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Message-Id: <1464091505-20943-1-git-send-email-nicstange@gmail.com> Date: Tue, 24 May 2016 14:05:05 +0200 From: Nicolai Stange <nicstange@...il.com> To: Greg Kroah-Hartman <gregkh@...uxfoundation.org> 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, Nicolai Stange <nicstange@...il.com> Subject: [PATCH v2] kernel/kcov: unproxify debugfs file's fops 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; } -- 2.8.2
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