diff mbox series

[v2] pidfd: getfd should always report ESRCH if a task is exiting

Message ID 20240206192357.81942-1-tycho@tycho.pizza
State New
Headers
Series [v2] pidfd: getfd should always report ESRCH if a task is exiting |

Commit Message

Tycho Andersen Feb. 6, 2024, 7:23 p.m. UTC 
  From: Tycho Andersen <tandersen@netflix.com>

We can get EBADF from __pidfd_fget() if a task is currently exiting, which
might be confusing. Let's check PF_EXITING, and just report ESRCH if so.

I chose PF_EXITING, because it is set in exit_signals(), which is called
before exit_files(). Since ->exit_status is mostly set after exit_files()
in exit_notify(), using that still leaves a window open for the race.

Signed-off-by: Tycho Andersen <tandersen@netflix.com>
v2: fix a race in the check by putting the check after __pidfd_fget()
    (thanks Oleg)
---
 kernel/pid.c                                  | 17 +++++++++-
 .../selftests/pidfd/pidfd_getfd_test.c        | 31 ++++++++++++++++++-
 2 files changed, 46 insertions(+), 2 deletions(-)


base-commit: 082d11c164aef02e51bcd9c7cbf1554a8e42d9b5

Comments

Christian Brauner Feb. 7, 2024, 9:15 a.m. UTC | #1 
On Tue, Feb 06, 2024 at 12:23:57PM -0700, Tycho Andersen wrote:
> From: Tycho Andersen <tandersen@netflix.com>
> 
> We can get EBADF from __pidfd_fget() if a task is currently exiting, which
> might be confusing. Let's check PF_EXITING, and just report ESRCH if so.
> 
> I chose PF_EXITING, because it is set in exit_signals(), which is called
> before exit_files(). Since ->exit_status is mostly set after exit_files()
> in exit_notify(), using that still leaves a window open for the race.
> 
> Signed-off-by: Tycho Andersen <tandersen@netflix.com>
> v2: fix a race in the check by putting the check after __pidfd_fget()
>     (thanks Oleg)
> ---
>  kernel/pid.c                                  | 17 +++++++++-
>  .../selftests/pidfd/pidfd_getfd_test.c        | 31 ++++++++++++++++++-
>  2 files changed, 46 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/pid.c b/kernel/pid.c
> index de0bf2f8d18b..a8cd6296ed6d 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -693,8 +693,23 @@ static int pidfd_getfd(struct pid *pid, int fd)
>  
>  	file = __pidfd_fget(task, fd);
>  	put_task_struct(task);
> -	if (IS_ERR(file))
> +	if (IS_ERR(file)) {
> +		/*
> +		 * It is possible that the target thread is exiting; it can be
> +		 * either:
> +		 * 1. before exit_signals(), which gives a real fd
> +		 * 2. before exit_files() takes the task_lock() gives a real fd
> +		 * 3. after exit_files() releases task_lock(), ->files is NULL;
> +		 *    this has PF_EXITING, since it was set in exit_signals(),
> +		 *    __pidfd_fget() returns EBADF.
> +		 * In case 3 we get EBADF, but that really means ESRCH, since
> +		 * the task is currently exiting and has freed its files
> +		 * struct, so we fix it up.
> +		 */
> +		if (task->flags & PF_EXITING && PTR_ERR(file) == -EBADF)
> +			return -ESRCH;

Isn't that a potential UAF because we called put_task_struct() above but
this is exiting task->flags afterwards?
Christian Brauner Feb. 7, 2024, 9:29 a.m. UTC | #2 
On Wed, Feb 07, 2024 at 10:15:25AM +0100, Christian Brauner wrote:
> On Tue, Feb 06, 2024 at 12:23:57PM -0700, Tycho Andersen wrote:
> > From: Tycho Andersen <tandersen@netflix.com>
> > 
> > We can get EBADF from __pidfd_fget() if a task is currently exiting, which
> > might be confusing. Let's check PF_EXITING, and just report ESRCH if so.
> > 
> > I chose PF_EXITING, because it is set in exit_signals(), which is called
> > before exit_files(). Since ->exit_status is mostly set after exit_files()
> > in exit_notify(), using that still leaves a window open for the race.
> > 
> > Signed-off-by: Tycho Andersen <tandersen@netflix.com>
> > v2: fix a race in the check by putting the check after __pidfd_fget()
> >     (thanks Oleg)
> > ---
> >  kernel/pid.c                                  | 17 +++++++++-
> >  .../selftests/pidfd/pidfd_getfd_test.c        | 31 ++++++++++++++++++-
> >  2 files changed, 46 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/pid.c b/kernel/pid.c
> > index de0bf2f8d18b..a8cd6296ed6d 100644
> > --- a/kernel/pid.c
> > +++ b/kernel/pid.c
> > @@ -693,8 +693,23 @@ static int pidfd_getfd(struct pid *pid, int fd)
> >  
> >  	file = __pidfd_fget(task, fd);
> >  	put_task_struct(task);
> > -	if (IS_ERR(file))
> > +	if (IS_ERR(file)) {
> > +		/*
> > +		 * It is possible that the target thread is exiting; it can be
> > +		 * either:
> > +		 * 1. before exit_signals(), which gives a real fd
> > +		 * 2. before exit_files() takes the task_lock() gives a real fd
> > +		 * 3. after exit_files() releases task_lock(), ->files is NULL;
> > +		 *    this has PF_EXITING, since it was set in exit_signals(),
> > +		 *    __pidfd_fget() returns EBADF.
> > +		 * In case 3 we get EBADF, but that really means ESRCH, since
> > +		 * the task is currently exiting and has freed its files
> > +		 * struct, so we fix it up.
> > +		 */
> > +		if (task->flags & PF_EXITING && PTR_ERR(file) == -EBADF)
> > +			return -ESRCH;
> 
> Isn't that a potential UAF because we called put_task_struct() above but
> this is exiting task->flags afterwards?

s/exiting/accessing/
Christian Brauner Feb. 7, 2024, 9:50 a.m. UTC | #3 
On Wed, Feb 07, 2024 at 10:29:51AM +0100, Christian Brauner wrote:
> On Wed, Feb 07, 2024 at 10:15:25AM +0100, Christian Brauner wrote:
> > On Tue, Feb 06, 2024 at 12:23:57PM -0700, Tycho Andersen wrote:
> > > From: Tycho Andersen <tandersen@netflix.com>
> > > 
> > > We can get EBADF from __pidfd_fget() if a task is currently exiting, which
> > > might be confusing. Let's check PF_EXITING, and just report ESRCH if so.
> > > 
> > > I chose PF_EXITING, because it is set in exit_signals(), which is called
> > > before exit_files(). Since ->exit_status is mostly set after exit_files()
> > > in exit_notify(), using that still leaves a window open for the race.
> > > 
> > > Signed-off-by: Tycho Andersen <tandersen@netflix.com>
> > > v2: fix a race in the check by putting the check after __pidfd_fget()
> > >     (thanks Oleg)
> > > ---
> > >  kernel/pid.c                                  | 17 +++++++++-
> > >  .../selftests/pidfd/pidfd_getfd_test.c        | 31 ++++++++++++++++++-
> > >  2 files changed, 46 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/pid.c b/kernel/pid.c
> > > index de0bf2f8d18b..a8cd6296ed6d 100644
> > > --- a/kernel/pid.c
> > > +++ b/kernel/pid.c
> > > @@ -693,8 +693,23 @@ static int pidfd_getfd(struct pid *pid, int fd)
> > >  
> > >  	file = __pidfd_fget(task, fd);
> > >  	put_task_struct(task);
> > > -	if (IS_ERR(file))
> > > +	if (IS_ERR(file)) {
> > > +		/*
> > > +		 * It is possible that the target thread is exiting; it can be
> > > +		 * either:
> > > +		 * 1. before exit_signals(), which gives a real fd
> > > +		 * 2. before exit_files() takes the task_lock() gives a real fd
> > > +		 * 3. after exit_files() releases task_lock(), ->files is NULL;
> > > +		 *    this has PF_EXITING, since it was set in exit_signals(),
> > > +		 *    __pidfd_fget() returns EBADF.
> > > +		 * In case 3 we get EBADF, but that really means ESRCH, since
> > > +		 * the task is currently exiting and has freed its files
> > > +		 * struct, so we fix it up.
> > > +		 */
> > > +		if (task->flags & PF_EXITING && PTR_ERR(file) == -EBADF)
> > > +			return -ESRCH;
> > 
> > Isn't that a potential UAF because we called put_task_struct() above but
> > this is exiting task->flags afterwards?
> 
> s/exiting/accessing/

