/dev/mem: Bail out upon SIGKILL.

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>

1 files changed, 21 insertions, 0 deletions

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index b08dc50f9f26..9eb564c002f6 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -97,6 +97,13 @@ void __weak unxlate_dev_mem_ptr(phys_addr_t phys, void *addr)
 }
 #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.
@@ -175,6 +182,8 @@ static ssize_t read_mem(struct file *file, char __user *buf,
                 p += sz;
                 count -= sz;
                 read += sz;
+                if (should_stop_iteration())
+                        break;
         }
         kfree(bounce);
 
@@ -251,6 +260,8 @@ static ssize_t write_mem(struct file *file, const char __user *buf,
                 p += sz;
                 count -= sz;
                 written += sz;
+                if (should_stop_iteration())
+                        break;
         }
 
         *ppos += written;
@@ -468,6 +479,10 @@ static ssize_t read_kmem(struct file *file, char __user *buf,
                         read += sz;
                         low_count -= sz;
                         count -= sz;
+                        if (should_stop_iteration()) {
+                                count = 0;
+                                break;
+                        }
                 }
         }
 
@@ -492,6 +507,8 @@ static ssize_t read_kmem(struct file *file, char __user *buf,
                         buf += sz;
                         read += sz;
                         p += sz;
+                        if (should_stop_iteration())
+                                break;
                 }
                 free_page((unsigned long)kbuf);
         }
@@ -544,6 +561,8 @@ static ssize_t do_write_kmem(unsigned long p, const char __user *buf,
                 p += sz;
                 count -= sz;
                 written += sz;
+                if (should_stop_iteration())
+                        break;
         }
 
         *ppos += written;
@@ -595,6 +614,8 @@ static ssize_t write_kmem(struct file *file, const char __user *buf,
                         buf += sz;
                         virtr += sz;
                         p += sz;
+                        if (should_stop_iteration())
+                                break;
                 }
                 free_page((unsigned long)kbuf);
         }