do_execve() must not clear fs->in_exec if it was set by another thread
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>
Acked-by: Roland McGrath <roland@redhat.com>
Acked-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/exec.c b/fs/exec.c
index 052a961..a2e6989 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1077,9 +1077,11 @@
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 @@
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 @@
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 @@
goto out;
/* execve succeeded */
- write_lock(¤t->fs->lock);
current->fs->in_exec = 0;
- write_unlock(¤t->fs->lock);
current->in_execve = 0;
mutex_unlock(¤t->cred_exec_mutex);
acct_update_integrals(current);
@@ -1377,9 +1379,8 @@
}
out_unmark:
- write_lock(¤t->fs->lock);
- current->fs->in_exec = 0;
- write_unlock(¤t->fs->lock);
+ if (clear_in_exec)
+ current->fs->in_exec = 0;
out_unlock:
current->in_execve = 0;