From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Mon, 26 Aug 2019 22:13:25 +0900
Subject: /dev/mem: Bail out upon SIGKILL.
Git-commit: 8619e5bdeee8b2c685d686281f2d2a6017c4bc15
Patch-mainline: 5.4-rc1
References: git-fixes

syzbot found that a thread can stall for minutes inside read_mem() or
write_mem() after that thread was killed by SIGKILL [1]. Reading from
iomem areas of /dev/mem can be slow, depending on the hardware.
While reading 2GB at one read() is legal, delaying termination of killed
thread for minutes is bad. Thus, allow reading/writing /dev/mem and
/dev/kmem to be preemptible and killable.

  [ 1335.912419][T20577] read_mem: sz=4096 count=2134565632
  [ 1335.943194][T20577] read_mem: sz=4096 count=2134561536
  [ 1335.978280][T20577] read_mem: sz=4096 count=2134557440
  [ 1336.011147][T20577] read_mem: sz=4096 count=2134553344
  [ 1336.041897][T20577] read_mem: sz=4096 count=2134549248

Theoretically, reading/writing /dev/mem and /dev/kmem can become
"interruptible". But this patch chose "killable". Future patch will make
them "interruptible" so that we can revert to "killable" if some program
regressed.

[1] https://syzkaller.appspot.com/bug?id=a0e3436829698d5824231251fad9d8e998f94f5e

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: stable <stable@vger.kernel.org>
Reported-by: syzbot <syzbot+8ab2d0f39fb79fe6ca40@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/1566825205-10703-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 drivers/char/mem.c |   21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -96,6 +96,13 @@ void __weak unxlate_dev_mem_ptr(phys_add
 }
 #endif
 
+static inline bool should_stop_iteration(void)
+{
+	if (need_resched())
+		cond_resched();
+	return fatal_signal_pending(current);
+}
+
 /*
  * This funcion reads the *physical* memory. The f_pos points directly to the
  * memory location.
@@ -174,6 +181,8 @@ static ssize_t read_mem(struct file *fil
 		p += sz;
 		count -= sz;
 		read += sz;
+		if (should_stop_iteration())
+			break;
 	}
 	kfree(bounce);
 
@@ -250,6 +259,8 @@ static ssize_t write_mem(struct file *fi
 		p += sz;
 		count -= sz;
 		written += sz;
+		if (should_stop_iteration())
+			break;
 	}
 
 	*ppos += written;
@@ -463,6 +474,10 @@ static ssize_t read_kmem(struct file *fi
 			read += sz;
 			low_count -= sz;
 			count -= sz;
+			if (should_stop_iteration()) {
+				count = 0;
+				break;
+			}
 		}
 	}
 
@@ -487,6 +502,8 @@ static ssize_t read_kmem(struct file *fi
 			buf += sz;
 			read += sz;
 			p += sz;
+			if (should_stop_iteration())
+				break;
 		}
 		free_page((unsigned long)kbuf);
 	}
@@ -539,6 +556,8 @@ static ssize_t do_write_kmem(unsigned lo
 		p += sz;
 		count -= sz;
 		written += sz;
+		if (should_stop_iteration())
+			break;
 	}
 
 	*ppos += written;
@@ -590,6 +609,8 @@ static ssize_t write_kmem(struct file *f
 			buf += sz;
 			virtr += sz;
 			p += sz;
+			if (should_stop_iteration())
+				break;
 		}
 		free_page((unsigned long)kbuf);
 	}