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

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2020-10-22 at 10:03 +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;
>  
>  	} else if (l->l_len < 0) {
>  		if (fl->fl_start + l->l_len < 0)

Wow, ok. Interesting that the order would have such an effect here, but
it seems legit. I'll plan to merge this for v5.11. Let me know if we
need to get this in earlier.

Thanks!
-- 
Jeff Layton <jlayton@kernel.org>