, but I have a
> local workaround, which avoids the memset() here by replacing it with
> an initializer.
> 
> The memset is required to avoid leaking stack data to user space
> and was added in commit 1f86840f8977 ("xfrm_user: fix info leak in
> copy_to_user_tmpl()"). Simply changing the initializer to set all fields
> still risks leaking data in the padding between them, which the compiler
> is free to do here. To work around that problem, explicit padding fields
> have to get added as well.
> 
> My first idea was that just adding the padding would avoid the warning
> as well, as the padding tends to confused the fortified string helpers,
> but it turns out that both changes are required here.
> 
> Since this is a false positive, a better fix would likely be to
> fix the compiler.

I have some observations and notes from my initial investigation into
this issue on our GitHub issue tracker but I have not produced a
minimized test case either.

https://github.com/ClangBuiltLinux/linux/issues/1985

> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  include/uapi/linux/xfrm.h | 3 +++
>  net/xfrm/xfrm_user.c      | 3 +--
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/xfrm.h b/include/uapi/linux/xfrm.h
> index 6a77328be114..99adac4fa648 100644
> --- a/include/uapi/linux/xfrm.h
> +++ b/include/uapi/linux/xfrm.h
> @@ -27,6 +27,7 @@ struct xfrm_id {
>  	xfrm_address_t	daddr;
>  	__be32		spi;
>  	__u8		proto;
> +	__u8		__pad[3];
>  };
>  
>  struct xfrm_sec_ctx {
> @@ -242,11 +243,13 @@ struct xfrm_user_sec_ctx {
>  struct xfrm_user_tmpl {
>  	struct xfrm_id		id;
>  	__u16			family;
> +	__u16			__pad1;
>  	xfrm_address_t		saddr;
>  	__u32			reqid;
>  	__u8			mode;
>  	__u8			share;
>  	__u8			optional;
> +	__u8			__pad2;
>  	__u32			aalgos;
>  	__u32			ealgos;
>  	__u32			calgos;
> diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
> index a5232dcfea46..e81f977e183c 100644
> --- a/net/xfrm/xfrm_user.c
> +++ b/net/xfrm/xfrm_user.c
> @@ -2011,7 +2011,7 @@ static int xfrm_add_policy(struct sk_buff *skb, struct nlmsghdr *nlh,
>  
>  static int copy_to_user_tmpl(struct xfrm_policy *xp, struct sk_buff *skb)
>  {
> -	struct xfrm_user_tmpl vec[XFRM_MAX_DEPTH];
> +	struct xfrm_user_tmpl vec[XFRM_MAX_DEPTH] = {};
>  	int i;
>  
>  	if (xp->xfrm_nr == 0)
> @@ -2021,7 +2021,6 @@ static int copy_to_user_tmpl(struct xfrm_policy *xp, struct sk_buff *skb)
>  		struct xfrm_user_tmpl *up = &vec[i];
>  		struct xfrm_tmpl *kp = &xp->xfrm_vec[i];
>  
> -		memset(up, 0, sizeof(*up));
>  		memcpy(&up->id, &kp->id, sizeof(up->id));
>  		up->family = kp->encap_family;
>  		memcpy(&up->saddr, &kp->saddr, sizeof(up->saddr));
> -- 
> 2.39.2
>