[PATCH 5.4 076/135] sched/psi: Fix OOB write when writing 0 bytes to PSI files

[Greg Kroah-Hartman]

From: Suren Baghdasaryan <surenb@xxxxxxxxxx>

commit 6fcca0fa48118e6d63733eb4644c6cd880c15b8f upstream.

Issuing write() with count parameter set to 0 on any file under
/proc/pressure/ will cause an OOB write because of the access to
buf[buf_size-1] when NUL-termination is performed. Fix this by checking
for buf_size to be non-zero.

Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200203212216.7076-1-surenb@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/sched/psi.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -1199,6 +1199,9 @@ static ssize_t psi_write(struct file *fi
 	if (static_branch_likely(&psi_disabled))
 		return -EOPNOTSUPP;
 
+	if (!nbytes)
+		return -EINVAL;
+
 	buf_size = min(nbytes, sizeof(buf));
 	if (copy_from_user(buf, user_buf, buf_size))
 		return -EFAULT;