[PATCH 1/2] do_execve() must not clear fs->in_exec if it was set by another thread

Oleg Nesterov
2009-04-23T23:08:26Z

If do_execve() fails after check_unsafe_exec(), it clears fs->in_exec
unconditionally. This is wrong if we race with our sub-thread which
also does do_execve:

Two threads T1 and T2 and another process P, all share the same
->fs.

T1 starts do_execve(BAD_FILE). It calls check_unsafe_exec(), since
->fs is shared, we set LSM_UNSAFE but not ->in_exec.

P exits and decrements fs->users.

T2 starts do_execve(), calls check_unsafe_exec(), now ->fs is not
shared, we set fs->in_exec.

T1 continues, open_exec(BAD_FILE) fails, we clear ->in_exec and
return to the user-space.

T1 does clone(CLONE_FS /* without CLONE_THREAD */).

T2 continues without LSM_UNSAFE_SHARE while ->fs is shared with
another process.

Change check_unsafe_exec() to return res = 1 if we set ->in_exec, and change
do_execve() to clear ->in_exec depending on res.

When do_execve() suceeds, it is safe to clear ->in_exec unconditionally.
It can be set only if we don't share ->fs with another process, and since
we already killed all sub-threads either ->in_exec == 0 or we are the
only user of this ->fs.

Also, we do not need fs->lock to clear fs->in_exec.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

fs/exec.c | 19 ++++++++++---------
fs/compat.c | 11 +++++------
2 files changed, 15 insertions(+), 15 deletions(-)

--- PTRACE/fs/exec.c~1_IN_EXEC 2009-04-06 00:03:41.000000000 +0200
+++ PTRACE/fs/exec.c 2009-04-24 00:01:53.000000000 +0200
@@ -1077,9 +1077,11 @@ int check_unsafe_exec(struct linux_binpr
if (p->fs->users > n_fs) {
bprm->unsafe |= LSM_UNSAFE_SHARE;
} else {
- if (p->fs->in_exec)
- res = -EAGAIN;
- p->fs->in_exec = 1;
+ res = -EAGAIN;
+ if (!p->fs->in_exec) {
+ p->fs->in_exec = 1;
+ res = 1;
+ }
}

unlock_task_sighand(p, &flags);
@@ -1284,6 +1286,7 @@ int do_execve(char * filename,
struct linux_binprm *bprm;
struct file *file;
struct files_struct *displaced;
+ bool clear_in_exec;
int retval;

retval = unshare_files(&displaced);
@@ -1306,8 +1309,9 @@ int do_execve(char * filename,
goto out_unlock;

retval = check_unsafe_exec(bprm);
- if (retval)
+ if (retval < 0)
goto out_unlock;
+ clear_in_exec = retval;

file = open_exec(filename);
retval = PTR_ERR(file);
@@ -1355,9 +1359,7 @@ int do_execve(char * filename,
goto out;

/* execve succeeded */
- write_lock(&current->fs->lock);
current->fs->in_exec = 0;
- write_unlock(&current->fs->lock);
current->in_execve = 0;
mutex_unlock(&current->cred_exec_mutex);
acct_update_integrals(current);
@@ -1377,9 +1379,8 @@ out_file:
}

out_unmark:
- write_lock(&current->fs->lock);
- current->fs->in_exec = 0;
- write_unlock(&current->fs->lock);
+ if (clear_in_exec)
+ current->fs->in_exec = 0;

out_unlock:
current->in_execve = 0;
--- PTRACE/fs/compat.c~1_IN_EXEC 2009-04-22 20:49:07.000000000 +0200
+++ PTRACE/fs/compat.c 2009-04-24 00:09:36.000000000 +0200
@@ -1476,6 +1476,7 @@ int compat_do_execve(char * filename,
struct linux_binprm *bprm;
struct file *file;
struct files_struct *displaced;
+ bool clear_in_exec;
int retval;

retval = unshare_files(&displaced);
@@ -1498,8 +1499,9 @@ int compat_do_execve(char * filename,
goto out_unlock;

retval = check_unsafe_exec(bprm);
- if (retval)
+ if (retval < 0)
goto out_unlock;
+ clear_in_exec = retval;

file = open_exec(filename);
retval = PTR_ERR(file);
@@ -1546,9 +1548,7 @@ 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);
current->in_execve = 0;
mutex_unlock(&current->cred_exec_mutex);
acct_update_integrals(current);
@@ -1568,9 +1568,8 @@ out_file:
}

out_unmark:
- write_lock(&current->fs->lock);
- current->fs->in_exec = 0;
- write_unlock(&current->fs->lock);
+ if (clear_in_exec)
+ current->fs->in_exec = 0;

out_unlock:
current->in_execve = 0;