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authorDmitry V. Levin <ldv@altlinux.org>2019-06-23 14:27:17 +0300
committerChristian Brauner <christian@brauner.io>2019-06-24 15:52:54 +0200
commit9014143bab2f3bc0b9e5db3bc8d00e2a43e50fbd (patch)
treed74f92ce4c3467e485a0097a1c8f5a7be484c206 /kernel/fork.c
parent4b972a01a7da614b4796475f933094751a295a2f (diff)
fork: don't check parent_tidptr with CLONE_PIDFD
Give userspace a cheap and reliable way to tell whether CLONE_PIDFD is supported by the kernel or not. The easiest way is to pass an invalid file descriptor value in parent_tidptr, perform the syscall and verify that parent_tidptr has been changed to a valid file descriptor value. CLONE_PIDFD uses parent_tidptr to return pidfds. CLONE_PARENT_SETTID will use parent_tidptr to return the tid of the parent. The two flags cannot be used together. Old kernels that only support CLONE_PARENT_SETTID will not verify the value pointed to by parent_tidptr. This behavior is unchanged even with the introduction of CLONE_PIDFD. However, if CLONE_PIDFD is specified the kernel will currently check the value pointed to by parent_tidptr before placing the pidfd in the memory pointed to. EINVAL will be returned if the value in parent_tidptr is not 0. If CLONE_PIDFD is supported and fd 0 is closed, then the returned pidfd can and likely will be 0 and parent_tidptr will be unchanged. This means userspace must either check CLONE_PIDFD support beforehand or check that fd 0 is not closed when invoking CLONE_PIDFD. The check for pidfd == 0 was introduced during the v5.2 merge window by commit b3e583825266 ("clone: add CLONE_PIDFD") to ensure that CLONE_PIDFD could be potentially extended by passing in flags through the return argument. However, that extension would look horrible, and with the upcoming introduction of the clone3 syscall in v5.3 there is no need to extend legacy clone syscall this way. (Even if it would need to be extended, CLONE_DETACHED can be reused with CLONE_PIDFD.) So remove the pidfd == 0 check. Userspace that needs to be portable to kernels without CLONE_PIDFD support can then be advised to initialize pidfd to -1 and check the pidfd value returned by CLONE_PIDFD. Fixes: b3e583825266 ("clone: add CLONE_PIDFD") Signed-off-by: Dmitry V. Levin <ldv@altlinux.org> Signed-off-by: Christian Brauner <christian@brauner.io>
Diffstat (limited to 'kernel/fork.c')
-rw-r--r--kernel/fork.c12
1 files changed, 0 insertions, 12 deletions
diff --git a/kernel/fork.c b/kernel/fork.c
index 75675b9bf6df..39a3adaa4ad1 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1822,8 +1822,6 @@ static __latent_entropy struct task_struct *copy_process(
}
if (clone_flags & CLONE_PIDFD) {
- int reserved;
-
/*
* - CLONE_PARENT_SETTID is useless for pidfds and also
* parent_tidptr is used to return pidfds.
@@ -1834,16 +1832,6 @@ static __latent_entropy struct task_struct *copy_process(
if (clone_flags &
(CLONE_DETACHED | CLONE_PARENT_SETTID | CLONE_THREAD))
return ERR_PTR(-EINVAL);
-
- /*
- * Verify that parent_tidptr is sane so we can potentially
- * reuse it later.
- */
- if (get_user(reserved, parent_tidptr))
- return ERR_PTR(-EFAULT);
-
- if (reserved != 0)
- return ERR_PTR(-EINVAL);
}
/*