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Date: Mon, 13 May 2024 13:01:57 -0700
From: Kees Cook <keescook@chromium.org>
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>, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH v2] fs: fix unintentional arithmetic wraparound in offset
 calculation
Message-ID: <202405131251.6FD48B6A8@keescook>
References: <20240509-b4-sio-read_write-v2-1-018fc1e63392@google.com>
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In-Reply-To: <20240509-b4-sio-read_write-v2-1-018fc1e63392@google.com>

On Thu, May 09, 2024 at 11:42:07PM +0000, Justin Stitt wrote:
> When running syzkaller with the newly reintroduced signed integer
> overflow sanitizer we encounter this report:
> 
> [   67.995501] UBSAN: signed-integer-overflow in ../fs/read_write.c:91:10
> [   68.000067] 9223372036854775807 + 4096 cannot be represented in type 'loff_t' (aka 'long long')
> [   68.006266] CPU: 4 PID: 10851 Comm: syz-executor.5 Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1
> [   68.012353] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> [   68.018983] Call Trace:
> [   68.020803]  <TASK>
> [   68.022540]  dump_stack_lvl+0x93/0xd0
> [   68.025222]  handle_overflow+0x171/0x1b0
> [   68.028053]  generic_file_llseek_size+0x35b/0x380
> 
> amongst others:
> UBSAN: signed-integer-overflow in ../fs/read_write.c:1657:12
> 142606336 - -9223372036854775807 cannot be represented in type 'loff_t' (aka 'long long')
> ...
> UBSAN: signed-integer-overflow in ../fs/read_write.c:1666:11
> 9223372036854775807 - -9223231299366420479 cannot be represented in type 'loff_t' (aka 'long long')
> 
> 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").
> 
> Fix the accidental overflow in these position and offset calculations
> by checking for negative position values, using check_add_overflow()
> helpers and clamping values to expected ranges.
> 
> Since @offset is later limited by @maxsize, we can proactively safeguard
> against exceeding that value (and by extension avoiding integer overflow):
> 	loff_t vfs_setpos(struct file *file, loff_t offset, loff_t maxsize)
> 	{
> 		if (offset < 0 && !unsigned_offsets(file))
> 			return -EINVAL;
> 		if (offset > maxsize)
> 			return -EINVAL;
> 		...
> 
> Link: https://github.com/llvm/llvm-project/pull/82432 [1]
> Closes: https://github.com/KSPP/linux/issues/358
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@google.com>
> ---
> Changes in v2:
> - fix some more cases syzkaller found in read_write.c
> - use min over min_t as the types are the same
> - Link to v1: https://lore.kernel.org/r/20240509-b4-sio-read_write-v1-1-06bec2022697@google.com
> ---
> Here's the syzkaller reproducer:
> | # {Threaded:false Repeat:false RepeatTimes:0 Procs:1 Slowdown:1 Sandbox:
> | # SandboxArg:0 Leak:false NetInjection:false NetDevices:false
> | # NetReset:false Cgroups:false BinfmtMisc:false CloseFDs:false KCSAN:false
> | # DevlinkPCI:false NicVF:false USB:false VhciInjection:false Wifi:false
> | # IEEE802154:false Sysctl:false Swap:false UseTmpDir:false
> | # HandleSegv:false Repro:false Trace:false LegacyOptions:{Collide:false
> | # Fault:false FaultCall:0 FaultNth:0}}
> | r0 = openat$sysfs(0xffffffffffffff9c, &(0x7f0000000000)='/sys/kernel/address_bits', 0x0, 0x98)
> | lseek(r0, 0x7fffffffffffffff, 0x2)
> 
> ... 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/read_write.c  | 18 +++++++++++-------
>  fs/remap_range.c | 12 ++++++------
>  2 files changed, 17 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/read_write.c b/fs/read_write.c
> index d4c036e82b6c..d116e6e3eb3d 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -88,7 +88,7 @@ generic_file_llseek_size(struct file *file, loff_t offset, int whence,
>  {
>  	switch (whence) {
>  	case SEEK_END:
> -		offset += eof;
> +		offset = min(offset, maxsize - eof) + eof;