So this is what I have applied currently where I moved the check into
__pidfd_fget() where it makes more sense imho. But please double check
that I didn't mess anything up:

From 7ab8f833aceb11c78627f4ea5d7e354314efa385 Mon Sep 17 00:00:00 2001
From: Tycho Andersen <tandersen@netflix.com>
Date: Wed, 7 Feb 2024 10:19:29 +0100
Subject: [PATCH 1/2] pidfd: getfd should always report ESRCH if a task is
 exiting

We can get EBADF from pidfd_getfd() if a task is currently exiting,
which might be confusing. Let's check PF_EXITING, and just report ESRCH
if so.

I chose PF_EXITING, because it is set in exit_signals(), which is called
before exit_files(). Since ->exit_status is mostly set after
exit_files() in exit_notify(), using that still leaves a window open for
the race.

Signed-off-by: Tycho Andersen <tandersen@netflix.com>
Link: https://lore.kernel.org/r/20240206192357.81942-1-tycho@tycho.pizza
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 kernel/pid.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index de0bf2f8d18b..c1d940fbd314 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -678,7 +678,26 @@ static struct file *__pidfd_fget(struct task_struct *task, int fd)
 
 	up_read(&task->signal->exec_update_lock);
 
-	return file ?: ERR_PTR(-EBADF);
+	if (!file) {
+		/*
+		 * It is possible that the target thread is exiting; it can be
+		 * either:
+		 * 1. before exit_signals(), which gives a real fd
+		 * 2. before exit_files() takes the task_lock() gives a real fd
+		 * 3. after exit_files() releases task_lock(), ->files is NULL;
+		 *    this has PF_EXITING, since it was set in exit_signals(),
+		 *    __pidfd_fget() returns EBADF.
+		 * In case 3 we get EBADF, but that really means ESRCH, since
+		 * the task is currently exiting and has freed its files
+		 * struct, so we fix it up.
+		 */
+		if (task->flags & PF_EXITING)
+			file = ERR_PTR(-ESRCH);
+		else
+			file = ERR_PTR(-EBADF);
+	}
+
+	return file;
 }
 
 static int pidfd_getfd(struct pid *pid, int fd)
Oleg Nesterov Feb. 7, 2024, 10:32 a.m. UTC | #4 
On 02/07, Christian Brauner wrote:
>
> So this is what I have applied currently where I moved the check into
> __pidfd_fget() where it makes more sense imho. But please double check
> that I didn't mess anything up:

LGTM.

You have already applied this patch, no need to add my ack, but FWIW

Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Christian Brauner Feb. 7, 2024, 11:17 a.m. UTC | #5 
On Wed, Feb 07, 2024 at 11:32:01AM +0100, Oleg Nesterov wrote:
> On 02/07, Christian Brauner wrote:
> >
> > So this is what I have applied currently where I moved the check into
> > __pidfd_fget() where it makes more sense imho. But please double check
> > that I didn't mess anything up:
> 
> LGTM.
> 
> You have already applied this patch, no need to add my ack, but FWIW
> 
> Reviewed-by: Oleg Nesterov <oleg@redhat.com>

Added. Thank you!
Tycho Andersen Feb. 7, 2024, 1:38 p.m. UTC | #6 
On Wed, Feb 07, 2024 at 10:50:26AM +0100, Christian Brauner wrote:
> So this is what I have applied currently where I moved the check into
> __pidfd_fget() where it makes more sense imho. But please double check
> that I didn't mess anything up:

Oof, good catch. Looks good to me.

