linux-kernel - Re: [PATCH 1/2] module: add syscall to load module from fd

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Date:	Fri, 7 Sep 2012 09:19:30 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	linux-kernel@...r.kernel.org,
	Serge Hallyn <serge.hallyn@...onical.com>,
	James Morris <james.l.morris@...cle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Eric Paris <eparis@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	linux-security-module@...r.kernel.org
Subject: Re: [PATCH 1/2] module: add syscall to load module from fd

On Thu, Sep 6, 2012 at 5:15 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
> Kees Cook <keescook@...omium.org> writes:
>> Instead of (or in addition to) kernel module signing, being able to reason
>> about the origin of a kernel module would be valuable in situations
>> where an OS already trusts a specific file system, file, etc, due to
>> things like security labels or an existing root of trust to a partition
>> through things like dm-verity.
>>
>> This introduces a new syscall (currently only on x86), similar to
>> init_module, that has only two arguments. The first argument is used as
>> a file descriptor to the module and the second argument is a pointer to
>> the NULL terminated string of module arguments.
>
> Thanks.  Minor comments follow:
>
>> +350  i386    init_module_from_fd     sys_init_module_from_fd
>
> The from_ seems redundant.

I'm happy to drop it.

>> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
>> index 19439c7..5386629 100644
>> --- a/include/linux/syscalls.h
>> +++ b/include/linux/syscalls.h
>> @@ -860,4 +860,5 @@ asmlinkage long sys_process_vm_writev(pid_t pid,
>>
>>  asmlinkage long sys_kcmp(pid_t pid1, pid_t pid2, int type,
>>                        unsigned long idx1, unsigned long idx2);
>> +asmlinkage long sys_init_module_from_fd(int len, const char __user *uargs);
>>  #endif
>
> You mean, "int fd"?

Whoops. Thanks, I'll fix that.

>> diff --git a/kernel/module.c b/kernel/module.c
>> index 4edbd9c..b080cf8 100644
>> --- a/kernel/module.c
>> +++ b/kernel/module.c
> ...
>> +/* Sets info->hdr and info->len. */
>> +int copy_module_from_fd(int fd, struct load_info *info)
>> +{
>> +     struct file *file;
>> +     int err;
>> +     struct kstat stat;
>> +     unsigned long size;
>> +     off_t pos;
>> +     ssize_t bytes = 0;
>> +
>> +     file = fget(fd);
>> +     if (!file)
>> +             return -ENOEXEC;
>> +
>> +     err = vfs_getattr(file->f_vfsmnt, file->f_dentry, &stat);
>> +     if (err)
>> +             goto out;
>> +
>> +     if (stat.size > INT_MAX) {
>> +             err = -ENOMEM;
>> +             goto out;
>>       }
>> +     size = stat.size;
>>
>> -     if (hdr->e_shoff >= len ||
>> -         hdr->e_shnum * sizeof(Elf_Shdr) > len - hdr->e_shoff) {
>> -             err = -ENOEXEC;
>> -             goto free_hdr;
>> +     info->hdr = vmalloc(size);
>> +     if (!info->hdr) {
>> +             err = -ENOMEM;
>> +             goto out;
>
> The stat.size > INT_MAX is redundant: vmalloc is quite careful on its
> checking of the size param.
>
> (We removed a similar test from the module.c code a few years ago).

Fair enough, I'll remove that too and send an updated version.

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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