linux-kernel - [PATCH] fs/proc: report eip/esp in /prod/PID/stat for coredumping

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Date:   Thu, 14 Sep 2017 11:42:17 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Tycho Andersen <tycho.andersen@...onical.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: [PATCH] fs/proc: report eip/esp in /prod/PID/stat for coredumping

Commit 0a1eb2d474ed ("fs/proc: Stop reporting eip and esp in
/proc/PID/stat") stopped reporting eip/esp because it is
racey and dangerous for executing tasks. The comment adds:

    As far as I know, there are no use programs that make any
    material use of these fields, so just get rid of them.

However, existing userspace core-dump-handler applications (for
example, minicoredumper) are using these fields since they
provide an excellent cross-platform interface to these valuable
pointers. So that commit introduced a user space visible
regression.

Partially revert the change and make the readout possible for
tasks with the proper permissions and only if the target task
has the PF_DUMPCORE flag set.

Reported-by: Marco Felsch <marco.felsch@...h.de>
Signed-off-by: John Ogness <john.ogness@...utronix.de>
Cc: stable@...r.kernel.org
Fixes: 0a1eb2d474ed ("fs/proc: Stop reporting eip and esp in> /proc/PID/stat")
---
 fs/proc/array.c |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 88c3555..696cc68 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -421,7 +421,15 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 		 * esp and eip are intentionally zeroed out.  There is no
 		 * non-racy way to read them without freezing the task.
 		 * Programs that need reliable values can use ptrace(2).
+		 *
+		 * The only exception is if the task is core dumping because
+		 * a program is not able to use ptrace(2) in that case. It is
+		 * safe because the task has stopped executing permanently.
 		 */
+		if (permitted && (task->flags & PF_DUMPCORE)) {
+			eip = KSTK_EIP(task);
+			esp = KSTK_ESP(task);
+		}
 	}
 
 	get_task_comm(tcomm, task);
-- 
1.7.10.4