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Message-ID: <20160524143955.GA28161@kroah.com> Date: Tue, 24 May 2016 07:39:55 -0700 From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> To: Nicolai Stange <nicstange@...il.com> Cc: Sasha Levin <sasha.levin@...cle.com>, Andrew Morton <akpm@...ux-foundation.org>, Dmitry Vyukov <dvyukov@...gle.com>, Kees Cook <keescook@...omium.org>, Andrey Ryabinin <aryabinin@...tuozzo.com>, James Morse <james.morse@....com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH v2] kernel/kcov: unproxify debugfs file's fops On Tue, May 24, 2016 at 02:05:05PM +0200, Nicolai Stange wrote: > Since commit 49d200deaa68 ("debugfs: prevent access to removed files' > private data"), a debugfs file's file_operations methods get proxied > through lifetime aware wrappers. > > However, only a certain subset of the file_operations members is supported > by debugfs and ->mmap isn't among them -- it appears to be NULL from the > VFS layer's perspective. > > This behaviour breaks the /sys/kernel/debug/kcov file introduced > concurrently with commit 5c9a8750a640 ("kernel: add kcov code coverage"). > > Since that file never gets removed, there is no file removal race and thus, > a lifetime checking proxy isn't needed. > > Avoid the proxying for /sys/kernel/debug/kcov by creating it via > debugfs_create_file_unsafe() rather than debugfs_create_file(). > > Fixes: 49d200deaa68 ("debugfs: prevent access to removed files' private > data") > Fixes: 5c9a8750a640 ("kernel: add kcov code coverage") > Signed-off-by: Nicolai Stange <nicstange@...il.com> > --- > The v1 thread can be found at > http://lkml.kernel.org/g/1464011147-31836-1-git-send-email-nicstange@gmail.com > > Changes to v1: > - Following the suggestion of Kees Cook, a comment explaining why the use > of debugfs_create_file_unsafe() is actually safe there has been added. > > This issue has been debugged and reported by > Sasha Levin <sasha.levin@...cle.com>: > http://lkml.kernel.org/g/573F4200.3080208@oracle.com > > Applicable to linux-next 20160524. > In particular, it depends on > - c64688081490 ("debugfs: add support for self-protecting attribute file > fops") > - 5c9a8750a640 ("kernel: add kcov code coverage") > > kernel/kcov.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/kernel/kcov.c b/kernel/kcov.c > index a02f2dd..8d44b3f 100644 > --- a/kernel/kcov.c > +++ b/kernel/kcov.c > @@ -264,7 +264,12 @@ static const struct file_operations kcov_fops = { > > static int __init kcov_init(void) > { > - if (!debugfs_create_file("kcov", 0600, NULL, NULL, &kcov_fops)) { > + /* > + * The kcov debugfs file won't ever get removed and thus, > + * there is no need to protect it against removal races. The > + * use of debugfs_create_file_unsafe() is actually safe here. > + */ > + if (!debugfs_create_file_unsafe("kcov", 0600, NULL, NULL, &kcov_fops)) { > pr_err("failed to create kcov in debugfs\n"); > return -ENOMEM; > } Thanks, I'll queue this up after 4.7-rc1 is out. greg k-h
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