On 15-12-31 04:30:19, Al Viro wrote:
> On Thu, Dec 31, 2015 at 03:24:53PM +1100, Stephen Rothwell wrote:
> > Hi James,
> >
> > Today's linux-next merge of the security tree got a conflict in:
> >
> > security/integrity/ima/ima_fs.c
> >
> > between commit:
> >
> > 3bc8f29b149e ("new helper: memdup_user_nul()")
> >
> > from the vfs tree and commit:
> >
> > 38d859f991f3 ("IMA: policy can now be updated multiple times")
> >
> > from the security tree.
> >
> > I fixed it up (hopefully, see below) and can carry the fix as necessary
> > (no action is required).
>
> > + res = mutex_lock_interruptible(&ima_write_mutex);
> > + if (res)
> > + return res;
> >
> > if (datalen >= PAGE_SIZE)
> > datalen = PAGE_SIZE - 1;
> >
> > /* No partial writes. */
> > + result = -EINVAL;
> > if (*ppos != 0)
> > - return -EINVAL;
> > + goto out;
> >
> > - result = -ENOMEM;
> > - data = kmalloc(datalen + 1, GFP_KERNEL);
> > - if (!data)
> > - goto out;
> > -
> > - *(data + datalen) = '\0';
> > -
> > - result = -EFAULT;
> > - if (copy_from_user(data, buf, datalen))
> > + data = memdup_user_nul(buf, datalen);
> > - if (IS_ERR(data))
> > - return PTR_ERR(data);
> > ++ if (IS_ERR(data)) {
> > ++ result = PTR_ERR(data);
> > + goto out;
> > ++ }
>
> Why do it in this order? With or without opencoding memdup_user_nul(),
> what's the point of taking the mutex before copying the data from
> userland? All it achieves is holding it longer, over the area that
> needs no exclusion whatsoever.
I introduced the write mutex when ima_write_policy() stopped being serialized by
other means. Come to think about it the semaphore could be taken right before
copy_from_user() so it is my fault, not Stephen's.
The patch, however, leaves out a bug where free without allocation can occur.
Look at *ppos evaluation. Instead of "goto out" it should be "return -EINVAL;".
This requires the mutex lock to be moved down, though.
cheers,
Petko
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