The limit checking in alloc_pipe_info() (used by pipe(2) and when
opening a FIFO) has the following problems:
(1) When checking capacity required for the new pipe, the checks
against the limit in /proc/sys/fs/pipe-user-pages-{soft,hard}
are made against existing consumption, and exclude the memory
required for the new pipe capacity. As a consequence: (1) the
memory allocation throttling provided by the soft limit does
not kick in quite as early as it should, and (2) the user can
overrun the hard limit.
(2) As currently implemented, accounting and checking against the limits
is done as follows:
(a) Test whether the user has exceeded the limit.
(b) Make new pipe buffer allocation.
(c) Account new allocation against the limits.
This is racey. Multiple processes may pass point (a)
simultaneously, and then allocate pipe buffers that are
accounted for only in step (c). The race means that the
user's pipe buffer allocation could be pushed over the limit
(by an arbitrary amount, depending on how unlucky we were in
the race). [Thanks to Vegard Nossum for spotting this point,
which I had missed.]
This patch addresses the above problems as follows:
* Alter the checks against limits to include the memory required for the
new pipe.
* Re-order the accounting step so that it precedes the buffer allocation.
If the accounting step determines that a limit has been reached, revert
the accounting and cause the operation to fail.
Cc: Willy Tarreau <w@xxxxxx>
Cc: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
Cc: socketpair@xxxxxxxxx
Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: linux-api@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
---
fs/pipe.c | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/fs/pipe.c b/fs/pipe.c
index 613c6b9..705d79f 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -632,24 +632,28 @@ struct pipe_inode_info *alloc_pipe_info(void)
if (pipe == NULL)
goto out_free_uid;
- if (!too_many_pipe_buffers_hard(user)) {
- if (too_many_pipe_buffers_soft(user))
- pipe_bufs = 1;
- pipe->bufs = kcalloc(pipe_bufs,
- sizeof(struct pipe_buffer),
- GFP_KERNEL_ACCOUNT);
- }
+ if (too_many_pipe_buffers_soft(user))
+ pipe_bufs = 1;
+
+ account_pipe_buffers(user, 0, pipe_bufs);
+
+ if (too_many_pipe_buffers_hard(user))
+ goto out_revert_acct;
+
+ pipe->bufs = kcalloc(pipe_bufs, sizeof(struct pipe_buffer),
+ GFP_KERNEL_ACCOUNT);
if (pipe->bufs) {
init_waitqueue_head(&pipe->wait);
pipe->r_counter = pipe->w_counter = 1;
pipe->buffers = pipe_bufs;
pipe->user = user;
- account_pipe_buffers(user, 0, pipe_bufs);
mutex_init(&pipe->mutex);
return pipe;
}
+out_revert_acct:
+ account_pipe_buffers(user, pipe_bufs, 0);
kfree(pipe);
out_free_uid:
free_uid(user);
--
2.5.5
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