Re: [PATCH] prctl: fix PR_SET_MM_AUXV kernel stack leak

Dan Carpenter
2021-03-15T10:30:18Z

On Sun, Mar 14, 2021 at 11:51:14PM +0300, Alexey Dobriyan wrote:
> prctl(PR_SET_MM, PR_SET_MM_AUXV, addr, 1);
>
> will copy 1 byte from userspace to (quite big) on-stack array
> and then stash everything to mm->saved_auxv.

What? It won't save everything, only the 1 byte. What am I not seeing?

kernel/sys.c
2073 static int prctl_set_auxv(struct mm_struct *mm, unsigned long addr,
2074 unsigned long len)
2075 {
2076 /*
2077 * This doesn't move the auxiliary vector itself since it's pinned to
2078 * mm_struct, but it permits filling the vector with new values. It's
2079 * up to the caller to provide sane values here, otherwise userspace
2080 * tools which use this vector might be unhappy.
2081 */
2082 unsigned long user_auxv[AT_VECTOR_SIZE] = {};
2083
2084 if (len > sizeof(user_auxv))
2085 return -EINVAL;
2086
2087 if (copy_from_user(user_auxv, (const void __user *)addr, len))
^^^^^^^^^ ^^^
Copies one byte from user space.

2088 return -EFAULT;
2089
2090 /* Make sure the last entry is always AT_NULL */
2091 user_auxv[AT_VECTOR_SIZE - 2] = 0;
2092 user_auxv[AT_VECTOR_SIZE - 1] = 0;
2093
2094 BUILD_BUG_ON(sizeof(user_auxv) != sizeof(mm->saved_auxv));
2095
2096 task_lock(current);
2097 memcpy(mm->saved_auxv, user_auxv, len);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Saves one byte to mm->saved_auxv.

2098 task_unlock(current);
2099
2100 return 0;
2101 }

regards,
dan carpenter