Re: [PATCH v2] fs: clear file privilege bits when mmap writing

[Kees Cook]

On Wed, Dec 2, 2015 at 4:18 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 2 Dec 2015 16:03:42 -0800 Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
>> Normally, when a user can modify a file that has setuid or setgid bits,
>> those bits are cleared when they are not the file owner or a member
>> of the group. This is enforced when using write and truncate but not
>> when writing to a shared mmap on the file. This could allow the file
>> writer to gain privileges by changing a binary without losing the
>> setuid/setgid/caps bits.
>>
>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>> during the page fault (due to mmap_sem being held during the fault).
>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>>
>> ...
>>
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -1340,6 +1340,17 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>>                       if (locks_verify_locked(file))
>>                               return -EAGAIN;
>>
>> +                     /*
>> +                      * If we must remove privs, we do it here since
>> +                      * doing it during page COW is expensive and
>> +                      * cannot hold inode->i_mutex.
>> +                      */
>> +                     if (prot & PROT_WRITE && !IS_NOSEC(inode)) {
>> +                             mutex_lock(&inode->i_mutex);
>> +                             file_remove_privs(file);
>> +                             mutex_unlock(&inode->i_mutex);
>> +                     }
>> +
>
> Still ignoring the file_remove_privs() return value.  If this is
> deliberate then a description of the reasons should be included?

Actually, there is a bigger problem:
https://lists.01.org/pipermail/lkp/2015-December/003185.html

[   37.741286] trinity-c0/742 is trying to acquire lock:
[   37.741982]  (&sb->s_type->i_mutex_key#8){+.+.+.}, at: [<811c3b34>]
do_mmap+0x544/0x670
[   37.752562]
[   37.752562] but task is already holding lock:
[   37.753442]  (&mm->mmap_sem){++++++}, at: [<811c3d70>]
SyS_remap_file_pages+0xe0/0x350

Jan, any thoughts on avoiding this?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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