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do_execve() must not clear fs->in_exec if it was set by another thread

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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>
This commit is contained in:
Oleg Nesterov 2009-04-24 01:01:56 +02:00 committed by Linus Torvalds
parent 0910697403
commit 8c652f96d3
2 changed files with 15 additions and 15 deletions
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11
fs/compat.c
View File

@ -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;

19
fs/exec.c
View File

@ -1077,9 +1077,11 @@ int check_unsafe_exec(struct linux_binprm *bprm)
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;