From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:53184 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726064AbeJPEru (ORCPT ); Tue, 16 Oct 2018 00:47:50 -0400 Date: Mon, 15 Oct 2018 14:00:52 -0700 From: Eric Biggers To: Jann Horn Cc: Al Viro , Miklos Szeredi , Jens Axboe , Jens Axboe , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Kees Cook Subject: Re: [PATCH 1/2] splice: don't merge into linked buffers Message-ID: <20181015210051.GA20855@gmail.com> References: <20181015150420.2096-1-jannh@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181015150420.2096-1-jannh@google.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Mon, Oct 15, 2018 at 05:04:18PM +0200, Jann Horn wrote: > Before this patch, it was possible for two pipes to affect each other after > data had been transferred between them with tee(): > > ============ > $ cat tee_test.c > > int main(void) { > int pipe_a[2]; > if (pipe(pipe_a)) err(1, "pipe"); > int pipe_b[2]; > if (pipe(pipe_b)) err(1, "pipe"); > if (write(pipe_a[1], "abcd", 4) != 4) err(1, "write"); > if (tee(pipe_a[0], pipe_b[1], 2, 0) != 2) err(1, "tee"); > if (write(pipe_b[1], "xx", 2) != 2) err(1, "write"); > > char buf[5]; > if (read(pipe_a[0], buf, 4) != 4) err(1, "read"); > buf[4] = 0; > printf("got back: '%s'\n", buf); > } > $ gcc -o tee_test tee_test.c > $ ./tee_test > got back: 'abxx' > $ > ============ > > Fix it by explicitly marking buffers as mergeable and clearing that flag in > splice_pipe_to_pipe() and link_pipe(). > > Cc: > Fixes: 7c77f0b3f920 ("splice: implement pipe to pipe splicing") > Fixes: 70524490ee2e ("[PATCH] splice: add support for sys_tee()") > Signed-off-by: Jann Horn > --- > Cleanup in the next patch, to simplify backporting. > > fs/pipe.c | 5 +++-- > fs/splice.c | 6 ++++++ > include/linux/pipe_fs_i.h | 8 ++++++++ > 3 files changed, 17 insertions(+), 2 deletions(-) > > diff --git a/fs/pipe.c b/fs/pipe.c > index bdc5d3c0977d..4e2eee77f855 100644 > --- a/fs/pipe.c > +++ b/fs/pipe.c > @@ -379,7 +379,8 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from) > struct pipe_buffer *buf = pipe->bufs + lastbuf; > int offset = buf->offset + buf->len; > > - if (buf->ops->can_merge && offset + chars <= PAGE_SIZE) { > + if (buf->ops->can_merge && offset + chars <= PAGE_SIZE && > + (buf->flags & PIPE_BUF_FLAG_MAYMERGE)) { > ret = pipe_buf_confirm(pipe, buf); > if (ret) > goto out; > @@ -439,7 +440,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from) > buf->ops = &anon_pipe_buf_ops; > buf->offset = 0; > buf->len = copied; > - buf->flags = 0; > + buf->flags = PIPE_BUF_FLAG_MAYMERGE; > if (is_packetized(filp)) { > buf->ops = &packet_pipe_buf_ops; > buf->flags = PIPE_BUF_FLAG_PACKET; > diff --git a/fs/splice.c b/fs/splice.c > index b3daa971f597..111977c80dfd 100644 > --- a/fs/splice.c > +++ b/fs/splice.c > @@ -1593,6 +1593,9 @@ static int splice_pipe_to_pipe(struct pipe_inode_info *ipipe, > */ > obuf->flags &= ~PIPE_BUF_FLAG_GIFT; > > + /* We can't merge data into a buffer we don't own. */ > + obuf->flags &= ~PIPE_BUF_FLAG_MAYMERGE; > + > obuf->len = len; > opipe->nrbufs++; > ibuf->offset += obuf->len; > @@ -1667,6 +1670,9 @@ static int link_pipe(struct pipe_inode_info *ipipe, > */ > obuf->flags &= ~PIPE_BUF_FLAG_GIFT; > > + /* We can't merge data into a buffer we don't own. */ > + obuf->flags &= ~PIPE_BUF_FLAG_MAYMERGE; > + > if (obuf->len > len) > obuf->len = len; > > diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h > index 5a3bb3b7c9ad..8893711f9171 100644 > --- a/include/linux/pipe_fs_i.h > +++ b/include/linux/pipe_fs_i.h > @@ -8,6 +8,14 @@ > #define PIPE_BUF_FLAG_ATOMIC 0x02 /* was atomically mapped */ > #define PIPE_BUF_FLAG_GIFT 0x04 /* page is a gift */ > #define PIPE_BUF_FLAG_PACKET 0x08 /* read() as a packet */ > +/* > + * Set this flag if the generic pipe read/write may coalesce data into an > + * existing buffer. If this is not set, a new pipe page segment is always used > + * for new data. > + * When pipe data is copied by reference (as in the tee() syscall), this flag > + * must be cleared on the copy. > + */ > +#define PIPE_BUF_FLAG_MAYMERGE 0x10 > > /** > * struct pipe_buffer - a linux kernel pipe buffer > -- > 2.19.0.605.g01d371f741-goog > Deja vu... https://marc.info/?l=linux-fsdevel&m=149003133809192 Thanks for fixing this; I think this is the right fix. I verified it works for my reproducer too. Did you check whether fuse_dev_splice_write() needs to clear PIPE_BUF_FLAG_MAYMERGE? - Eric