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

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Date: Fri, 16 Feb 2024 13:19:20 -0800
From: Kees Cook <keescook@...omium.org>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Steffen Klassert <steffen.klassert@...unet.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Leon Romanovsky <leon@...nel.org>, Lin Ma <linma@....edu.cn>,
	Simon Horman <horms@...nel.org>, Breno Leitao <leitao@...ian.org>,
	Tobias Brunner <tobias@...ongswan.org>,
	Raed Salem <raeds@...dia.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] [RFC] xfrm: work around a clang-19 fortifiy-string
 false-positive

On Fri, Feb 16, 2024 at 09:26:40PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> 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.

Per C11, padding bits are zero initialized if there is an initializer,
so "= { }" here should be sufficient -- no need to add the struct
members.

> Since this is a false positive, a better fix would likely be to
> fix the compiler.

As Nathan has found, this appears to be a loop unrolling bug in Clang.
https://github.com/ClangBuiltLinux/linux/issues/1985

The shorter fix (in the issue) is to explicitly range-check before
the loop:

       if (xp->xfrm_nr > XFRM_MAX_DEPTH)
               return -ENOBUFS;

-- 
Kees Cook