[PATCH] proc: prevent changes to overridden credentials

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Prevent userspace from changing the the /proc/PID/attr values if the
task's credentials are currently overriden.  This not only makes sense
conceptually, it also prevents some really bizarre error cases caused
when trying to commit credentials to a task with overridden
credentials.

Cc: <stable@xxxxxxxxxxxxxxx>
Reported-by: "chengjian (D)" <cj.chengjian@xxxxxxxxxx>
Signed-off-by: Paul Moore <paul@xxxxxxxxxxxxxx>
---
 fs/proc/base.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ddef482f1334..87ba007b86db 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2539,6 +2539,11 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 		rcu_read_unlock();
 		return -EACCES;
 	}
+	/* Prevent changes to overridden credentials. */
+	if (current_cred() != current_real_cred()) {
+		rcu_read_unlock();
+		return -EBUSY;
+	}
 	rcu_read_unlock();
 
 	if (count > PAGE_SIZE)





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