watch_queue: Fix to always request a pow-of-2 pipe ring size

commit 96a4d8912b28451cd62825fd7caa0e66e091d938 upstream.

The pipe ring size must always be a power of 2 as the head and tail
pointers are masked off by AND'ing with the size of the ring - 1.
watch_queue_set_size(), however, lets you specify any number of notes
between 1 and 511.  This number is passed through to pipe_resize_ring()
without checking/forcing its alignment.

Fix this by rounding the number of slots required up to the nearest
power of two.  The request is meant to guarantee that at least that many
notifications can be generated before the queue is full, so rounding
down isn't an option, but, alternatively, it may be better to give an
error if we aren't allowed to allocate that much ring space.

Fixes: c73be61cede5 ("pipe: Add general notification queue support")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index dfb3a7e28280469416826a264414960cd10648f7..4bcd400984a7a4cf0705c465aa6b9f593db41877 100644 (file)
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -244,7 +244,7 @@ long watch_queue_set_size(struct pipe_inode_info *pipe, unsigned int nr_notes)
                goto error;
        }
 
-       ret = pipe_resize_ring(pipe, nr_notes);
+       ret = pipe_resize_ring(pipe, roundup_pow_of_two(nr_notes));
        if (ret < 0)
                goto error;