Make file credentials available to the seqfile interfaces (34dbbcdb) · Commits · 戴 / test

A lot of seqfile users seem to be using things like %pK that uses the credentials of the current process, but that is actually completely wrong for filesystem interfaces. The unix semantics for permission checking files is to check permissions at _open_ time, not at read or write time, and that is not just a small detail: passing off stdin/stdout/stderr to a suid application and making the actual IO happen in privileged context is a classic exploit technique. So if we want to be able to look at permissions at read time, we need to use the file open credentials, not the current ones. Normal file accesses can just use "f_cred" (or any of the helper functions that do that, like file_ns_capable()), but the seqfile interfaces do not have any such options. It turns out that seq_file _does_ save away the user_ns information of the file, though. Since user_ns is just part of the full credential information, replace that special case with saving off the cred pointer instead, and suddenly seq_file has all the permission information it needs. Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

fs/seq_file.c

+4 −3

Original line number	Diff line number	Diff line
		@@ -72,9 +72,10 @@ int seq_open(struct file file, const struct seq_operations op)

		mutex_init(&p->lock);
		p->op = op;
		#ifdef CONFIG_USER_NS
		p->user_ns = file->f_cred->user_ns;
		#endif

		// No refcounting: the lifetime of 'p' is constrained
		// to the lifetime of the file.
		p->file = file;

		/*
		* Wrappers around seq_open(e.g. swaps_open) need to be

include/linux/seq_file.h

+4 −9

Original line number	Diff line number	Diff line
		@@ -7,13 +7,10 @@
		#include <linux/mutex.h>
		#include <linux/cpumask.h>
		#include <linux/nodemask.h>
		#include <linux/fs.h>
		#include <linux/cred.h>

		struct seq_operations;
		struct file;
		struct path;
		struct inode;
		struct dentry;
		struct user_namespace;

		struct seq_file {
		char *buf;
		@@ -27,9 +24,7 @@ struct seq_file {
		struct mutex lock;
		const struct seq_operations *op;
		int poll_event;
		#ifdef CONFIG_USER_NS
		struct user_namespace *user_ns;
		#endif
		const struct file *file;
		void *private;
		};

		@@ -147,7 +142,7 @@ int seq_release_private(struct inode , struct file );
		static inline struct user_namespace seq_user_ns(struct seq_file seq)
		{
		#ifdef CONFIG_USER_NS
		return seq->user_ns;
		return seq->file->f_cred->user_ns;
		#else
		extern struct user_namespace init_user_ns;
		return &init_user_ns;

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