linux-kernel - [PATCH] cifs: clear PF_MEMALLOC before exiting demultiplex thread

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Message-Id: <20200308043645.1034870-1-ebiggers@kernel.org>
Date:   Sat,  7 Mar 2020 20:36:45 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     linux-cifs@...r.kernel.org
Cc:     linux-ext4@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        linux-kernel@...r.kernel.org
Subject: [PATCH] cifs: clear PF_MEMALLOC before exiting demultiplex thread

From: Eric Biggers <ebiggers@...gle.com>

Leaving PF_MEMALLOC set when exiting a kthread causes it to remain set
during do_exit().  That can confuse things.  For example, if BSD process
accounting is enabled, then it's possible for do_exit() to end up
calling ext4_write_inode().  That triggers the
WARN_ON_ONCE(current->flags & PF_MEMALLOC) there, as it assumes
(appropriately) that inodes aren't written when allocating memory.

This case was reported by syzbot at
https://lkml.kernel.org/r/0000000000000e7156059f751d7b@google.com.

Fix this in cifs_demultiplex_thread() by using the helper functions to
save and restore PF_MEMALLOC.

Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
 fs/cifs/connect.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
index 4804d1df8c1c..beab1dc2dc01 100644
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -1164,8 +1164,9 @@ cifs_demultiplex_thread(void *p)
 	struct task_struct *task_to_wake = NULL;
 	struct mid_q_entry *mids[MAX_COMPOUND];
 	char *bufs[MAX_COMPOUND];
+	unsigned int noreclaim_flag;

-	current->flags |= PF_MEMALLOC;
+	noreclaim_flag = memalloc_noreclaim_save();
 	cifs_dbg(FYI, "Demultiplex PID: %d\n", task_pid_nr(current));

 	length = atomic_inc_return(&tcpSesAllocCount);
@@ -1320,6 +1321,7 @@ cifs_demultiplex_thread(void *p)
 		set_current_state(TASK_RUNNING);
 	}

+	memalloc_noreclaim_restore(noreclaim_flag);
 	module_put_and_exit(0);
 }

-- 
2.25.1