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Date: Tue, 14 May 2024 16:33:15 +0200
From: Jan Kara <jack@suse.cz>
To: Justin Stitt <justinstitt@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>,
	Nathan Chancellor <nathan@kernel.org>,
	Bill Wendling <morbo@google.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev, linux-hardening@vger.kernel.org,
	Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v2] fs: remove accidental overflow during wraparound check
Message-ID: <20240514143315.wxs3hnetssth2xt5@quack3>
References: <20240513-b4-sio-vfs_fallocate-v2-1-db415872fb16@google.com>
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In-Reply-To: <20240513-b4-sio-vfs_fallocate-v2-1-db415872fb16@google.com>

On Mon 13-05-24 17:50:30, 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
> ...
> [  195.490053] ------------[ cut here ]------------
> [  195.493146] UBSAN: signed-integer-overflow in ../fs/open.c:321:61
> [  195.497030] 9223372036854775807 + 562984447377399 cannot be represented in type 'loff_t' (aka 'long long)
> [  195.502940] CPU: 1 PID: 703 Comm: syz-executor.0 Not tainted 6.8.0-rc2-00039-g14de58dbe653-dirty #11
> [  195.508395] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> [  195.514075] Call Trace:
> [  195.515636]  <TASK>
> [  195.517000]  dump_stack_lvl+0x93/0xd0
> [  195.519255]  handle_overflow+0x171/0x1b0
> [  195.521677]  vfs_fallocate+0x4cb/0x4f0
> [  195.524033]  __x64_sys_fallocate+0xb2/0xf0
> 
> Historically, the signed integer overflow sanitizer did not work in the
> kernel due to its interaction with `-fwrapv` but this has since been
> changed [1] in the newest version of Clang. It was re-enabled in the
> kernel with Commit 557f8c582a9ba8ab ("ubsan: Reintroduce signed overflow
> sanitizer").
> 
> Let's use the check_add_overflow helper to first verify the addition
> stays within the bounds of its type (long long); then we can use that
> sum for the following check.
> 
> Link: https://github.com/llvm/llvm-project/pull/82432 [1]
> Closes: https://github.com/KSPP/linux/issues/356
> Cc: linux-hardening@vger.kernel.org
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Justin Stitt <justinstitt@google.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
> Changes in v2:
> - drop the sum < 0 check (thanks Jan)
> - carry along Kees' RB tag
> - Link to v1: https://lore.kernel.org/r/20240507-b4-sio-vfs_fallocate-v1-1-322f84b97ad5@google.com
> ---
> Here's the syzkaller reproducer:
> r0 = openat(0xffffffffffffff9c, &(0x7f0000000040)='./file1\x00', 0x42, 0x0)
> fallocate(r0, 0x10, 0x7fffffffffffffff, 0x2000807fffff7)
> 
> ... which was used against Kees' tree here (v6.8rc2):
> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=wip/v6.9-rc2/unsigned-overflow-sanitizer
> 
> ... with this config:
> https://gist.github.com/JustinStitt/824976568b0f228ccbcbe49f3dee9bf4
> ---
>  fs/open.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/open.c b/fs/open.c
> index ee8460c83c77..23849d487479 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -247,6 +247,7 @@ int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
>  {
>  	struct inode *inode = file_inode(file);
>  	long ret;
> +	loff_t sum;
>  
>  	if (offset < 0 || len <= 0)
>  		return -EINVAL;
> @@ -319,8 +320,11 @@ 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;
> +
> +	if (sum > inode->i_sb->s_maxbytes)
>  		return -EFBIG;
>  
>  	if (!file->f_op->fallocate)
> 
> ---
> base-commit: 0106679839f7c69632b3b9833c3268c316c0a9fc
> change-id: 20240507-b4-sio-vfs_fallocate-7b5223ba3a81
> 
> Best regards,
> --
> Justin Stitt <justinstitt@google.com>
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR