Fix: compat_rw_copy_check_uvector() misuse in aio, readv, writev, and security keys

Looking at mm/process_vm_access.c:process_vm_rw() and comparing it to
compat_process_vm_rw() shows that the compatibility code requires an
explicit "access_ok()" check before calling
compat_rw_copy_check_uvector(). The same difference seems to appear when
we compare fs/read_write.c:do_readv_writev() to
fs/compat.c:compat_do_readv_writev().

This subtle difference between the compat and non-compat requirements
should probably be debated, as it seems to be error-prone. In fact,
there are two others sites that use this function in the Linux kernel,
and they both seem to get it wrong:

Now shifting our attention to fs/aio.c, we see that aio_setup_iocb()
also ends up calling compat_rw_copy_check_uvector() through
aio_setup_vectored_rw(). Unfortunately, the access_ok() check appears to
be missing. Same situation for
security/keys/compat.c:compat_keyctl_instantiate_key_iov().

I propose that we add the access_ok() check directly into
compat_rw_copy_check_uvector(), so callers don't have to worry about it,
and it therefore makes the compat call code similar to its non-compat
counterpart. Place the access_ok() check in the same location where
copy_from_user() can trigger a -EFAULT error in the non-compat code, so
the ABI behaviors are alike on both compat and non-compat.

While we are here, fix compat_do_readv_writev() so it checks for
compat_rw_copy_check_uvector() negative return values.

And also, fix a memory leak in compat_keyctl_instantiate_key_iov() error
handling.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 8 deletions

diff --git a/fs/compat.c b/fs/compat.c
index fe40fde29111..d487985dd0ea 100644
--- a/fs/compat.c
+++ b/fs/compat.c
@@ -558,6 +558,10 @@ ssize_t compat_rw_copy_check_uvector(int type,
         }
         *ret_pointer = iov;
 
+        ret = -EFAULT;
+        if (!access_ok(VERIFY_READ, uvector, nr_segs*sizeof(*uvector)))
+                goto out;
+
         /*
          * Single unix specification:
          * We should -EINVAL if an element length is not >= 0 and fitting an
@@ -1080,17 +1084,12 @@ static ssize_t compat_do_readv_writev(int type, struct file *file,
         if (!file->f_op)
                 goto out;
 
-        ret = -EFAULT;
-        if (!access_ok(VERIFY_READ, uvector, nr_segs*sizeof(*uvector)))
-                goto out;
-
-        tot_len = compat_rw_copy_check_uvector(type, uvector, nr_segs,
+        ret = compat_rw_copy_check_uvector(type, uvector, nr_segs,
                                                UIO_FASTIOV, iovstack, &iov);
-        if (tot_len == 0) {
-                ret = 0;
+        if (ret <= 0)
                 goto out;
-        }
 
+        tot_len = ret;
         ret = rw_verify_area(type, file, pos, tot_len);
         if (ret < 0)
                 goto out;