This seems effectively unchanged compared to v1?

https://lore.kernel.org/all/CAFhGd8qbUYXmgiFuLGQ7dWXFUtZacvT82wD4jSS-xNTvtzXKGQ@mail.gmail.com/

>  		break;
>  	case SEEK_CUR:
>  		/*
> @@ -105,7 +105,8 @@ generic_file_llseek_size(struct file *file, loff_t offset, int whence,
>  		 * like SEEK_SET.
>  		 */
>  		spin_lock(&file->f_lock);
> -		offset = vfs_setpos(file, file->f_pos + offset, maxsize);
> +		offset = vfs_setpos(file, min(file->f_pos, maxsize - offset) +
> +					      offset, maxsize);
>  		spin_unlock(&file->f_lock);
>  		return offset;
>  	case SEEK_DATA:
> @@ -1416,7 +1417,7 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
>  	struct inode *inode_in = file_inode(file_in);
>  	struct inode *inode_out = file_inode(file_out);
>  	uint64_t count = *req_count;
> -	loff_t size_in;
> +	loff_t size_in, in_sum, out_sum;
>  	int ret;
>  
>  	ret = generic_file_rw_checks(file_in, file_out);
> @@ -1450,8 +1451,8 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
>  	if (IS_SWAPFILE(inode_in) || IS_SWAPFILE(inode_out))
>  		return -ETXTBSY;
>  
> -	/* Ensure offsets don't wrap. */
> -	if (pos_in + count < pos_in || pos_out + count < pos_out)
> +	if (check_add_overflow(pos_in, count, &in_sum) ||
> +	    check_add_overflow(pos_out, count, &out_sum))
>  		return -EOVERFLOW;

I like these changes -- they make this much more readable.

>  
>  	/* Shorten the copy to EOF */
> @@ -1467,8 +1468,8 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
>  
>  	/* Don't allow overlapped copying within the same file. */
>  	if (inode_in == inode_out &&
> -	    pos_out + count > pos_in &&
> -	    pos_out < pos_in + count)
> +	    out_sum > pos_in &&
> +	    pos_out < in_sum)
>  		return -EINVAL;
>  
>  	*req_count = count;
> @@ -1649,6 +1650,9 @@ int generic_write_check_limits(struct file *file, loff_t pos, loff_t *count)
>  	loff_t max_size = inode->i_sb->s_maxbytes;
>  	loff_t limit = rlimit(RLIMIT_FSIZE);
>  
> +	if (pos < 0)
> +		return -EINVAL;
> +
>  	if (limit != RLIM_INFINITY) {
>  		if (pos >= limit) {
>  			send_sig(SIGXFSZ, current, 0);
> diff --git a/fs/remap_range.c b/fs/remap_range.c
> index de07f978ce3e..4570be4ef463 100644
> --- a/fs/remap_range.c
> +++ b/fs/remap_range.c
> @@ -36,7 +36,7 @@ static int generic_remap_checks(struct file *file_in, loff_t pos_in,
>  	struct inode *inode_out = file_out->f_mapping->host;
>  	uint64_t count = *req_count;
>  	uint64_t bcount;
> -	loff_t size_in, size_out;
> +	loff_t size_in, size_out, in_sum, out_sum;
>  	loff_t bs = inode_out->i_sb->s_blocksize;
>  	int ret;
>  
> @@ -44,17 +44,17 @@ static int generic_remap_checks(struct file *file_in, loff_t pos_in,
>  	if (!IS_ALIGNED(pos_in, bs) || !IS_ALIGNED(pos_out, bs))
>  		return -EINVAL;
>  
> -	/* Ensure offsets don't wrap. */
> -	if (pos_in + count < pos_in || pos_out + count < pos_out)
> -		return -EINVAL;
> +	if (check_add_overflow(pos_in, count, &in_sum) ||
> +	    check_add_overflow(pos_out, count, &out_sum))
> +		return -EOVERFLOW;

Yeah, this is a good error code change. This is ultimately exposed via
copy_file_range, where this error is documented as possible.

-Kees

-- 
Kees Cook