Created attachment 757373 [details]
PoC

In ./fs/ext4/balloc.c the "test_root" inline function is vulnerable to
'infinite' loop.

static inline int test_root(ext4_group_t a, int b)
{
    int num = b;

    while (a > num)
        num *= b;
    return num == a;
}

The 'a' argument is controlled from the userspace and its type is an
'unsigned int'. If 'a' value is 0xffffffff, we will seldom break the
'while' condition. With CAP_SYS_RESOURCE, an unprivileged user could 
use this flaw to cause a denial of service.

We can trigger this 'infinite' loop with the attached PoC.