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Subject[PATCH 3.16 22/72] /dev/mem: Bail out upon SIGKILL.
3.16.79-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

commit 8619e5bdeee8b2c685d686281f2d2a6017c4bc15 upstream.

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>
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: Ben Hutchings <ben@decadent.org.uk>
---
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(unsigned
{
}

+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.
@@ -162,6 +169,8 @@ static ssize_t read_mem(struct file *fil
p += sz;
count -= sz;
read += sz;
+ if (should_stop_iteration())
+ break;
}

*ppos += read;
@@ -233,6 +242,8 @@ static ssize_t write_mem(struct file *fi
p += sz;
count -= sz;
written += sz;
+ if (should_stop_iteration())
+ break;
}

*ppos += written;
@@ -436,6 +447,10 @@ static ssize_t read_kmem(struct file *fi
read += sz;
low_count -= sz;
count -= sz;
+ if (should_stop_iteration()) {
+ count = 0;
+ break;
+ }
}
}

@@ -460,6 +475,8 @@ static ssize_t read_kmem(struct file *fi
buf += sz;
read += sz;
p += sz;
+ if (should_stop_iteration())
+ break;
}
free_page((unsigned long)kbuf);
}
@@ -510,6 +527,8 @@ static ssize_t do_write_kmem(unsigned lo
p += sz;
count -= sz;
written += sz;
+ if (should_stop_iteration())
+ break;
}

*ppos += written;
@@ -561,6 +580,8 @@ static ssize_t write_kmem(struct file *f
buf += sz;
virtr += sz;
p += sz;
+ if (should_stop_iteration())
+ break;
}
free_page((unsigned long)kbuf);
}
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