[PATCH] bsdacct: switch credentials for writing to the accounting file

[PATCH] bsdacct: switch credentials for writing to the accounting file

[Michal Schmidt]

When process accounting is enabled, every exiting process writes a log
to the account file. In addition, every once in a while one of the
exiting processes checks whether there's enough free space for the log.

SELinux policy may or may not allow the exiting process to stat the fs.
So unsuspecting processes start generating AVC denials just because
someone enabled process accounting.

For these filesystem operations, the exiting process's credentials
should be temporarily switched to that of the process which enabled
accounting, because it's really that process who wanted to have the
accounting information logged.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
---

 kernel/acct.c |   25 ++++++++++++++++++++-----
 1 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/kernel/acct.c b/kernel/acct.c
index 9f33910..54956cc 100644
--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -76,7 +76,7 @@ int acct_parm[3] = {4, 2, 30};
  * External references and all of the globals.
  */
 static void do_acct_process(struct bsd_acct_struct *acct,
-		struct pid_namespace *ns, struct file *);
+		struct pid_namespace *ns, struct file *, const struct cred *);
 
 /*
  * This structure is used so that all the data protected by lock
@@ -90,6 +90,7 @@ struct bsd_acct_struct {
 	struct pid_namespace	*ns;
 	struct timer_list	timer;
 	struct list_head	list;
+	const struct cred	*cred;
 };
 
 static DEFINE_SPINLOCK(acct_lock);
@@ -181,20 +182,24 @@ static void acct_file_reopen(struct bsd_acct_struct *acct, struct file *file,
 {
 	struct file *old_acct = NULL;
 	struct pid_namespace *old_ns = NULL;
+	const struct cred *old_cred = NULL;
 
 	if (acct->file) {
 		old_acct = acct->file;
 		old_ns = acct->ns;
+		old_cred = acct->cred;
 		del_timer(&acct->timer);
 		acct->active = 0;
 		acct->needcheck = 0;
 		acct->file = NULL;
 		acct->ns = NULL;
+		acct->cred = NULL;
 		list_del(&acct->list);
 	}
 	if (file) {
 		acct->file = file;
 		acct->ns = ns;
+		acct->cred = get_current_cred();
 		acct->needcheck = 0;
 		acct->active = 1;
 		list_add(&acct->list, &acct_list);
@@ -206,7 +211,8 @@ static void acct_file_reopen(struct bsd_acct_struct *acct, struct file *file,
 	if (old_acct) {
 		mnt_unpin(old_acct->f_path.mnt);
 		spin_unlock(&acct_lock);
-		do_acct_process(acct, old_ns, old_acct);
+		do_acct_process(acct, old_ns, old_acct, old_cred);
+		put_cred(old_cred);
 		filp_close(old_acct, NULL);
 		spin_lock(&acct_lock);
 	}
@@ -481,7 +487,7 @@ static u32 encode_float(u64 value)
  *  do_acct_process does all actual work. Caller holds the reference to file.
  */
 static void do_acct_process(struct bsd_acct_struct *acct,
-		struct pid_namespace *ns, struct file *file)
+	struct pid_namespace *ns, struct file *file, const struct cred *cred)
 {
 	struct pacct_struct *pacct = &current->signal->pacct;
 	acct_t ac;
@@ -491,13 +497,17 @@ static void do_acct_process(struct bsd_acct_struct *acct,
 	u64 run_time;
 	struct timespec uptime;
 	struct tty_struct *tty;
+	const struct cred *orig_cred;
+
+	/* Perform file operations on behalf of whoever enabled accounting */
+	orig_cred = override_creds(cred);
 
 	/*
 	 * First check to see if there is enough free_space to continue
 	 * the process accounting system.
 	 */
 	if (!check_free_space(acct, file))
-		return;
+		goto out;
 
 	/*
 	 * Fill the accounting struct with the needed info as recorded
@@ -578,6 +588,8 @@ static void do_acct_process(struct bsd_acct_struct *acct,
 			       sizeof(acct_t), &file->f_pos);
 	current->signal->rlim[RLIMIT_FSIZE].rlim_cur = flim;
 	set_fs(fs);
+out:
+	revert_creds(orig_cred);
 }
 
 /**
@@ -636,6 +648,7 @@ static void acct_process_in_ns(struct pid_namespace *ns)
 {
 	struct file *file = NULL;
 	struct bsd_acct_struct *acct;
+	const struct cred *cred;
 
 	acct = ns->bacct;
 	/*
@@ -651,9 +664,11 @@ static void acct_process_in_ns(struct pid_namespace *ns)
 		return;
 	}
 	get_file(file);
+	cred = get_cred(acct->cred);
 	spin_unlock(&acct_lock);
 
-	do_acct_process(acct, ns, file);
+	do_acct_process(acct, ns, file, cred);
+	put_cred(cred);
 	fput(file);
 }
 

Re: [PATCH] bsdacct: switch credentials for writing to the accounting file

[David Howells]

Michal Schmidt <mschmidt@redhat.com> wrote:

> When process accounting is enabled, every exiting process writes a log
> to the account file. In addition, every once in a while one of the
> exiting processes checks whether there's enough free space for the log.
> 
> SELinux policy may or may not allow the exiting process to stat the fs.
> So unsuspecting processes start generating AVC denials just because
> someone enabled process accounting.
> 
> For these filesystem operations, the exiting process's credentials
> should be temporarily switched to that of the process which enabled
> accounting, because it's really that process who wanted to have the
> accounting information logged.
> 
> Signed-off-by: Michal Schmidt <mschmidt@redhat.com>

Do you really need to keep creds in acct?  Can you used acct->file->f_cred
instead?  Those are the credentials of the process that opened the file, so
acct->cred may be redundant.

Other than that, it looks reasonable.

David

[PATCH v2] bsdacct: switch credentials for writing to the accounting file

[Michal Schmidt]

When process accounting is enabled, every exiting process writes a log
to the account file. In addition, every once in a while one of the
exiting processes checks whether there's enough free space for the log.

SELinux policy may or may not allow the exiting process to stat the fs.
So unsuspecting processes start generating AVC denials just because
someone enabled process accounting.

For these filesystem operations, the exiting process's credentials
should be temporarily switched to that of the process which enabled
accounting, because it's really that process who wanted to have the
accounting information logged.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
---

Dne Thu, 20 Aug 2009 11:02:06 +0100 David Howells napsal:
> Do you really need to keep creds in acct?  Can you used
> acct->file->f_cred instead?  Those are the credentials of the process
> that opened the file, so acct->cred may be redundant.
> 
> Other than that, it looks reasonable.

You're right, I didn't realize that. That makes the patch much simpler.

 kernel/acct.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/kernel/acct.c b/kernel/acct.c
index 9f33910..9a4715a 100644
--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -491,13 +491,17 @@ static void do_acct_process(struct bsd_acct_struct *acct,
 	u64 run_time;
 	struct timespec uptime;
 	struct tty_struct *tty;
+	const struct cred *orig_cred;
+
+	/* Perform file operations on behalf of whoever enabled accounting */
+	orig_cred = override_creds(file->f_cred);
 
 	/*
 	 * First check to see if there is enough free_space to continue
 	 * the process accounting system.
 	 */
 	if (!check_free_space(acct, file))
-		return;
+		goto out;
 
 	/*
 	 * Fill the accounting struct with the needed info as recorded
@@ -578,6 +582,8 @@ static void do_acct_process(struct bsd_acct_struct *acct,
 			       sizeof(acct_t), &file->f_pos);
 	current->signal->rlim[RLIMIT_FSIZE].rlim_cur = flim;
 	set_fs(fs);
+out:
+	revert_creds(orig_cred);
 }
 
 /**
-- 
1.6.2.5

Re: [PATCH v2] bsdacct: switch credentials for writing to the accounting file

[David Howells]

Michal Schmidt <mschmidt@redhat.com> wrote:

> When process accounting is enabled, every exiting process writes a log
> to the account file. In addition, every once in a while one of the
> exiting processes checks whether there's enough free space for the log.
> 
> SELinux policy may or may not allow the exiting process to stat the fs.
> So unsuspecting processes start generating AVC denials just because
> someone enabled process accounting.
> 
> For these filesystem operations, the exiting process's credentials
> should be temporarily switched to that of the process which enabled
> accounting, because it's really that process who wanted to have the
> accounting information logged.
> 
> Signed-off-by: Michal Schmidt <mschmidt@redhat.com>

Acked-by: David Howells <dhowells@redhat.com>