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From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-arch <linux-arch@vger.kernel.org>,
	Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrey Konovalov <andreyknvl@google.com>,
	Vineet Gupta <Vineet.Gupta1@synopsys.com>,
	Aleksa Sarai <cyphar@cyphar.com>, Ingo Molnar <mingo@kernel.org>,
	Khalid Aziz <khalid.aziz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-snps-arc@lists.infradead.org,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 2/4] lib/strncpy_from_user: Remove redundant user space pointer range check
Date: Tue, 14 Jan 2020 23:46:00 +0000
Message-ID: <20200114234600.GD8904@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CAHk-=wgoc5DaF6=WxsAcft_Lp4XUYTiRhhCJGcmM5PwEDXY6Gw@mail.gmail.com>

On Tue, Jan 14, 2020 at 01:22:07PM -0800, Linus Torvalds wrote:

> The fact is, copying a string from user space is *very* different from
> copying a fixed number of bytes, and that whole dance with
> 
>         max_addr = user_addr_max();
> 
> is absolutely required and necessary.
> 
> You completely broke string copying.

BTW, a quick grep through the callers has found something odd -
static ssize_t kmemleak_write(struct file *file, const char __user *user_buf,
                              size_t size, loff_t *ppos)
{
        char buf[64];
        int buf_size;
        int ret;

        buf_size = min(size, (sizeof(buf) - 1));
        if (strncpy_from_user(buf, user_buf, buf_size) < 0)
                return -EFAULT;
        buf[buf_size] = 0;

What the hell?  If somebody is calling write(fd, buf, n) they'd
better be ready to see any byte from buf[0] up to buf[n - 1]
fetched, and if something is unmapped - deal with -EFAULT.
Is something really doing that and if so, why does kmemleak
try to accomodate that idiocy?

The same goes for several more ->write() instances - mtrr_write(),
armada_debugfs_crtc_reg_write() and cio_ignore_write(); IMO that's
seriously misguided (and cio one ought use vmemdup_user() instead
of what it's doing)...


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Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-14 20:08 [RFC 0/4] Switching ARC to optimized generic strncpy_from_user Vineet Gupta
2020-01-14 20:08 ` [RFC 1/4] asm-generic/uaccess: don't define inline functions if noinline lib/* in use Vineet Gupta
2020-01-14 20:57   ` Arnd Bergmann
2020-01-15 23:01     ` Vineet Gupta
2020-01-16 11:43       ` Arnd Bergmann
2020-01-14 21:32   ` Linus Torvalds
2020-01-15  9:08     ` Arnd Bergmann
2020-01-15 14:12       ` Al Viro
2020-01-15 14:21         ` Arnd Bergmann
2020-01-14 20:08 ` [RFC 2/4] lib/strncpy_from_user: Remove redundant user space pointer range check Vineet Gupta
2020-01-14 21:22   ` Linus Torvalds
2020-01-14 21:52     ` Vineet Gupta
2020-01-14 23:46     ` Al Viro [this message]
2020-01-15 14:42   ` Andrey Konovalov
2020-01-15 23:00     ` Vineet Gupta
2020-01-14 20:08 ` [RFC 3/4] ARC: uaccess: remove noinline variants of __strncpy_from_user() and friends Vineet Gupta
2020-01-14 20:08 ` [RFC 4/4] ARC: uaccess: use optimized generic __strnlen_user/__strncpy_from_user Vineet Gupta
2020-01-14 20:42   ` Arnd Bergmann
2020-01-14 21:36     ` Vineet Gupta
2020-01-14 21:49       ` Linus Torvalds
2020-01-14 22:14         ` Vineet Gupta

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