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Date:   Thu, 21 Oct 2021 09:12:47 -0700
From:   Kees Cook <keescook@chromium.org>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        David Miller <davem@davemloft.net>, sparclinux@vger.kernel.org
Subject: Re: [PATCH 08/20] signal/sparc: In setup_tsb_params convert open
 coded BUG into BUG
Message-ID: <202110210910.7861F4041@keescook>
References: <87y26nmwkb.fsf@disp2133>
 <20211020174406.17889-8-ebiederm@xmission.com>
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On Wed, Oct 20, 2021 at 12:43:54PM -0500, Eric W. Biederman wrote:
> The function setup_tsb_params has exactly one caller tsb_grow.  The
> function tsb_grow passes in a tsb_bytes value that is between 8192 and
> 1048576 inclusive, and is guaranteed to be a power of 2.  The function
> setup_tsb_params verifies this property with a switch statement and
> then prints an error and causes the task to exit if this is not true.
> 
> In practice that print statement can never be reached because tsb_grow
> never passes in a bad tsb_size.  So if tsb_size ever gets a bad value
> that is a kernel bug.
> 
> So replace the do_exit which is effectively an open coded version of
> BUG() with an actuall call to BUG().  Making it clearer that this
> is a case that can never, and should never happen.
> 
> Cc: David Miller <davem@davemloft.net>
> Cc: sparclinux@vger.kernel.org
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
>  arch/sparc/mm/tsb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/sparc/mm/tsb.c b/arch/sparc/mm/tsb.c
> index 0dce4b7ff73e..912205787161 100644
> --- a/arch/sparc/mm/tsb.c
> +++ b/arch/sparc/mm/tsb.c
> @@ -266,7 +266,7 @@ static void setup_tsb_params(struct mm_struct *mm, unsigned long tsb_idx, unsign
>  	default:
>  		printk(KERN_ERR "TSB[%s:%d]: Impossible TSB size %lu, killing process.\n",
>  		       current->comm, current->pid, tsb_bytes);
> -		do_exit(SIGSEGV);
> +		BUG();
>  	}
>  	tte |= pte_sz_bits(page_sz);

Given the other uses of BUG() here, this seems okay.

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook