Re: [PATCH] locks: Fix UBSAN undefined behaviour in flock64_to_posix_lock

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Oct 22, 2020 at 10:03:41AM +0800, Luo Meng wrote:
> When the sum of fl->fl_start and l->l_len overflows,
> UBSAN shows the following warning:
> 
> UBSAN: Undefined behaviour in fs/locks.c:482:29
> signed integer overflow: 2 + 9223372036854775806
> cannot be represented in type 'long long int'
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0xe4/0x14e lib/dump_stack.c:118
>  ubsan_epilogue+0xe/0x81 lib/ubsan.c:161
>  handle_overflow+0x193/0x1e2 lib/ubsan.c:192
>  flock64_to_posix_lock fs/locks.c:482 [inline]
>  flock_to_posix_lock+0x595/0x690 fs/locks.c:515
>  fcntl_setlk+0xf3/0xa90 fs/locks.c:2262
>  do_fcntl+0x456/0xf60 fs/fcntl.c:387
>  __do_sys_fcntl fs/fcntl.c:483 [inline]
>  __se_sys_fcntl fs/fcntl.c:468 [inline]
>  __x64_sys_fcntl+0x12d/0x180 fs/fcntl.c:468
>  do_syscall_64+0xc8/0x5a0 arch/x86/entry/common.c:293
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> Fix it by moving -1 forward.
> 
> Signed-off-by: Luo Meng <luomeng12@huawei.com>
> ---
>  fs/locks.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index 1f84a03601fe..8489787ca97e 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -542,7 +542,7 @@ static int flock64_to_posix_lock(struct file *filp, struct file_lock *fl,
>  	if (l->l_len > 0) {
>  		if (l->l_len - 1 > OFFSET_MAX - fl->fl_start)
>  			return -EOVERFLOW;
> -		fl->fl_end = fl->fl_start + l->l_len - 1;
> +		fl->fl_end = fl->fl_start - 1 + l->l_len;
>  

Given what the bounds check just above does, wouldn't it make more sense to
parenthesize 'l->l_len - 1' instead?  So:

		fl->fl_end = fl->fl_start + (l->l_len - 1);

Also FWIW, the Linux kernel uses the -fwrapv compiler flag, so signed integer
overflow is defined.  IMO it's still best avoided though...

- Eric