linux-hardening - Re: [PATCH] fs: remove accidental overflow during wraparound check

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Date: Thu, 9 May 2024 17:53:56 +0200
From: Jan Kara <jack@...e.cz>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Nathan Chancellor <nathan@...nel.org>,
	Bill Wendling <morbo@...gle.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] fs: remove accidental overflow during wraparound check

On Tue 07-05-24 23:17:57, Justin Stitt wrote:
> Running syzkaller with the newly enabled signed integer overflow
> sanitizer produces this report:
> 
> [  195.401651] ------------[ cut here ]------------
> [  195.404808] UBSAN: signed-integer-overflow in ../fs/open.c:321:15
> [  195.408739] 9223372036854775807 + 562984447377399 cannot be represented in type 'loff_t' (aka 'long long')
> [  195.414683] CPU: 1 PID: 703 Comm: syz-executor.0 Not tainted 6.8.0-rc2-00039-g14de58dbe653-dirty #11
> [  195.420138] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> [  195.425804] Call Trace:
> [  195.427360]  <TASK>
> [  195.428791]  dump_stack_lvl+0x93/0xd0
> [  195.431150]  handle_overflow+0x171/0x1b0
> [  195.433640]  vfs_fallocate+0x459/0x4f0

Well, we compile the kernel with -fno-strict-overflow for a reason so I
wouldn't consider this a bug. But check_add_overflow() is easier to digest
since we don't have to worry about type details so I'm for this change.

> @@ -319,8 +320,12 @@ int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
>  	if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
>  		return -ENODEV;
>  
> -	/* Check for wrap through zero too */
> -	if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))
> +	/* Check for wraparound */
> +	if (check_add_overflow(offset, len, &sum))
> +		return -EFBIG;
> +
> +	/* Now, check bounds */
> +	if (sum > inode->i_sb->s_maxbytes || sum < 0)
>  		return -EFBIG;

But why do you check for sum < 0? We know from previous checks offset >= 0
&& len > 0 so unless we overflow, sum is guaranteed to be > 0.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR