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Subject[patch 40/58] New locking/refcounting for fs_struct
2.6.29-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Al Viro <viro@zeniv.linux.org.uk>

commit 498052bba55ecaff58db6a1436b0e25bfd75a7ff upstream.

* all changes of current->fs are done under task_lock and write_lock of
old fs->lock
* refcount is not atomic anymore (same protection)
* its decrements are done when removing reference from current; at the
same time we decide whether to free it.
* put_fs_struct() is gone
* new field - ->in_exec. Set by check_unsafe_exec() if we are trying to do
execve() and only subthreads share fs_struct. Cleared when finishing exec
(success and failure alike). Makes CLONE_FS fail with -EAGAIN if set.
* check_unsafe_exec() may fail with -EAGAIN if another execve() from subthread
is in progress.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
fs/compat.c | 16 +++++++++-
fs/exec.c | 31 +++++++++++++++++---
fs/fs_struct.c | 69 ++++++++++++++++++++++++++++++++--------------
fs/internal.h | 2 -
fs/proc/task_nommu.c | 2 -
include/linux/fs_struct.h | 8 ++---
kernel/fork.c | 37 ++++++++++++++++++------
7 files changed, 121 insertions(+), 44 deletions(-)

--- a/fs/compat.c
+++ b/fs/compat.c
@@ -51,6 +51,7 @@
#include <linux/poll.h>
#include <linux/mm.h>
#include <linux/eventpoll.h>
+#include <linux/fs_struct.h>

#include <asm/uaccess.h>
#include <asm/mmu_context.h>
@@ -1412,12 +1413,15 @@ int compat_do_execve(char * filename,
bprm->cred = prepare_exec_creds();
if (!bprm->cred)
goto out_unlock;
- check_unsafe_exec(bprm);
+
+ retval = check_unsafe_exec(bprm);
+ if (retval)
+ goto out_unlock;

file = open_exec(filename);
retval = PTR_ERR(file);
if (IS_ERR(file))
- goto out_unlock;
+ goto out_unmark;

sched_exec();

@@ -1459,6 +1463,9 @@ int compat_do_execve(char * filename,
goto out;

/* execve succeeded */
+ write_lock(&current->fs->lock);
+ current->fs->in_exec = 0;
+ write_unlock(&current->fs->lock);
mutex_unlock(&current->cred_exec_mutex);
acct_update_integrals(current);
free_bprm(bprm);
@@ -1476,6 +1483,11 @@ out_file:
fput(bprm->file);
}

+out_unmark:
+ write_lock(&current->fs->lock);
+ current->fs->in_exec = 0;
+ write_unlock(&current->fs->lock);
+
out_unlock:
mutex_unlock(&current->cred_exec_mutex);

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1049,16 +1049,18 @@ EXPORT_SYMBOL(install_exec_creds);
* - the caller must hold current->cred_exec_mutex to protect against
* PTRACE_ATTACH
*/
-void check_unsafe_exec(struct linux_binprm *bprm)
+int check_unsafe_exec(struct linux_binprm *bprm)
{
struct task_struct *p = current, *t;
unsigned long flags;
unsigned n_fs, n_sighand;
+ int res = 0;

bprm->unsafe = tracehook_unsafe_exec(p);

n_fs = 1;
n_sighand = 1;
+ write_lock(&p->fs->lock);
lock_task_sighand(p, &flags);
for (t = next_thread(p); t != p; t = next_thread(t)) {
if (t->fs == p->fs)
@@ -1066,11 +1068,19 @@ void check_unsafe_exec(struct linux_binp
n_sighand++;
}

- if (atomic_read(&p->fs->count) > n_fs ||
- atomic_read(&p->sighand->count) > n_sighand)
+ if (p->fs->users > n_fs ||
+ atomic_read(&p->sighand->count) > n_sighand) {
bprm->unsafe |= LSM_UNSAFE_SHARE;
+ } else {
+ if (p->fs->in_exec)
+ res = -EAGAIN;
+ p->fs->in_exec = 1;
+ }

unlock_task_sighand(p, &flags);
+ write_unlock(&p->fs->lock);
+
+ return res;
}

/*
@@ -1285,12 +1295,15 @@ int do_execve(char * filename,
bprm->cred = prepare_exec_creds();
if (!bprm->cred)
goto out_unlock;
- check_unsafe_exec(bprm);
+
+ retval = check_unsafe_exec(bprm);
+ if (retval)
+ goto out_unlock;

file = open_exec(filename);
retval = PTR_ERR(file);
if (IS_ERR(file))
- goto out_unlock;
+ goto out_unmark;

sched_exec();

@@ -1333,6 +1346,9 @@ int do_execve(char * filename,
goto out;

/* execve succeeded */
+ write_lock(&current->fs->lock);
+ current->fs->in_exec = 0;
+ write_unlock(&current->fs->lock);
mutex_unlock(&current->cred_exec_mutex);
acct_update_integrals(current);
free_bprm(bprm);
@@ -1350,6 +1366,11 @@ out_file:
fput(bprm->file);
}

+out_unmark:
+ write_lock(&current->fs->lock);
+ current->fs->in_exec = 0;
+ write_unlock(&current->fs->lock);
+
out_unlock:
mutex_unlock(&current->cred_exec_mutex);

--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -72,25 +72,27 @@ void chroot_fs_refs(struct path *old_roo
path_put(old_root);
}

-void put_fs_struct(struct fs_struct *fs)
+void free_fs_struct(struct fs_struct *fs)
{
- /* No need to hold fs->lock if we are killing it */
- if (atomic_dec_and_test(&fs->count)) {
- path_put(&fs->root);
- path_put(&fs->pwd);
- kmem_cache_free(fs_cachep, fs);
- }
+ path_put(&fs->root);
+ path_put(&fs->pwd);
+ kmem_cache_free(fs_cachep, fs);
}

void exit_fs(struct task_struct *tsk)
{
- struct fs_struct * fs = tsk->fs;
+ struct fs_struct *fs = tsk->fs;

if (fs) {
+ int kill;
task_lock(tsk);
+ write_lock(&fs->lock);
tsk->fs = NULL;
+ kill = !--fs->users;
+ write_unlock(&fs->lock);
task_unlock(tsk);
- put_fs_struct(fs);
+ if (kill)
+ free_fs_struct(fs);
}
}

@@ -99,7 +101,8 @@ struct fs_struct *copy_fs_struct(struct
struct fs_struct *fs = kmem_cache_alloc(fs_cachep, GFP_KERNEL);
/* We don't need to lock fs - think why ;-) */
if (fs) {
- atomic_set(&fs->count, 1);
+ fs->users = 1;
+ fs->in_exec = 0;
rwlock_init(&fs->lock);
fs->umask = old->umask;
read_lock(&old->lock);
@@ -114,28 +117,54 @@ struct fs_struct *copy_fs_struct(struct

int unshare_fs_struct(void)
{
- struct fs_struct *fsp = copy_fs_struct(current->fs);
- if (!fsp)
+ struct fs_struct *fs = current->fs;
+ struct fs_struct *new_fs = copy_fs_struct(fs);
+ int kill;
+
+ if (!new_fs)
return -ENOMEM;
- exit_fs(current);
- current->fs = fsp;
+
+ task_lock(current);
+ write_lock(&fs->lock);
+ kill = !--fs->users;
+ current->fs = new_fs;
+ write_unlock(&fs->lock);
+ task_unlock(current);
+
+ if (kill)
+ free_fs_struct(fs);
+
return 0;
}
EXPORT_SYMBOL_GPL(unshare_fs_struct);

/* to be mentioned only in INIT_TASK */
struct fs_struct init_fs = {
- .count = ATOMIC_INIT(1),
+ .users = 1,
.lock = __RW_LOCK_UNLOCKED(init_fs.lock),
.umask = 0022,
};

void daemonize_fs_struct(void)
{
- struct fs_struct *fs;
+ struct fs_struct *fs = current->fs;

- exit_fs(current); /* current->fs->count--; */
- fs = &init_fs;
- current->fs = fs;
- atomic_inc(&fs->count);
+ if (fs) {
+ int kill;
+
+ task_lock(current);
+
+ write_lock(&init_fs.lock);
+ init_fs.users++;
+ write_unlock(&init_fs.lock);
+
+ write_lock(&fs->lock);
+ current->fs = &init_fs;
+ kill = !--fs->users;
+ write_unlock(&fs->lock);
+
+ task_unlock(current);
+ if (kill)
+ free_fs_struct(fs);
+ }
}
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -44,7 +44,7 @@ extern void __init chrdev_init(void);
/*
* exec.c
*/
-extern void check_unsafe_exec(struct linux_binprm *);
+extern int check_unsafe_exec(struct linux_binprm *);

/*
* namespace.c
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -49,7 +49,7 @@ void task_mem(struct seq_file *m, struct
else
bytes += kobjsize(mm);

- if (current->fs && atomic_read(&current->fs->count) > 1)
+ if (current->fs && current->fs->users > 1)
sbytes += kobjsize(current->fs);
else
bytes += kobjsize(current->fs);
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -4,12 +4,10 @@
#include <linux/path.h>

struct fs_struct {
- atomic_t count; /* This usage count is used by check_unsafe_exec() for
- * security checking purposes - therefore it may not be
- * incremented, except by clone(CLONE_FS).
- */
+ int users;
rwlock_t lock;
int umask;
+ int in_exec;
struct path root, pwd;
};

@@ -19,7 +17,7 @@ extern void exit_fs(struct task_struct *
extern void set_fs_root(struct fs_struct *, struct path *);
extern void set_fs_pwd(struct fs_struct *, struct path *);
extern struct fs_struct *copy_fs_struct(struct fs_struct *);
-extern void put_fs_struct(struct fs_struct *);
+extern void free_fs_struct(struct fs_struct *);
extern void daemonize_fs_struct(void);
extern int unshare_fs_struct(void);

--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -678,11 +678,19 @@ fail_nomem:

static int copy_fs(unsigned long clone_flags, struct task_struct *tsk)
{
+ struct fs_struct *fs = current->fs;
if (clone_flags & CLONE_FS) {
- atomic_inc(&current->fs->count);
+ /* tsk->fs is already what we want */
+ write_lock(&fs->lock);
+ if (fs->in_exec) {
+ write_unlock(&fs->lock);
+ return -EAGAIN;
+ }
+ fs->users++;
+ write_unlock(&fs->lock);
return 0;
}
- tsk->fs = copy_fs_struct(current->fs);
+ tsk->fs = copy_fs_struct(fs);
if (!tsk->fs)
return -ENOMEM;
return 0;
@@ -1518,12 +1526,16 @@ static int unshare_fs(unsigned long unsh
{
struct fs_struct *fs = current->fs;

- if ((unshare_flags & CLONE_FS) &&
- (fs && atomic_read(&fs->count) > 1)) {
- *new_fsp = copy_fs_struct(current->fs);
- if (!*new_fsp)
- return -ENOMEM;
- }
+ if (!(unshare_flags & CLONE_FS) || !fs)
+ return 0;
+
+ /* don't need lock here; in the worst case we'll do useless copy */
+ if (fs->users == 1)
+ return 0;
+
+ *new_fsp = copy_fs_struct(fs);
+ if (!*new_fsp)
+ return -ENOMEM;

return 0;
}
@@ -1639,8 +1651,13 @@ SYSCALL_DEFINE1(unshare, unsigned long,

if (new_fs) {
fs = current->fs;
+ write_lock(&fs->lock);
current->fs = new_fs;
- new_fs = fs;
+ if (--fs->users)
+ new_fs = NULL;
+ else
+ new_fs = fs;
+ write_unlock(&fs->lock);
}

if (new_mm) {
@@ -1679,7 +1696,7 @@ bad_unshare_cleanup_sigh:

bad_unshare_cleanup_fs:
if (new_fs)
- put_fs_struct(new_fs);
+ free_fs_struct(new_fs);

bad_unshare_cleanup_thread:
bad_unshare_out:



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