linux-kernel - Re: [PATCH v5] fs: clear file privilege bits when mmap writing

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Date:	Thu, 10 Dec 2015 08:06:35 +0100
From:	Willy Tarreau <w@....eu>
To:	Kees Cook <keescook@...omium.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	yalin wang <yalin.wang2010@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] fs: clear file privilege bits when mmap writing

Hi Kees,

Why not add a new file flag instead ?

Something like this (editing your patch by hand to illustrate) :

diff --git a/fs/file_table.c b/fs/file_table.c
index ad17e05ebf95..3a7eee76ea90 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -191,6 +191,17 @@ static void __fput(struct file *file)
 
 	might_sleep();
 
+	/*
+	 * XXX: While avoiding mmap_sem, we've already been written to.
+	 * We must ignore the return value, since we can't reject the
+	 * write.
+	 */
+	if (unlikely(file->f_flags & FL_DROP_PRIVS)) {
+		mutex_lock(&inode->i_mutex);
+		file_remove_privs(file);
+		mutex_unlock(&inode->i_mutex);
+	}
+
 	fsnotify_close(file);
 	/*
 	 * The function eventpoll_release() should be the first called
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3aa514254161..409bd7047e7e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -913,3 +913,4 @@
 #define FL_OFDLCK       1024    /* lock is "owned" by struct file */
 #define FL_LAYOUT       2048    /* outstanding pNFS layout */
+#define FL_DROP_PRIVS   4096    /* lest something weird decides that 2 is OK */
 
diff --git a/mm/memory.c b/mm/memory.c
index c387430f06c3..08a77e0cf65f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2036,6 +2036,7 @@ static inline int wp_page_reuse(struct mm_struct *mm,
 
 		if (!page_mkwrite)
 			file_update_time(vma->vm_file);
+		vma->vm_file->f_flags |= FL_DROP_PRIVS;
 	}
 
 	return VM_FAULT_WRITE;

Willy

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