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>

1 files changed, 5 insertions, 6 deletions

diff --git a/fs/compat.c b/fs/compat.c
index 379a399bf5c3..681ed81e6be0 100644
--- a/fs/compat.c
+++ b/fs/compat.c
@@ -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;