Thanks,

Tycho
diff mbox series

Patch

diff --git a/kernel/pid.c b/kernel/pid.c
index de0bf2f8d18b..a8cd6296ed6d 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -693,8 +693,23 @@  static int pidfd_getfd(struct pid *pid, int fd)
 
 	file = __pidfd_fget(task, fd);
 	put_task_struct(task);
-	if (IS_ERR(file))
+	if (IS_ERR(file)) {
+		/*
+		 * It is possible that the target thread is exiting; it can be
+		 * either:
+		 * 1. before exit_signals(), which gives a real fd
+		 * 2. before exit_files() takes the task_lock() gives a real fd
+		 * 3. after exit_files() releases task_lock(), ->files is NULL;
+		 *    this has PF_EXITING, since it was set in exit_signals(),
+		 *    __pidfd_fget() returns EBADF.
+		 * In case 3 we get EBADF, but that really means ESRCH, since
+		 * the task is currently exiting and has freed its files
+		 * struct, so we fix it up.
+		 */
+		if (task->flags & PF_EXITING && PTR_ERR(file) == -EBADF)
+			return -ESRCH;
 		return PTR_ERR(file);
+	}
 
 	ret = receive_fd(file, NULL, O_CLOEXEC);
 	fput(file);
diff --git a/tools/testing/selftests/pidfd/pidfd_getfd_test.c b/tools/testing/selftests/pidfd/pidfd_getfd_test.c
index 0930e2411dfb..cd51d547b751 100644
--- a/tools/testing/selftests/pidfd/pidfd_getfd_test.c
+++ b/tools/testing/selftests/pidfd/pidfd_getfd_test.c
@@ -5,6 +5,7 @@ 
 #include <fcntl.h>
 #include <limits.h>
 #include <linux/types.h>
+#include <poll.h>
 #include <sched.h>
 #include <signal.h>
 #include <stdio.h>
@@ -129,6 +130,7 @@  FIXTURE(child)
 	 * When it is closed, the child will exit.
 	 */
 	int sk;
+	bool ignore_child_result;
 };
 
 FIXTURE_SETUP(child)
@@ -165,10 +167,14 @@  FIXTURE_SETUP(child)
 
 FIXTURE_TEARDOWN(child)
 {
+	int ret;
+
 	EXPECT_EQ(0, close(self->pidfd));
 	EXPECT_EQ(0, close(self->sk));
 
-	EXPECT_EQ(0, wait_for_pid(self->pid));
+	ret = wait_for_pid(self->pid);
+	if (!self->ignore_child_result)
+		EXPECT_EQ(0, ret);
 }
 
 TEST_F(child, disable_ptrace)
@@ -235,6 +241,29 @@  TEST(flags_set)
 	EXPECT_EQ(errno, EINVAL);
 }
 
+TEST_F(child, no_strange_EBADF)
+{
+	struct pollfd fds;
+
+	self->ignore_child_result = true;
+
+	fds.fd = self->pidfd;
+	fds.events = POLLIN;
+
+	ASSERT_EQ(kill(self->pid, SIGKILL), 0);
+	ASSERT_EQ(poll(&fds, 1, 5000), 1);
+
+	/*
+	 * It used to be that pidfd_getfd() could race with the exiting thread
+	 * between exit_files() and release_task(), and get a non-null task
+	 * with a NULL files struct, and you'd get EBADF, which was slightly
+	 * confusing.
+	 */
+	errno = 0;
+	EXPECT_EQ(sys_pidfd_getfd(self->pidfd, self->remote_fd, 0), -1);
+	EXPECT_EQ(errno, ESRCH);
+}
+
 #if __NR_pidfd_getfd == -1
 int main(void)
 {