linux-kernel - RE: [PATCH bpf v2 1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator

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Message-ID: <cbe00e1421db4566a076102506b6d670@AcuMS.aculab.com>
Date:   Thu, 5 Nov 2020 09:00:00 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Daniel Xu' <dxu@...uu.xyz>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>
CC:     "kernel-team@...com" <kernel-team@...com>
Subject: RE: [PATCH bpf v2 1/2] lib/strncpy_from_user.c: Don't overcopy bytes
 after NUL terminator

From: Daniel Xu
> Sent: 05 November 2020 02:26
...
> --- a/lib/strncpy_from_user.c
> +++ b/lib/strncpy_from_user.c
> @@ -35,17 +35,22 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src,
>  		goto byte_at_a_time;
> 
>  	while (max >= sizeof(unsigned long)) {
> -		unsigned long c, data;
> +		unsigned long c, data, mask, *out;
> 
>  		/* Fall back to byte-at-a-time if we get a page fault */
>  		unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);

It's not related to this change, but since both addresses
are aligned (checked earlier) a page fault on the word read
is fatal.

	David

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